/*
 * DiskQueue.actor.cpp
 *
 * This source file is part of the FoundationDB open source project
 *
 * Copyright 2013-2022 Apple Inc. and the FoundationDB project authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "fdbserver/IDiskQueue.h"
#include "flow/IAsyncFile.h"
#include "fdbserver/Knobs.h"
#include "fdbrpc/simulator.h"
#include "crc32/crc32c.h"
#include "flow/genericactors.actor.h"
#include "flow/xxhash.h"

#include "flow/actorcompiler.h" // This must be the last #include.

typedef bool (*compare_pages)(void*, void*);
typedef int64_t loc_t;

FDB_DEFINE_BOOLEAN_PARAM(CheckHashes);

// 0 -> 0
// 1 -> 4k
// 4k -> 4k
int64_t pageCeiling(int64_t loc) {
	return (loc + _PAGE_SIZE - 1) / _PAGE_SIZE * _PAGE_SIZE;
}

// 0 -> 0
// 1 -> 0
// 4k -> 4k
int64_t pageFloor(int64_t loc) {
	return loc / _PAGE_SIZE * _PAGE_SIZE;
}

struct StringBuffer {
	Standalone<StringRef> str;
	int reserved;
	UID id;

	StringBuffer(UID fromFileID) : reserved(0), id(fromFileID) {}

	int size() const { return str.size(); }
	StringRef& ref() { return str; }
	void clear() {
		str = Standalone<StringRef>();
		reserved = 0;
	}
	void clearReserve(int size) {
		str = Standalone<StringRef>();
		reserved = size;
		ref() = StringRef(new (str.arena()) uint8_t[size], 0);
	}
	void append(StringRef x) { memcpy(append(x.size()), x.begin(), x.size()); }
	void* append(int bytes) {
		ASSERT(str.size() + bytes <= reserved);
		void* p = const_cast<uint8_t*>(str.end());
		ref() = StringRef(str.begin(), str.size() + bytes);
		return p;
	}
	StringRef pop_front(int bytes) {
		ASSERT(bytes <= str.size());
		StringRef result = str.substr(0, bytes);
		ref() = str.substr(bytes);
		return result;
	}
	void alignReserve(int alignment, int size) {
		ASSERT(alignment && (alignment & (alignment - 1)) == 0); // alignment is a power of two

		if (size >= reserved) {
			// SOMEDAY: Use a new arena and discard the old one after copying?
			reserved = std::max(size, reserved * 2);
			if (reserved > 1e9) {
				printf("WOAH! Huge allocation\n");
				TraceEvent(SevError, "StringBufferHugeAllocation", id)
				    .detail("Alignment", alignment)
				    .detail("Reserved", reserved)
				    .backtrace();
			}
			uint8_t* b = new (str.arena()) uint8_t[reserved + alignment - 1];
			uint8_t* e = b + (reserved + alignment - 1);

			uint8_t* p = (uint8_t*)(int64_t(b + alignment - 1) &
			                        ~(alignment - 1)); // first multiple of alignment greater than or equal to b
			ASSERT(p >= b && p + reserved <= e && int64_t(p) % alignment == 0);

			if (str.size() > 0) {
				memcpy(p, str.begin(), str.size());
			}
			ref() = StringRef(p, str.size());
		}
	}
};

struct SyncQueue : ReferenceCounted<SyncQueue> {
	SyncQueue(int outstandingLimit, Reference<IAsyncFile> file) : outstandingLimit(outstandingLimit), file(file) {
		for (int i = 0; i < outstandingLimit; i++)
			outstanding.push_back(Void());
	}

	Future<Void> onSync() { // Future is set when all writes completed before the call to onSync are complete
		if (outstanding.size() <= outstandingLimit)
			outstanding.push_back(waitAndSync(this));
		return outstanding.back();
	}

private:
	int outstandingLimit;
	Deque<Future<Void>> outstanding;
	Reference<IAsyncFile> file;

	ACTOR static Future<Void> waitAndSync(SyncQueue* self) {
		wait(self->outstanding.front());
		self->outstanding.pop_front();
		wait(self->file->sync());
		return Void();
	}
};

// We use a Tracked instead of a Reference when the shutdown/destructor code would need to wait() on pending file
// operations (e.g., read).
template <typename T>
class Tracked {
protected:
	struct TrackMe : NonCopyable {
		T* self;
		explicit TrackMe(T* self) : self(self) {
			self->actorCount++;
			if (self->actorCount == 1)
				self->actorCountIsZero.set(false);
		}
		~TrackMe() {
			self->actorCount--;
			if (self->actorCount == 0)
				self->actorCountIsZero.set(true);
		}
	};

	Future<Void> onSafeToDestruct() {
		if (actorCountIsZero.get()) {
			return Void();
		} else {
			return actorCountIsZero.onChange();
		}
	}

private:
	int actorCount = 0;
	AsyncVar<bool> actorCountIsZero = true;
};

// DiskQueue uses two files to implement a dynamically resizable ring buffer, where files only allow append and read
// operations.
//    To increase the ring buffer size, it creates a ring buffer in the other file.
//    After finish reading the current file, it switch to use the other file as the ring buffer.
class RawDiskQueue_TwoFiles : public Tracked<RawDiskQueue_TwoFiles> {
public:
	RawDiskQueue_TwoFiles(std::string basename, std::string fileExtension, UID dbgid, int64_t fileSizeWarningLimit)
	  : basename(basename), fileExtension(fileExtension), dbgid(dbgid), dbg_file0BeginSeq(0),
	    fileSizeWarningLimit(fileSizeWarningLimit), onError(delayed(error.getFuture())), onStopped(stopped.getFuture()),
	    readyToPush(Void()), lastCommit(Void()), isFirstCommit(true), readingBuffer(dbgid), readingFile(-1),
	    readingPage(-1), writingPos(-1), fileExtensionBytes(SERVER_KNOBS->DISK_QUEUE_FILE_EXTENSION_BYTES),
	    fileShrinkBytes(SERVER_KNOBS->DISK_QUEUE_FILE_SHRINK_BYTES) {
		if (BUGGIFY)
			fileExtensionBytes = _PAGE_SIZE * deterministicRandom()->randomSkewedUInt32(1, 10 << 10);
		if (BUGGIFY)
			fileShrinkBytes = _PAGE_SIZE * deterministicRandom()->randomSkewedUInt32(1, 10 << 10);
		files[0].dbgFilename = filename(0);
		files[1].dbgFilename = filename(1);
		// We issue reads into firstPages, so it needs to be 4k aligned.
		firstPages.reserve(firstPages.arena(), 2);
		void* pageMemory = operator new(sizeof(Page) * 3, firstPages.arena());
		// firstPages is assumed to always be a valid page, and our initialization here is the only
		// time that it would not contain a valid page.  Whenever DiskQueue reaches in to look at
		// these bytes, it only cares about `seq`, and having that be all 0xFF's means uninitialized
		// pages will look like the ultimate end of the disk queue, rather than the beginning of it.
		// This makes code fail in more immediate and obvious ways.
		firstPages[0] = (Page*)((((uintptr_t)pageMemory + 4095) / 4096) * 4096);
		memset(firstPages[0], 0xFF, sizeof(Page));
		firstPages[1] = (Page*)((uintptr_t)firstPages[0] + 4096);
		memset(firstPages[1], 0xFF, sizeof(Page));
		stallCount.init(LiteralStringRef("RawDiskQueue.StallCount"));
	}

	Future<Void> pushAndCommit(StringRef pageData, StringBuffer* pageMem, uint64_t poppedPages) {
		return pushAndCommit(this, pageData, pageMem, poppedPages);
	}

	void stall() {
		stallCount++;
		readyToPush = lastCommit;
	}

	Future<Standalone<StringRef>> readFirstAndLastPages(compare_pages compare) {
		return readFirstAndLastPages(this, compare);
	}

	void setStartPage(int file, int64_t page) {
		TraceEvent("RDQSetStart", dbgid)
		    .detail("FileNum", file)
		    .detail("PageNum", page)
		    .detail("File0Name", files[0].dbgFilename);
		readingFile = file;
		readingPage = page;
	}

	Future<Void> setPoppedPage(int file, int64_t page, int64_t debugSeq) {
		return setPoppedPage(this, file, page, debugSeq);
	}

	// FIXME: let the caller pass in where to write the data.
	Future<Standalone<StringRef>> read(int file, int page, int nPages) { return read(this, file, page, nPages); }
	Future<Standalone<StringRef>> readNextPage() { return readNextPage(this); }
	Future<Void> truncateBeforeLastReadPage() { return truncateBeforeLastReadPage(this); }

	Future<Void> getError() { return onError; }
	Future<Void> onClosed() { return onStopped; }
	void dispose() { shutdown(this, true); }
	void close() { shutdown(this, false); }

