Alter pre-update handling during downgrade script generation

Before this change during downgrade script generation we would
always fetch the pre-update script from the previous version and
prepend it to the generated scripts. This limits what can be
referenced in the pre-update script and also what is possible
within the downgrade itself.
This patch splits the pre-update script into a generic part that
is used for update/downgrade and an update specific part. We could
later also add a downgrade specific part but currently it is not
needed. This change is necessary because we reference a timescaledb
view in the pre-update script which prevents changes to that view.

cmake/GenerateScripts.cmake

@@ -158,19 +158,11 @@ function(generate_downgrade_script)
   include(
     ${CMAKE_BINARY_DIR}/v${_downgrade_TARGET_VERSION}/cmake/ScriptFiles.cmake)
 
-  set(_downgrade_PRE_FILES ${PRE_UPDATE_FILES})
+  set(_downgrade_PRE_FILES ${PRE_DOWNGRADE_FILES})
   set(_downgrade_POST_FILES "${PRE_INSTALL_FUNCTION_FILES};${SOURCE_FILES}" ${SET_POST_UPDATE_STAGE}
                             ${POST_UPDATE_FILES} ${UNSET_UPDATE_STAGE})
 
-  # Fetch prolog and epilog from target version.
-  git_versioned_get(
-    VERSION
-    ${_downgrade_TARGET_VERSION}
-    FILES
-    ${_downgrade_PRE_FILES}
-    RESULT_FILES
-    _prolog_files
-    IGNORE_ERRORS)
+  # Fetch epilog from target version.
   git_versioned_get(
     VERSION
     ${_downgrade_TARGET_VERSION}
@@ -180,7 +172,11 @@ function(generate_downgrade_script)
     _epilog_files
     IGNORE_ERRORS)
 
-  set(_files ${_prolog_files})
+  foreach(_downgrade_file ${_downgrade_PRE_FILES})
+    get_filename_component(_downgrade_filename ${_downgrade_file} NAME)
+    configure_file(${_downgrade_file} ${_downgrade_INPUT_DIRECTORY}/${_downgrade_filename} COPYONLY)
+    list(APPEND _files ${_downgrade_INPUT_DIRECTORY}/${_downgrade_filename})
+  endforeach()
   foreach(_downgrade_file ${_downgrade_FILES})
     list(APPEND _files ${_downgrade_INPUT_DIRECTORY}/${_downgrade_file})
   endforeach()

cmake/ScriptFiles.cmake

@@ -77,7 +77,8 @@ list(APPEND SOURCE_FILES
 
 # These files should be pre-pended to update scripts so that they are executed
 # before anything else during updates
-set(PRE_UPDATE_FILES updates/pre-update.sql)
+set(PRE_UPDATE_FILES updates/pre-version-change.sql updates/pre-update.sql)
+set(PRE_DOWNGRADE_FILES updates/pre-version-change.sql)
 
 # The POST_UPDATE_FILES should be executed as the last part of the update
 # script. sets state for executing POST_UPDATE_FILES during ALTER EXTENSION

sql/updates/pre-update.sql

@@ -2,54 +2,7 @@
 -- Please see the included NOTICE for copyright information and
 -- LICENSE-APACHE for a copy of the license.
 
--- This file is always prepended to all upgrade and downgrade scripts.
-SET LOCAL search_path TO pg_catalog, pg_temp;
-
--- Disable parallel execution for the duration of the update process.
--- This avoids version mismatch errors that would have beeen triggered by the
--- parallel workers in ts_extension_check_version().
-SET LOCAL max_parallel_workers = 0;
-
--- Triggers should be disabled during upgrades to avoid having them
--- invoke functions that might load an old version of the shared
--- library before those functions have been updated.
-DROP EVENT TRIGGER IF EXISTS timescaledb_ddl_command_end;
-DROP EVENT TRIGGER IF EXISTS timescaledb_ddl_sql_drop;
-
--- Since we want to call the new version of restart_background_workers we
--- create a function that points to that version. The proper restart_background_workers
--- may either be in _timescaledb_internal or in _timescaledb_functions
--- depending on the version we are upgrading from and we can't make
--- the move in this location as the new schema might not have been set up.
-CREATE FUNCTION _timescaledb_internal._tmp_restart_background_workers()
-RETURNS BOOL
-AS '@LOADER_PATHNAME@', 'ts_bgw_db_workers_restart'
-LANGUAGE C VOLATILE;
-SELECT _timescaledb_internal._tmp_restart_background_workers();
-DROP FUNCTION _timescaledb_internal._tmp_restart_background_workers();
-
--- Table for ACL and initprivs of tables.
-CREATE TABLE _timescaledb_internal.saved_privs(
-       tmpnsp name,
-       tmpname name,
-       tmpacl aclitem[],
-       tmpini aclitem[],
-       UNIQUE (tmpnsp, tmpname));
-
--- We save away both the ACL and the initprivs for all tables and
--- views in the extension (but not for chunks and internal objects) so
--- that we can restore them to the proper state after the update.
-INSERT INTO _timescaledb_internal.saved_privs
-SELECT nspname, relname, relacl, initprivs
-  FROM pg_class cl JOIN pg_namespace ns ON ns.oid = relnamespace
-                   JOIN pg_