	StorageBytes getStorageBytes() const {
		int64_t free;
		int64_t total;

		g_network->getDiskBytes(parentDirectory(basename), free, total);

		return StorageBytes(free,
		                    total,
		                    files[0].size + files[1].size,
		                    free); // TODO: we could potentially do better in the available field by accounting for the
		                           // unused pages at the end of the file
	}

	// private:
	struct Page {
		uint8_t data[_PAGE_SIZE];
	};

	struct File {
		Reference<IAsyncFile> f;
		int64_t size; // always a multiple of _PAGE_SIZE, even if the physical file isn't for some reason
		int64_t popped;
		std::string dbgFilename;
		Reference<SyncQueue> syncQueue;

		File() : size(-1), popped(-1) {}

		void setFile(Reference<IAsyncFile> f) {
			this->f = f;
			this->syncQueue = makeReference<SyncQueue>(1, f);
		}
	};
	File files[2]; // After readFirstAndLastPages(), files[0] is logically before files[1] (pushes are always into
	               // files[1])
	Standalone<VectorRef<Page*>> firstPages;

	std::string basename;
	std::string fileExtension;
	std::string filename(int i) const { return basename + format("%d.%s", i, fileExtension.c_str()); }

	UID dbgid;
	int64_t dbg_file0BeginSeq;
	int64_t fileSizeWarningLimit;

	Promise<Void> error, stopped;
	Future<Void> onError, onStopped;

	Future<Void> readyToPush;
	Future<Void> lastCommit;
	bool isFirstCommit;

	StringBuffer readingBuffer; // Pages that have been read and not yet returned
	int readingFile; // File index where the next page (after readingBuffer) should be read from, i.e.,
	                 // files[readingFile]. readingFile = 2 if recovery is complete (all files have been read).
	int64_t readingPage; // Page within readingFile that is the next page after readingBuffer

	int64_t writingPos; // Position within files[1] that will be next written

	int64_t fileExtensionBytes;
	int64_t fileShrinkBytes;

	Int64MetricHandle stallCount;

	Future<Void> truncateFile(int file, int64_t pos) { return truncateFile(this, file, pos); }

	// FIXME: Merge this function with IAsyncFileSystem::incrementalDeleteFile().
	ACTOR static void incrementalTruncate(Reference<IAsyncFile> file) {
		state int64_t remainingFileSize = wait(file->size());

		for (; remainingFileSize > 0; remainingFileSize -= FLOW_KNOBS->INCREMENTAL_DELETE_TRUNCATE_AMOUNT) {
			wait(file->truncate(remainingFileSize));
			wait(file->sync());
			wait(delay(FLOW_KNOBS->INCREMENTAL_DELETE_INTERVAL));
		}

		TraceEvent("DiskQueueReplaceTruncateEnded").detail("Filename", file->getFilename());
	}

#if defined(_WIN32)
	ACTOR static Future<Reference<IAsyncFile>> replaceFile(Reference<IAsyncFile> toReplace) {
		// Windows doesn't support a rename over an open file.
		wait(toReplace->truncate(4 << 10));
		return toReplace;
	}
#else
	ACTOR static Future<Reference<IAsyncFile>> replaceFile(Reference<IAsyncFile> toReplace) {
		incrementalTruncate(toReplace);

		Reference<IAsyncFile> _replacement = wait(IAsyncFileSystem::filesystem()->open(
		    toReplace->getFilename(),
		    IAsyncFile::OPEN_ATOMIC_WRITE_AND_CREATE | IAsyncFile::OPEN_CREATE | IAsyncFile::OPEN_READWRITE |
		        IAsyncFile::OPEN_UNCACHED | IAsyncFile::OPEN_UNBUFFERED | IAsyncFile::OPEN_LOCK,
		    0600));
		state Reference<IAsyncFile> replacement = _replacement;
		wait(replacement->sync());

		return replacement;
	}
#endif

	Future<Future<Void>> push(StringRef pageData, std::vector<Reference<SyncQueue>>* toSync) {
		return push(this, pageData, toSync);
	}

	ACTOR static Future<Future<Void>> push(RawDiskQueue_TwoFiles* self,
	                                       StringRef pageData,
	                                       std::vector<Reference<SyncQueue>>* toSync) {
		// Write the given data (pageData) to the queue files, swapping or extending them if necessary.
		// Don't do any syncs, but push the modified file(s) onto toSync.
		ASSERT(self->readingFile == 2);
		ASSERT(pageData.size() % _PAGE_SIZE == 0);
		ASSERT(int64_t(pageData.begin()) % _PAGE_SIZE == 0);
		ASSERT(self->writingPos % _PAGE_SIZE == 0);
		ASSERT(self->files[0].size % _PAGE_SIZE == 0 && self->files[1].size % _PAGE_SIZE == 0);

		state std::vector<Future<Void>> waitfor;

		if (pageData.size() + self->writingPos > self->files[1].size) {
			if (self->files[0].popped == self->files[0].size) {
				// Finish self->files[1] and swap
				int p = self->files[1].size - self->writingPos;
				if (p > 0) {
					toSync->push_back(self->files[1].syncQueue);
					/*TraceEvent("RDQWriteAndSwap", this->dbgid).detail("File1name", self->files[1].dbgFilename).detail("File1size", self->files[1].size)
					    .detail("WritingPos", self->writingPos).detail("WritingBytes", p);*/
					waitfor.push_back(self->files[1].f->write(pageData.begin(), p, self->writingPos));
					pageData = pageData.substr(p);
				}

				self->dbg_file0BeginSeq += self->files[0].size;
				std::swap(self->files[0], self->files[1]);
				std::swap(self->firstPages[0], self->firstPages[1]);
				self->files[1].popped = 0;
				self->writingPos = 0;
				*self->firstPages[1] = *(const Page*)pageData.begin();

				const int64_t activeDataVolume = pageCeiling(self->files[0].size - self->files[0].popped +
				                                             self->fileExtensionBytes + self->fileShrinkBytes);
				const int64_t desiredMaxFileSize =
				    pageCeiling(std::max(activeDataVolume, SERVER_KNOBS->TLOG_HARD_LIMIT_BYTES * 2));
				const bool frivolouslyTruncate = BUGGIFY_WITH_PROB(0.1);
				if (self->files[1].size > desiredMaxFileSize || frivolouslyTruncate) {
					// Either shrink self->files[1] to the size of self->files[0], or chop off fileShrinkBytes
					int64_t maxShrink =
					    pageFloor(std::max(self->files[1].size - desiredMaxFileSize, self->fileShrinkBytes));
					if ((maxShrink > SERVER_KNOBS->DISK_QUEUE_MAX_TRUNCATE_BYTES) ||
					    (frivolouslyTruncate && deterministicRandom()->random01() < 0.3)) {
						CODE_PROBE(true, "Replacing DiskQueue file");
						TraceEvent("DiskQueueReplaceFile", self->dbgid)
						    .detail("Filename", self->files[1].f->getFilename())
						    .detail("OldFileSize", self->files[1].size)
						    .detail("ElidedTruncateSize", maxShrink);
						Reference<IAsyncFile> newFile = wait(replaceFile(self->files[1].f));
						self->files[1].setFile(newFile);
						waitfor.push_back(self->files[1].f->truncate(self->fileExtensionBytes));
						self->files[1].size = self->fileExtensionBytes;
					} else {
						CODE_PROBE(true, "Truncating DiskQueue file");
						const int64_t startingSize = self->files[1].size;
						self->files[1].size -= std::min(maxShrink, self->files[1].size);
						self->files[1].size = std::max(self->files[1].size, self->fileExtensionBytes);
						TraceEvent("DiskQueueTruncate", self->dbgid)
						    .detail("Filename", self->files[1].f->getFilename())
						    .detail("OldFileSize", startingSize)
						    .detail("NewFileSize", self->files[1].size);
						waitfor.push_back(self->files[1].f->truncate(self->files[1].size));
					}
				}
			} else {
				// Extend self->files[1] to accomodate the new write and about 10MB or 2x current size for future
				// writes.
				/*TraceEvent("RDQExtend", this->dbgid).detail("File1name", self->files[1].dbgFilename).detail("File1size", self->files[1].size)
				    .detail("ExtensionBytes", fileExtensionBytes);*/
				int64_t minExtension = pageData.size() + self->writingPos - self->files[1].size;
				self->files[1].size += std::min(std::max(self->fileExtensionBytes, minExtension),
				                                self->files[0].size + self->files[1].size + minExtension);
				waitfor.push_back(self->files[1].f->truncate(self->files[1].size));

				if (self->fileSizeWarningLimit > 0 && self->files[1].size > self->fileSizeWarningLimit) {
					TraceEvent(SevWarnAlways, "DiskQueueFileTooLarge", self->dbgid)
					    .suppressFor(1.0)
					    .detail("Filename", self->filename(1))
					    .detail("Size", self->files[1].size);
				}
			}
		} else if (self->writingPos == 0) {
			// If this is the first write to a brand new disk queue file.
			*self->firstPages[1] = *(const Page*)pageData.begin();
		}

		/*TraceEvent("RDQWrite", this->dbgid).detail("File1name", self->files[1].dbgFilename).detail("File1size", self->files[1].size)
		    .detail("WritingPos", self->writingPos).detail("WritingBytes", pageData.size());*/
		self->files[1].size = std::max(self->files[1].size, self->writingPos + pageData.size());
		toSync->push_back(self->files[1].syncQueue);
		waitfor.push_back(self->files[1].f->write(pageData.begin(), pageData.size(), self->writingPos));
		self->writingPos += pageData.size();

		return waitForAll(waitfor);
	}

	// Write the given data (pageData) to the queue files of self, sync data to disk, and delete the memory (pageMem)
	// that hold the pageData
	ACTOR static UNCANCELLABLE Future<Void> pushAndCommit(RawDiskQueue_TwoFiles* self,
	                                                      StringRef pageData,
	                                                      StringBuffer* pageMem,
	                                                      uint64_t poppedPages) {
		state Promise<Void> pushing, committed;
		state Promise<Void> errorPromise = self->error;
		state std::string filename = self->files[0].dbgFilename;
		state UID dbgid = self->dbgid;
		state std::vector<Reference<SyncQueue>> syncFiles;
		state Future<Void> lastCommit = self->lastCommit;
		try {
			// pushing might need to wait for previous pushes to start (to maintain order) or for
			// a previous commit to finish if stall() was called
			Future<Void> ready = self->readyToPush;
			self->readyToPush = pushing.getFuture();
			self->lastCommit = committed.getFuture();

			// the first commit must complete before we can pipeline other commits so that we will always have a valid
			// page to binary search to
			if (self->isFirstCommit) {
				self->isFirstCommit = false;
				self->readyToPush = self->lastCommit;
			}

			wait(ready);

			CODE_PROBE(pageData.size() > sizeof(Page), "push more than one page of data");

			Future<Void> pushed = wait(self->push(pageData, &syncFiles));
			pushing.send(Void());
			ASSERT(syncFiles.size() >= 1 && syncFiles.size() <= 2);
			CODE_PROBE(2 == syncFiles.size(), "push spans both files");
			wait(pushed);

			delete pageMem;
			pageMem = 0;

			Future<Void> sync = syncFiles[0]->onSync();
			for (int i = 1; i < syncFiles.size(); i++)
				sync = sync && syncFiles[i]->onSync();
			wait(sync);
			wait(lastCommit);

			// Calling check_yield instead of yield to avoid a destruction ordering problem in simulation
			if (g_network->check_yield(g_network->getCurrentTask())) {
				wait(delay(0, g_network->getCurrentTask()));
			}

			self->updatePopped(poppedPages * sizeof(Page));

			/*TraceEvent("RDQCommitEnd", self->dbgid).detail("DeltaPopped", poppedPages*sizeof(Page)).detail("PoppedCommitted", self->dbg_file0BeginSeq + self->files[0].popped + self->files[1].popped)
			    .detail("File0Size", self->files[0].size).detail("File1Size", self->files[1].size)
			    .detail("File0Name", self->files[0].dbgFilename).detail("SyncedFiles", syncFiles.size());*/

			committed.send(Void());
		} catch (Error& e) {
			delete pageMem;
			CODE_PROBE(true, "push error");
			CODE_PROBE(2 == syncFiles.size(), "push spanning both files error");
			TraceEvent(SevError, "RDQPushAndCommitError", dbgid)
			    .errorUnsuppressed(e)
			    .detail("InitialFilename0", filename);

			if (errorPromise.canBeSet())
				errorPromise.sendError(e);
			if (pushing.canBeSet())
				pushing.sendError(e);
			if (committed.canBeSet())
				committed.sendError(e);

			throw e;
		}
		return Void();
	}

	void updatePopped(int64_t popped) {
		int64_t pop0 = std::min(popped, files[0].size - files[0].popped);
		files[0].popped += pop0;
		files[1].popped += popped - pop0;
	}

	// Set the starting point of the ring buffer, i.e., the first useful page to be read (and poped)
	ACTOR static Future<Void> setPoppedPage(RawDiskQueue_TwoFiles* self, int file, int64_t page, int64_t debugSeq) {
		self->files[file].popped = page * sizeof(Page);
		if (file)
			self->files[0].popped = self->files[0].size;
		else
			self->files[1].popped = 0;
		self->dbg_file0BeginSeq = debugSeq - self->files[1].popped - self->files[0].popped;

		// If we are starting in file 1, we truncate file 0 in case it has been corrupted.
		//  In particular, we are trying to avoid a dropped or corrupted write to the first page of file 0 causing it to
		//  be sequenced before file 1, when in fact it contains many pages that follow file 1.  These ok pages may be
		//  incorrectly read if the machine dies after overwritting the first page of file 0 and is then recovered
		if (file == 1)
			wait(self->truncateFile(self, 0, 0));

		return Void();
	}

	ACTOR static Future<Void> openFiles(RawDiskQueue_TwoFiles* self) {
		state std::vector<Future<Reference<IAsyncFile>>> fs;
		fs.reserve(2);
		for (int i = 0; i < 2; i++)
			fs.push_back(IAsyncFileSystem::filesystem()->open(self->filename(i),
			                                                  IAsyncFile::OPEN_READWRITE | IAsyncFile::OPEN_UNCACHED |
			                                                      IAsyncFile::OPEN_UNBUFFERED | IAsyncFile::OPEN_LOCK,
			                                                  0));
		wait(waitForAllReady(fs));

		// Treatment of errors here is important.  If only one of the two files is present
		// (due to a power failure during creation or deletion, or administrative error) we don't want to
		// open the queue!

		if (!fs[0].isError() && !fs[1].isError()) {
			// Both files were opened OK: success
		} else if (fs[0].isError() && fs[0].getError().code() == error_code_file_not_found && fs[1].isError() &&
		           fs[1].getError().code() == error_code_file_not_found) {
			// Neither file was found: we can create a new queue
			// OPEN_ATOMIC_WRITE_AND_CREATE defers creation (using a .part file) until the calls to sync() below
			TraceEvent("DiskQueueCreate").detail("File0", self->filename(0));
			for (int i = 0; i < 2; i++)
				fs[i] = IAsyncFileSystem::filesystem()->open(
				    self->filename(i),
				    IAsyncFile::OPEN_ATOMIC_WRITE_AND_CREATE | IAsyncFile::OPEN_CREATE | IAsyncFile::OPEN_READWRITE |
				        IAsyncFile::OPEN_UNCACHED | IAsyncFile::OPEN_UNBUFFERED | IAsyncFile::OPEN_LOCK,
				    0600);

			// Any error here is fatal
			wait(waitForAll(fs));

			// sync on each file to actually create it will be done below
		} else {
			// One file had a more serious error or one file is present and the other is not.  Die.
			if (!fs[0].isError() || (fs[1].isError() && fs[1].getError().code() != error_code_file_not_found))
				throw fs[1].getError();
			else
				throw fs[0].getError();
		}

		// fsync both files.  This is necessary to trigger atomic file creation in the creation case above.
		// It also permits the recovery code to assume that whatever it reads is durable.  Otherwise a prior
		// process could have written (but not synchronized) data to the file which we will read but which
		// might not survive a reboot.  The recovery code assumes otherwise and could corrupt the disk.
		std::vector<Future<Void>> syncs;
		syncs.reserve(fs.size());
		for (int i = 0; i < fs.size(); i++)
			syncs.push_back(fs[i].get()->sync());
		wait(waitForAll(syncs));

		// Successfully opened or created; fill in self->files[]
		for (int i = 0; i < 2; i++)
			self->files[i].setFile(fs[i].get());

		return Void();
	}

	ACTOR static void shutdown(RawDiskQueue_TwoFiles* self, bool deleteFiles) {
		// Wait for all reads and writes on the file, and all actors referencing self, to be finished
		state Error error = success();
		try {
			wait(success(errorOr(self->lastCommit)));
			// Wait for the pending operations (e.g., read) to finish before we destroy the DiskQueue, because
			// tLog, instead of DiskQueue, hold the future of the pending operations.
			wait(self->onSafeToDestruct());

			for (int i = 0; i < 2; i++)
				self->files[i].f.clear();

			if (deleteFiles) {
				TraceEvent("DiskQueueShutdownDeleting", self->dbgid)
				    .detail("File0", self->filename(0))
				    .detail("File1", self->filename(1));
				wait(IAsyncFileSystem::filesystem()->incrementalDeleteFile(self->filename(0), false));
				wait(IAsyncFileSystem::filesystem()->incrementalDeleteFile(self->filename(1), true));
			}
			TraceEvent("DiskQueueShutdownComplete", self->dbgid)
			    .detail("DeleteFiles", deleteFiles)
			    .detail("File0", self->filename(0));
		} catch (Error& e) {
			TraceEvent(SevError, "DiskQueueShutdownError", self->dbgid)
			    .errorUnsuppressed(e)
			    .detail("Reason", e.code() == error_code_platform_error ? "could not delete database" : "unknown");
			error = e;
		}

		if (error.code() != error_code_actor_cancelled) {
			if (self->stopped.canBeSet())
				self->stopped.send(Void());
			if (self->error.canBeSet())
				self->error.send(Never());
			delete self;
		}
	}

	// Return the most recently written page, the page with largest seq number
	ACTOR static UNCANCELLABLE Future<Standalone<StringRef>> readFirstAndLastPages(RawDiskQueue_TwoFiles* self,
	                                                                               compare_pages compare) {
		state TrackMe trackMe(self);

		try {
			// Open both files or create both files
			wait(openFiles(self));

			// Get the file sizes
			std::vector<Future<int64_t>> fsize;
			fsize.reserve(2);
			for (int i = 0; i < 2; i++)
				fsize.push_back(self->files[i].f->size());
			std::vector<int64_t> file_sizes = wait(getAll(fsize));
			for (int i = 0; i < 2; i++) {
				// SOMEDAY: If the file size is not a multiple of page size, it may never be shortened.  Change this?
				self->files[i].size = file_sizes[i] - file_sizes[i] % sizeof(Page);
				ASSERT(self->files[i].size % sizeof(Page) == 0);
			}

			// Read the first pages
			std::vector<Future<int>> reads;
			for (int i = 0; i < 2; i++)
				if (self->files[i].size > 0)
					reads.push_back(self->files[i].f->read(self->firstPages[i], sizeof(Page), 0));
			wait(waitForAll(reads));

			// Determine which file comes first
			if (compare(self->firstPages[1], self->firstPages[0])) {
				std::swap(self->firstPages[0], self->firstPages[1]);
				std::swap(self->files[0], self->files[1]);
			}

			if (!compare(self->firstPages[0], self->firstPages[0])) {
				memset(self->firstPages[0], 0xFF, sizeof(Page));
			}

			if (!compare(self->firstPages[1], self->firstPages[1])) {
				// Both files are invalid... the queue is empty!
				// Begin pushing at the beginning of files[1]

				// Truncate both files, since perhaps only the first pages are corrupted.  This avoids cases where
				// overwritting the first page and then terminating makes subsequent pages valid upon recovery.
				std::vector<Future<Void>> truncates;
				for (int i = 0; i < 2; ++i)
					if (self->files[i].size > 0)
						truncates.push_back(self->truncateFile(self, i, 0));

				wait(waitForAll(truncates));

				self->files[0].popped = self->files[0].size;
				self->files[1].popped = 0;
				memset(self->firstPages[1], 0xFF, sizeof(Page));
				self->writingPos = 0;
				self->readingFile = 2;
				return Standalone<StringRef>();
			}

			// A page in files[1] is "valid" iff compare(self->firstPages[1], page)
			// Binary search to find a page in files[1] that is "valid" but the next page is not valid
			// Invariant: the page at begin is valid, and the page at end is invalid
			state int64_t begin = 0;
			state int64_t end = self->files[1].size / sizeof(Page);
			state Standalone<StringRef> middlePageAllocation = makeAlignedString(sizeof(Page), sizeof(Page));
			state Page* middlePage = (Page*)middlePageAllocation.begin();
			while (begin + 1 != end) {
				state int64_t middle = (begin + end) / 2;
				ASSERT(middle > begin && middle < end); // So the loop always changes begin or end

				int len = wait(self->files[1].f->read(middlePage, sizeof(Page), middle * sizeof(Page)));
				ASSERT(len == sizeof(Page));

				bool middleValid = compare(self->firstPages[1], middlePage);

				TraceEvent("RDQBS", self->dbgid)
				    .detail("Begin", begin)
				    .detail("End", end)
				    .detail("Middle", middle)
				    .detail("Valid", middleValid)
				    .detail("File0Name", self->files[0].dbgFilename);

				if (middleValid)
					begin = middle;
				else
					end = middle;
			}
			// Now by the invariant and the loop condition, begin is a valid page and begin+1 is an invalid page
			// Check that begin+1 is invalid
			int len1 = wait(self->files[1].f->read(middlePage, sizeof(Page), (begin + 1) * sizeof(Page)));
			ASSERT(!(len1 == sizeof(Page) && compare(self->firstPages[1], middlePage)));

			// Read it
			int len2 = wait(self->files[1].f->read(middlePage, sizeof(Page), begin * sizeof(Page)));
			ASSERT(len2 == sizeof(Page) && compare(self->firstPages[1], middlePage));

			TraceEvent("RDQEndFound", self->dbgid)
			    .detail("File0Name", self->files[0].dbgFilename)
			    .detail("Pos", begin)
			    .detail("FileSize", self->files[1].size);

			return middlePageAllocation;
		} catch (Error& e) {
			bool ok = e.code() == error_code_file_not_found;
			TraceEvent(ok ? SevInfo : SevError, "RDQReadFirstAndLastPagesError", self->dbgid)
			    .errorUnsuppressed(e)
			    .detail("File0Name", self->files[0].dbgFilename);
			if (!self->error.isSet())
				self->error.sendError(e);
			throw;
		}
	}

	// Read nPages from pageOffset*sizeof(Page) offset in file self->files[file]
	ACTOR static Future<Standalone<StringRef>> read(RawDiskQueue_TwoFiles* self, int file, int pageOffset, int nPages) {
		state TrackMe trackMe(self);
		state const size_t bytesRequested = nPages * sizeof(Page);
		state Standalone<StringRef> result = makeAlignedString(sizeof(Page), bytesRequested);
		if (file == 1)
			ASSERT_WE_THINK(pageOffset * sizeof(Page) + bytesRequested <= self->writingPos);
		int bytesRead =
		    wait(self->files[file].f->read(mutateString(result), bytesRequested, pageOffset * sizeof(Page)));
		ASSERT_WE_THINK(bytesRead == bytesRequested);
		return result;
	}

	Future<int> fillReadingBuffer() {
		// If we're right at the end of a file...
		if (readingPage * sizeof(Page) >= (size_t)files[readingFile].size) {
			readingFile++;
			readingPage = 0;
			if (readingFile >= 2) {
				// Recovery complete
				readingBuffer.clear();
				writingPos = files[1].size;
				return 0;
			}
		}

		// Read up to 1MB into readingBuffer
		int len = std::min<int64_t>((files[readingFile].size / sizeof(Page) - readingPage) * sizeof(Page),
		                            BUGGIFY_WITH_PROB(1.0) ? sizeof(Page) * deterministicRandom()->randomInt(1, 4)
		                                                   : (1 << 20));
		readingBuffer.clear();
		readingBuffer.alignReserve(sizeof(Page), len);
		void* p = readingBuffer.append(len);

		auto pos = readingPage * sizeof(Page);
		readingPage += len / sizeof(Page);
		ASSERT(int64_t(p) % sizeof(Page) == 0);
		return files[readingFile].f->read(p, len, pos);
	}

	ACTOR static UNCANCELLABLE Future<Standalone<StringRef>> readNextPage(RawDiskQueue_TwoFiles* self) {
		state TrackMe trackMe(self);

		try {
			ASSERT(self->readingFile < 2);
			ASSERT(self->files[0].f && self->files[1].f);

			if (!self->readingBuffer.size()) {
				state Future<Void> f = Void();
				// if (BUGGIFY) f = delay( deterministicRandom()->random01() * 0.1 );

				int read = wait(self->fillReadingBuffer());
				ASSERT(read == self->readingBuffer.size());

				wait(f);
			}
			if (!self->readingBuffer.size())
				return Standalone<StringRef>();

			ASSERT(self->readingBuffer.size() >= sizeof(Page));
			Standalone<StringRef> result = self->readingBuffer.pop_front(sizeof(Page));
			return result;
		} catch (Error& e) {
			CODE_PROBE(true, "Read next page error");
			TraceEvent(SevError, "RDQReadNextPageError", self->dbgid)
			    .errorUnsuppressed(e)
			    .detail("File0Name", self->files[0].dbgFilename);
			if (!self->error.isSet())
				self->error.sendError(e);
			throw;
		}
	}

	// Set zero and free the memory from pos to the end of file self->files[file].
	ACTOR static UNCANCELLABLE Future<Void> truncateFile(RawDiskQueue_TwoFiles* self, int file, int64_t pos) {
		state TrackMe trackMe(self);
		TraceEvent("DQTruncateFile", self->dbgid)
		    .detail("File", file)
		    .detail("Pos", pos)
		    .detail("File0Name", self->files[0].dbgFilename);
		state Reference<IAsyncFile> f =
		    self->files[file].f; // Hold onto a reference in the off-chance that the DQ is removed from underneath us.
		if (pos == 0) {
			memset(self->firstPages[file], 0xFF, _PAGE_SIZE);
		}
		wait(f->zeroRange(pos, self->files[file].size - pos));
		wait(self->files[file].syncQueue->onSync());
		// We intentionally don't return the f->zero future, so that TrackMe is destructed after f->zero finishes.
		return Void();
	}

	ACTOR static Future<Void> truncateBeforeLastReadPage(RawDiskQueue_TwoFiles* self) {
		try {
			state int file = self->readingFile;
			state int64_t pos = (self->readingPage - self->readingBuffer.size() / sizeof(Page) - 1) * sizeof(Page);
			state std::vector<Future<Void>> commits;
			state bool swap = file == 0;

			CODE_PROBE(file == 0, "truncate before last read page on file 0");
			CODE_PROBE(file == 1 && pos != self->files[1].size, "truncate before last read page on file 1");

			self->readingFile = 2;
			self->readingBuffer.clear();
			self->writingPos = pos;

			while (file < 2) {
				commits.push_back(self->truncateFile(self, file, pos));
				file++;
				pos = 0;
			}

			wait(waitForAll(commits));

			if (swap) {
				std::swap(self->files[0], self->files[1]);
				std::swap(self->firstPages[0], self->firstPages[1]);
				self->files[0].popped = self->files[0].size;
			}

			return Void();
		} catch (Error& e) {
			TraceEvent(SevError, "RDQTruncateBeforeLastReadPageError", self->dbgid)
			    .error(e)
			    .detail("File0Name", self->files[0].dbgFilename);
			if (!self->error.isSet())
				self->error.sendError(e);
			throw;
		}
	}
};

class DiskQueue final : public IDiskQueue, public Tracked<DiskQueue> {
public:
	// FIXME: Is setting lastCommittedSeq to -1 instead of 0 necessary?
	DiskQueue(std::string basename,
	          std::string fileExtension,
	          UID dbgid,
	          DiskQueueVersion diskQueueVersion,
	          int64_t fileSizeWarningLimit)
	  : rawQueue(new RawDiskQueue_TwoFiles(basename, fileExtension, dbgid, fileSizeWarningLimit)), dbgid(dbgid),
	    diskQueueVersion(diskQueueVersion), anyPopped(false), warnAlwaysForMemory(true), nextPageSeq(0), poppedSeq(0),
	    lastPoppedSeq(0), lastCommittedSeq(-1), pushed_page_buffer(nullptr), recovered(false), initialized(false),
	    nextReadLocation(-1), readBufPage(nullptr), readBufPos(0) {}

	location push(StringRef contents) override {
		ASSERT(recovered);
		uint8_t const* begin = contents.begin();
		uint8_t const* end = contents.end();
		CODE_PROBE(contents.size() && pushedPageCount(), "More than one push between commits");

		bool pushAtEndOfPage = contents.size() >= 4 && pushedPageCount() && backPage().remainingCapacity() < 4;
		CODE_PROBE(pushAtEndOfPage, "Push right at the end of a page, possibly splitting size");
		while (begin != end) {
			if (!pushedPageCount() || !backPage().remainingCapacity())
				addEmptyPage();

			auto& p = backPage();
			int s = std::min<int>(p.remainingCapacity(), end - begin);
			memcpy(p.payload + p.payloadSize, begin, s);
			p.payloadSize += s;
			begin += s;
		}
		return endLocation();
	}

	void pop(location upTo) override {
		ASSERT(!upTo.hi);
		ASSERT(!recovered || upTo.lo <= endLocation());

		// SS can pop pages that have not been sync.ed to disk because of concurrency:
		//   SS can read (i.e., pop) data at the same time or before tLog syncs the page to disk.
		//   This is rare in real situation but common in simulation.
		// The following ASSERT is NOT part of the intended contract of IDiskQueue, but alerts the user to a known bug
		// where popping
		//  into uncommitted pages can cause a durability failure.
		// FIXME: Remove this ASSERT when popping into uncommitted pages is fixed
		if (upTo.lo > lastCommittedSeq) {
			TraceEvent(SevError, "DQPopUncommittedData", dbgid)
			    .detail("UpTo", upTo)
			    .detail("LastCommittedSeq", lastCommittedSeq)
			    .detail("File0Name", rawQueue->files[0].dbgFilename);
		}
		if (upTo.lo > poppedSeq) {
			poppedSeq = upTo.lo;
			anyPopped = true;
		}
	}

	Future<Standalone<StringRef>> read(location from, location to, CheckHashes ch) override {
		return read(this, from, to, ch);
	}

	int getMaxPayload() const { return Page::maxPayload; }

	// Always commit an entire page. Commit overhead is the unused space in a to-be-committed page
	int getCommitOverhead() const override {
		if (!pushedPageCount()) {
			if (!anyPopped)
				return 0;

			// To mark pages are poped, we push an empty page to specify that following pages were poped.
			// maxPayLoad is the max. payload size, i.e., (page_size - page_header_size).
			return Page::maxPayload;
		} else
			return backPage().remainingCapacity();
	}

	Future<Void> commit() override {
		ASSERT(recovered);
		if (!pushedPageCount()) {
			if (!anyPopped)
				return Void();
			addEmptyPage(); // To remove poped pages, we push an empty page to specify that pages behind it were poped.
		}
		anyPopped = false;
		backPage().popped = poppedSeq;
		backPage().zeroPad();
		backPage().updateHash();

		// Warn users that we pushed too many pages. 8000 is an arbitrary value.
		if (pushedPageCount() >= 8000) {
			TraceEvent(warnAlwaysForMemory ? SevWarnAlways : SevWarn, "DiskQueueMemoryWarning", dbgid)
			    .suppressFor(1.0)
			    .detail("PushedPages", pushedPageCount())
			    .detail("NextPageSeq", nextPageSeq)
			    .detail("Details", format("%d pages", pushedPageCount()))
			    .detail("File0Name", rawQueue->files[0].dbgFilename);
			if (g_network->isSimulated())
				warnAlwaysForMemory = false;
		}

		/*TraceEvent("DQCommit", dbgid).detail("Pages", pushedPageCount()).detail("LastPoppedSeq", lastPoppedSeq).detail("PoppedSeq", poppedSeq).detail("NextPageSeq", nextPageSeq)
		    .detail("RawFile0Size", rawQueue->files[0].size).detail("RawFile1Size",
		   rawQueue->files[1].size).detail("WritingPos", rawQueue->writingPos) .detail("RawFile0Name",
		   rawQueue->files[0].dbgFilename);*/

		lastCommittedSeq = backPage().endSeq();
		auto f = rawQueue->pushAndCommit(
		    pushed_page_buffer->ref(), pushed_page_buffer, poppedSeq / sizeof(Page) - lastPoppedSeq / sizeof(Page));
		lastPoppedSeq = poppedSeq;
		pushed_page_buffer = 0;
		return f;
	}

	void stall() { rawQueue->stall(); }

	Future<bool> initializeRecovery(location recoverAt) override { return initializeRecovery(this, recoverAt); }
	Future<Standalone<StringRef>> readNext(int bytes) override { return readNext(this, bytes); }

	// FIXME: getNextReadLocation should ASSERT( initialized ), but the memory storage engine needs
	// to be changed to understand the new intiailizeRecovery protocol.
	location getNextReadLocation() const override { return nextReadLocation; }
	location getNextCommitLocation() const override {
		ASSERT(initialized);
		return lastCommittedSeq + sizeof(Page);
	}
	location getNextPushLocation() const override {
		ASSERT(initialized);
		return endLocation();
	}

	Future<Void> getError() const override { return rawQueue->getError(); }
	Future<Void> onClosed() const override { return rawQueue->onClosed(); }

	void dispose() override {
		TraceEvent("DQDestroy", dbgid)
		    .detail("LastPoppedSeq", lastPoppedSeq)
		    .detail("PoppedSeq", poppedSeq)
		    .detail("NextPageSeq", nextPageSeq)
		    .detail("File0Name", rawQueue->files[0].dbgFilename);
		dispose(this);
	}

	void close() override {
		TraceEvent("DQClose", dbgid)
		    .detail("LastPoppedSeq", lastPoppedSeq)
		    .detail("PoppedSeq", poppedSeq)
		    .detail("NextPageSeq", nextPageSeq)
		    .detail("PoppedCommitted",
		            rawQueue->dbg_file0BeginSeq + rawQueue->files[0].popped + rawQueue->files[1].popped)
		    .detail("File0Name", rawQueue->files[0].dbgFilename);
		close(this);
	}

	StorageBytes getStorageBytes() const override { return rawQueue->getStorageBytes(); }

private:
	ACTOR static void dispose(DiskQueue* self) {
		wait(self->onSafeToDestruct());
		TraceEvent("DQDestroyDone", self->dbgid).detail("File0Name", self->rawQueue->files[0].dbgFilename);
		self->rawQueue->dispose();
		delete self;
	}

	ACTOR static void close(DiskQueue* self) {
		wait(self->onSafeToDestruct());
		TraceEvent("DQCloseDone", self->dbgid).detail("File0Name", self->rawQueue->files[0].dbgFilename);
		self->rawQueue->close();
		delete self;
	}

#pragma pack(push, 1)
	struct PageHeader {
		union {
			UID hash;
			struct {
				union {
					uint64_t hash64;
					struct {
						uint32_t hash32;
						uint32_t _unused;
					};
				};
				uint16_t magic;
				uint16_t implementationVersion;
			};
		};
		uint64_t seq; // seq is the index of the virtually infinite disk queue file. Its unit is bytes.
		uint64_t popped;
		int payloadSize;
	};
	// The on disk format depends on the size of PageHeader.
	static_assert(sizeof(PageHeader) == 36, "PageHeader must be 36 bytes");

	struct Page : PageHeader {
		static const int maxPayload = _PAGE_SIZE - sizeof(PageHeader);
		uint8_t payload[maxPayload];

		DiskQueueVersion diskQueueVersion() const { return static_cast<DiskQueueVersion>(implementationVersion); }
		int remainingCapacity() const { return maxPayload - payloadSize; }
		uint64_t endSeq() const { return seq + sizeof(PageHeader) + payloadSize; }
		UID checksum_hashlittle2() const {
			// SOMEDAY: Better hash?
			uint32_t part[2] = { 0x12345678, 0xbeefabcd };
			hashlittle2(&seq, sizeof(Page) - sizeof(UID), &part[0], &part[1]);
			return UID(int64_t(part[0]) << 32 | part[1], 0xFDB);
		}
		uint32_t checksum_crc32c() const {
			return crc32c_append(0xfdbeefdb, (uint8_t*)&_unused, sizeof(Page) - sizeof(uint32_t));
		}
		uint64_t checksum_xxhash3() const {
			return XXH3_64bits(static_cast<const void*>(&magic), sizeof(Page) - sizeof(uint64_t));
		}
		void updateHash() {
			switch (diskQueueVersion()) {
			case DiskQueueVersion::V0: {
				hash = checksum_hashlittle2();
				return;
			}
			case DiskQueueVersion::V1: {
				hash32 = checksum_crc32c();
				return;
			}
			case DiskQueueVersion::V2:
			default: {
				hash64 = checksum_xxhash3();
				return;
			}
			}
		}
		bool checkHash() {
			switch (diskQueueVersion()) {
			case DiskQueueVersion::V0: {
				return hash == checksum_hashlittle2();
			}
			case DiskQueueVersion::V1: {
				return hash32 == checksum_crc32c();
			}
			case DiskQueueVersion::V2: {
				return hash64 == checksum_xxhash3();
			}
			default:
				return false;
			}
		}
		void zeroPad() { memset(payload + payloadSize, 0, maxPayload - payloadSize); }
	};
	static_assert(sizeof(Page) == _PAGE_SIZE, "Page must be 4k");
#pragma pack(pop)

	loc_t endLocation() const { return pushedPageCount() ? backPage().endSeq() : nextPageSeq; }

	void addEmptyPage() {
		if (pushedPageCount()) {
			backPage().updateHash();
			ASSERT(backPage().payloadSize == Page::maxPayload);
		}

		// pushed_pages.resize( pushed_pages.arena(), pushed_pages.size()+1 );
		if (!pushed_page_buffer)
			pushed_page_buffer = new StringBuffer(dbgid);
		pushed_page_buffer->alignReserve(sizeof(Page), pushed_page_buffer->size() + sizeof(Page));
		pushed_page_buffer->append(sizeof(Page));

		ASSERT(nextPageSeq % sizeof(Page) == 0);

		auto& p = backPage();
		memset(static_cast<void*>(&p), 0, sizeof(Page)); // FIXME: unnecessary?
		p.magic = 0xFDB;
		switch (diskQueueVersion) {
		case DiskQueueVersion::V0:
			p.implementationVersion = 0;
			break;
		case DiskQueueVersion::V1:
			p.implementationVersion = 1;
			break;
		case DiskQueueVersion::V2:
			p.implementationVersion = 2;
			break;
		}
		p.payloadSize = 0;
		p.seq = nextPageSeq;
		nextPageSeq += sizeof(Page);
		p.popped = poppedSeq;

		if (pushedPageCount() == 8000) {
			TraceEvent("DiskQueueHighPageCount", dbgid)
			    .detail("PushedPages", pushedPageCount())
			    .detail("NextPageSeq", nextPageSeq)
			    .detail("File0Name", rawQueue->files[0].dbgFilename);
		}
	}

	ACTOR static void verifyCommit(DiskQueue* self,
	                               Future<Void> commitSynced,
	                               StringBuffer* buffer,
	                               loc_t start,
	                               loc_t end) {
		state TrackMe trackme(self);
		try {
			wait(commitSynced);
			Standalone<StringRef> pagedData = wait(readPages(self, start, end));
			const int startOffset = start % _PAGE_SIZE;
			const int dataLen = end - start;
			ASSERT(pagedData.substr(startOffset, dataLen).compare(buffer->ref().substr(0, dataLen)) == 0);
		} catch (Error& e) {
			if (e.code() != error_code_io_error) {
				delete buffer;
				throw;
			}
		}
		delete buffer;
	}

	// Read pages from [start, end) bytes
	ACTOR static Future<Standalone<StringRef>> readPages(DiskQueue* self, location start, location end) {
		state TrackMe trackme(self);
		state int fromFile;
		state int toFile;
		state int64_t fromPage;
		state int64_t toPage;
		state uint64_t file0size =
		    self->rawQueue->files[0].size ? self->firstPages(1).seq - self->firstPages(0).seq : self->firstPages(1).seq;
		ASSERT(end > start);
		ASSERT(start.lo >= self->firstPages(0).seq || start.lo >= self->firstPages(1).seq);
		self->findPhysicalLocation(start.lo, &fromFile, &fromPage, nullptr);
		self->findPhysicalLocation(end.lo - 1, &toFile, &toPage, nullptr);
		if (fromFile == 0) {
			ASSERT(fromPage < file0size / _PAGE_SIZE);
		}
		if (toFile == 0) {
			ASSERT(toPage < file0size / _PAGE_SIZE);
		}
		// FIXME I think there's something with nextReadLocation we can do here when initialized && !recovered.
		if (fromFile == 1 && self->recovered) {
			ASSERT(fromPage < self->rawQueue->writingPos / _PAGE_SIZE);
		}
		if (toFile == 1 && self->recovered) {
			ASSERT(toPage < self->rawQueue->writingPos / _PAGE_SIZE);
		}
		if (fromFile == toFile) {
			ASSERT(toPage >= fromPage);
			Standalone<StringRef> pagedData = wait(self->rawQueue->read(fromFile, fromPage, toPage - fromPage + 1));
			if (std::min(self->firstPages(0).seq, self->firstPages(1).seq) > start.lo) {
				// Simulation allows for reads to be delayed and executed after overlapping subsequent
				// write operations.  This means that by the time our read was executed, it's possible
				// that both disk queue files have been completely overwritten.
				// I'm not clear what is the actual contract for read/write in this case, so simulation
				// might be a bit overly aggressive here, but it's behavior we need to tolerate.
				throw io_error();
			}
			ASSERT(((Page*)pagedData.begin())->seq == pageFloor(start.lo));
			ASSERT(pagedData.size() == (toPage - fromPage + 1) * _PAGE_SIZE);

			ASSERT(((Page*)pagedData.end() - 1)->seq == pageFloor(end.lo - 1));
			return pagedData;
		} else {
			ASSERT(fromFile == 0);
			state Standalone<StringRef> firstChunk;
			state Standalone<StringRef> secondChunk;
			wait(store(firstChunk, self->rawQueue->read(fromFile, fromPage, (file0size / sizeof(Page)) - fromPage)) &&
			     store(secondChunk, self->rawQueue->read(toFile, 0, toPage + 1)));
			if (std::min(self->firstPages(0).seq, self->firstPages(1).seq) > start.lo) {
				// See above.
				throw io_error();
			}
			ASSERT(firstChunk.size() == ((file0size / sizeof(Page)) - fromPage) * _PAGE_SIZE);
			ASSERT(((Page*)firstChunk.begin())->seq == pageFloor(start.lo));
			ASSERT(secondChunk.size() == (toPage + 1) * _PAGE_SIZE);
			ASSERT(((Page*)secondChunk.end() - 1)->seq == pageFloor(end.lo - 1));
			return firstChunk.withSuffix(secondChunk);
		}
	}

	ACTOR static Future<Standalone<StringRef>> read(DiskQueue* self, location start, location end, CheckHashes ch) {
		// This `state` is unnecessary, but works around pagedData wrongly becoming const
		// due to the actor compiler.
		state Standalone<StringRef> pagedData = wait(readPages(self, start, end));
		ASSERT(start.lo % sizeof(Page) == 0 || start.lo % sizeof(Page) >= sizeof(PageHeader));
		int startingOffset = start.lo % sizeof(Page);
		if (startingOffset > 0)
			startingOffset -= sizeof(PageHeader);
		ASSERT(end.lo % sizeof(Page) == 0 || end.lo % sizeof(Page) > sizeof(PageHeader));
		int endingOffset = end.lo % sizeof(Page);
		if (endingOffset == 0)
			endingOffset = sizeof(Page);
		if (endingOffset > 0)
			endingOffset -= sizeof(PageHeader);

		if (pageFloor(end.lo - 1) == pageFloor(start.lo)) {
			// start and end are on the same page
			ASSERT(pagedData.size() == sizeof(Page));
			Page* data = reinterpret_cast<Page*>(const_cast<uint8_t*>(pagedData.begin()));
			if (ch && !data->checkHash())
				throw io_error();
			if (!ch && data->payloadSize > Page::maxPayload)
				throw io_error();
			pagedData.contents() = pagedData.substr(sizeof(PageHeader) + startingOffset, endingOffset - startingOffset);
			return pagedData;
		} else {
			// Reusing pagedData wastes # of pages * sizeof(PageHeader) bytes, but means
			// we don't have to double allocate in a hot, memory hungry call.
			uint8_t* buf = mutateString(pagedData);
			Page* data = reinterpret_cast<Page*>(const_cast<uint8_t*>(pagedData.begin()));
			if (ch && !data->checkHash())
				throw io_error();
			if (!ch && data->payloadSize > Page::maxPayload)
				throw io_error();

			// Only start copying from `start` in the first page.
			if (data->payloadSize > startingOffset) {
				const int length = data->payloadSize - startingOffset;
				memmove(buf, data->payload + startingOffset, length);
				buf += length;
			}
			data++;
			if (ch && !data->checkHash())
				throw io_error();
			if (!ch && data->payloadSize > Page::maxPayload)
				throw io_error();

			// Copy all the middle pages
			while (data->seq != pageFloor(end.lo - 1)) {
				// These pages can have varying amounts of data, as pages with partial
				// data will be zero-filled when commit is called.
				const int length = data->payloadSize;
				memmove(buf, data->payload, length);
				buf += length;
				data++;
				if (ch && !data->checkHash())
					throw io_error();
				if (!ch && data->payloadSize > Page::maxPayload)
					throw io_error();
			}

			// Copy only until `end` in the last page.
			const int length = data->payloadSize;
			memmove(buf, data->payload, std::min(endingOffset, length));
			buf += std::min(endingOffset, length);

			memset(buf, 0, pagedData.size() - (buf - pagedData.begin()));
			Standalone<StringRef> unpagedData = pagedData.substr(0, buf - pagedData.begin());
			return unpagedData;
		}
	}

	void readFromBuffer(StringBuffer* result, int* bytes) {
		// extract up to bytes from readBufPage into result
		int len = std::min(readBufPage->payloadSize - readBufPos, *bytes);
		if (len <= 0)
			return;

		result->append(StringRef(readBufPage->payload + readBufPos, len));

		readBufPos += len;
		*bytes -= len;
		nextReadLocation += len;
	}

	ACTOR static Future<Standalone<StringRef>> readNext(DiskQueue* self, int bytes) {
		state StringBuffer result(self->dbgid);
		ASSERT(bytes >= 0);
		result.clearReserve(bytes);

		ASSERT(!self->recovered);

		if (!self->initialized) {
			bool recoveryComplete = wait(initializeRecovery(self, 0));

			if (recoveryComplete) {
				ASSERT(self->poppedSeq <= self->endLocation());

				return Standalone<StringRef>();
			}
		}

		loop {
			if (self->readBufPage) {
				self->readFromBuffer(&result, &bytes);
				// if done, return
				if (!bytes)
					return result.str;
				ASSERT(self->readBufPos == self->readBufPage->payloadSize);
				self->readBufPage = 0;
				self->nextReadLocation += sizeof(Page) - self->readBufPos;
				self->readBufPos = 0;
			}

			Standalone<StringRef> page = wait(self->rawQueue->readNextPage());
			if (!page.size()) {
				TraceEvent("DQRecEOF", self->dbgid)
				    .detail("NextReadLocation", self->nextReadLocation)
				    .detail("File0Name", self->rawQueue->files[0].dbgFilename);
				break;
			}
			ASSERT(page.size() == sizeof(Page));

			self->readBufArena = page.arena();
			self->readBufPage = (Page*)page.begin();
			if (!self->readBufPage->checkHash() || self->readBufPage->seq < pageFloor(self->nextReadLocation)) {
				TraceEvent("DQRecInvalidPage", self->dbgid)
				    .detail("NextReadLocation", self->nextReadLocation)
				    .detail("HashCheck", self->readBufPage->checkHash())
				    .detail("Seq", self->readBufPage->seq)
				    .detail("Expect", pageFloor(self->nextReadLocation))
				    .detail("File0Name", self->rawQueue->files[0].dbgFilename);
				wait(self->rawQueue->truncateBeforeLastReadPage());
				break;
			}
			//TraceEvent("DQRecPage", self->dbgid).detail("NextReadLoc", self->nextReadLocation).detail("Seq", self->readBufPage->seq).detail("Pop", self->readBufPage->popped).detail("Payload", self->readBufPage->payloadSize).detail("File0Name", self->rawQueue->files[0].dbgFilename);
			ASSERT(self->readBufPage->seq == pageFloor(self->nextReadLocation));
			self->lastPoppedSeq = self->readBufPage->popped;
		}

		// Recovery complete.
		// The fully durable popped point is self->lastPoppedSeq; tell the raw queue that.
		int f;
		int64_t p;
		bool poppedNotDurable = self->lastPoppedSeq / sizeof(Page) != self->poppedSeq / sizeof(Page);
		CODE_PROBE(poppedNotDurable, "DiskQueue: Recovery popped position not fully durable");
		self->findPhysicalLocation(self->lastPoppedSeq, &f, &p, "lastPoppedSeq");
		wait(self->rawQueue->setPoppedPage(f, p, pageFloor(self->lastPoppedSeq)));

		// Writes go at the end of our reads (but on the next page)
		self->nextPageSeq = pageFloor(self->nextReadLocation);
		if (self->nextReadLocation % sizeof(Page) > sizeof(PageHeader))
			self->nextPageSeq += sizeof(Page);

		TraceEvent("DQRecovered", self->dbgid)
		    .detail("LastPoppedSeq", self->lastPoppedSeq)
		    .detail("PoppedSeq", self->poppedSeq)
		    .detail("NextPageSeq", self->nextPageSeq)
		    .detail("File0Name", self->rawQueue->files[0].dbgFilename);
		self->recovered = true;
		ASSERT(self->poppedSeq <= self->endLocation());

		CODE_PROBE(result.size() == 0, "End of queue at border between reads");
		CODE_PROBE(result.size() != 0, "Partial read at end of queue");

		// The next read location isn't necessarily the end of the last commit, but this is sufficient for helping us
		// check an ASSERTion
		self->lastCommittedSeq = self->nextReadLocation;

		return result.str;
	}

	// recoverAt is the minimum position in the disk queue file that needs to be read to restore log's states.
	// This allows log to read only a small portion of the most recent data from a large (e.g., 10GB) disk file.
	// This is particularly useful for logSpilling feature.
	ACTOR static Future<bool> initializeRecovery(DiskQueue* self, location recoverAt) {
		if (self->initialized) {
			return self->recovered;
		}
		Standalone<StringRef> lastPageData = wait(self->rawQueue->readFirstAndLastPages(&comparePages));
		self->initialized = true;

		if (!lastPageData.size()) {
			// There are no valid pages, so apparently this is a completely empty queue
			self->nextReadLocation = 0;
			self->lastCommittedSeq = 0;
			self->recovered = true;
			return true;
		}

		Page* lastPage = (Page*)lastPageData.begin();
		self->poppedSeq = lastPage->popped;
		self->nextReadLocation = std::max(recoverAt.lo, self->poppedSeq);

		/*
		state std::auto_ptr<Page> testPage(new Page);
		state int fileNum;
		for( fileNum=0; fileNum<2; fileNum++) {
		    state int sizeNum;
		    for( sizeNum=0; sizeNum < self->rawQueue->files[fileNum].size; sizeNum += sizeof(Page) ) {
		        wait(success( self->rawQueue->files[fileNum].f->read( testPage.get(), sizeof(Page), sizeNum ) ));
		        TraceEvent("PageData").detail("File", self->rawQueue->files[fileNum].dbgFilename).detail("SizeNum", sizeNum).detail("Seq", testPage->seq).detail("Hash", testPage->checkHash()).detail("Popped", testPage->popped);
		    }
		}
		*/

		int file;
		int64_t page;
		self->findPhysicalLocation(self->nextReadLocation, &file, &page, "FirstReadLocation");
		self->rawQueue->setStartPage(file, page);

		self->readBufPos = self->nextReadLocation % sizeof(Page) - sizeof(PageHeader);
		if (self->readBufPos < 0) {
			self->nextReadLocation -= self->readBufPos;
			self->readBufPos = 0;
		}
		TraceEvent("DQRecStart", self->dbgid)
		    .detail("ReadBufPos", self->readBufPos)
		    .detail("NextReadLoc", self->nextReadLocation)
		    .detail("Popped", self->poppedSeq)
		    .detail("MinRecoverAt", recoverAt)
		    .detail("File0Name", self->rawQueue->files[0].dbgFilename);

		return false;
	}

	Page& firstPages(int i) {
		ASSERT(initialized);
		return *(Page*)rawQueue->firstPages[i];
	}

	void findPhysicalLocation(loc_t loc, int* file, int64_t* page, const char* context) {
		bool ok = false;

		if (context)
			TraceEvent(SevInfo, "FindPhysicalLocation", dbgid)
			    .detail("Page0Valid", firstPages(0).checkHash())
			    .detail("Page0Seq", firstPages(0).seq)
			    .detail("Page1Valid", firstPages(1).checkHash())
			    .detail("Page1Seq", firstPages(1).seq)
			    .detail("Location", loc)
			    .detail("Context", context)
			    .detail("File0Name", rawQueue->files[0].dbgFilename);

		for (int i = 1; i >= 0; i--) {
			ASSERT_WE_THINK(firstPages(i).checkHash());
			if (firstPages(i).seq <= (size_t)loc) {
				*file = i;
				*page = (loc - firstPages(i).seq) / sizeof(Page);
				if (context)
					TraceEvent("FoundPhysicalLocation", dbgid)
					    .detail("PageIndex", i)
					    .detail("PageLocation", *page)
					    .detail("SizeofPage", sizeof(Page))
					    .detail("PageSequence", firstPages(i).seq)
					    .detail("Location", loc)
					    .detail("Context", context)
					    .detail("File0Name", rawQueue->files[0].dbgFilename);
				ok = true;
				break;
			}
		}
		if (!ok)
			TraceEvent(SevError, "DiskQueueLocationError", dbgid)
			    .detail("Page0Valid", firstPages(0).checkHash())
			    .detail("Page0Seq", firstPages(0).seq)
			    .detail("Page1Valid", firstPages(1).checkHash())
			    .detail("Page1Seq", firstPages(1).seq)
			    .detail("Location", loc)
			    .detail("Context", context ? context : "")
			    .detail("File0Name", rawQueue->files[0].dbgFilename);
		ASSERT(ok);
	}

	// isValid(firstPage) == compare(firstPage, firstPage)
	// isValid(otherPage) == compare(firstPage, otherPage)
	// Swap file1, file2 if comparePages( file2.firstPage, file1.firstPage )
	static bool comparePages(void* v1, void* v2) {
		Page* p1 = (Page*)v1;
		Page* p2 = (Page*)v2;
		return p2->checkHash() && (p2->seq >= p1->seq || !p1->checkHash());
	}

	RawDiskQueue_TwoFiles* rawQueue;
	UID dbgid;
	DiskQueueVersion diskQueueVersion;

	bool anyPopped; // pop() has been called since the most recent call to commit()
	bool warnAlwaysForMemory;
	loc_t nextPageSeq, poppedSeq;
	loc_t lastPoppedSeq; // poppedSeq the last time commit was called.
	loc_t lastCommittedSeq; // The seq location where the last commit finishes at.

	// Buffer of pushed pages that haven't been committed.  The last one (backPage()) is still mutable.
	StringBuffer* pushed_page_buffer;
	Page& backPage() {
		ASSERT(pushedPageCount());
		return ((Page*)pushed_page_buffer->ref().end())[-1];
	}
	Page const& backPage() const { return ((Page*)pushed_page_buffer->ref().end())[-1]; }
	int pushedPageCount() const { return pushed_page_buffer ? pushed_page_buffer->size() / sizeof(Page) : 0; }

	// Recovery state
	bool recovered;
	bool initialized;
	loc_t nextReadLocation;
	Arena readBufArena;
	Page* readBufPage;
	int readBufPos;
};

// A class wrapping DiskQueue which durably allows uncommitted data to be popped.
// This works by performing two commits when uncommitted data is popped:
//	Commit 1 - pop only previously committed data and push new data (i.e., commit uncommitted data)
//  Commit 2 - finish pop into uncommitted data
class DiskQueue_PopUncommitted final : public IDiskQueue {

public:
	DiskQueue_PopUncommitted(std::string basename,
	                         std::string fileExtension,
	                         UID dbgid,
	                         DiskQueueVersion diskQueueVersion,
	                         int64_t fileSizeWarningLimit)
	  : queue(new DiskQueue(basename, fileExtension, dbgid, diskQueueVersion, fileSizeWarningLimit)), pushed(0),
	    popped(0), committed(0){};

	// IClosable
	Future<Void> getError() const override { return queue->getError(); }
	Future<Void> onClosed() const override { return queue->onClosed(); }
	void dispose() override {
		queue->dispose();
		delete this;
	}
	void close() override {
		queue->close();
		delete this;
	}

	// IDiskQueue
	Future<bool> initializeRecovery(location recoverAt) override { return queue->initializeRecovery(recoverAt); }
	Future<Standalone<StringRef>> readNext(int bytes) override { return readNext(this, bytes); }

	location getNextReadLocation() const override { return queue->getNextReadLocation(); }

	Future<Standalone<StringRef>> read(location start, location end, CheckHashes ch) override {
		return queue->read(start, end, ch);
	}
	location getNextCommitLocation() const override { return queue->getNextCommitLocation(); }
	location getNextPushLocation() const override { return queue->getNextPushLocation(); }

	location push(StringRef contents) override {
		pushed = queue->push(contents);
		return pushed;
	}

	void pop(location upTo) override {
		popped = std::max(popped, upTo);
		ASSERT_WE_THINK(committed >= popped);
		queue->pop(std::min(committed, popped));
	}

	int getCommitOverhead() const override {
		return queue->getCommitOverhead() + (popped > committed ? queue->getMaxPayload() : 0);
	}

	Future<Void> commit() override {
		location pushLocation = pushed;
		location popLocation = popped;

		Future<Void> commitFuture = queue->commit();

		bool updatePop = popLocation > committed;
		committed = pushLocation;

		if (updatePop) {
			ASSERT_WE_THINK(false);
			ASSERT(popLocation <= committed);

			queue->stall(); // Don't permit this pipelined commit to write anything to disk until the previous commit is
			                // totally finished
			pop(popLocation);
			commitFuture = commitFuture && queue->commit();
		} else {
			CODE_PROBE(true, "No uncommitted data was popped");
		}

		return commitFuture;
	}

	StorageBytes getStorageBytes() const override { return queue->getStorageBytes(); }

private:
	DiskQueue* queue;
	location pushed;
	location popped;
	location committed;

	ACTOR static Future<Standalone<StringRef>> readNext(DiskQueue_PopUncommitted* self, int bytes) {
		Standalone<StringRef> str = wait(self->queue->readNext(bytes));
		if (str.size() < bytes)
			self->pushed = self->getNextReadLocation();

		return str;
	}
};

IDiskQueue* openDiskQueue(std::string basename,
                          std::string ext,
                          UID dbgid,
                          DiskQueueVersion dqv,
                          int64_t fileSizeWarningLimit) {
	return new DiskQueue_PopUncommitted(basename, ext, dbgid, dqv, fileSizeWarningLimit);
}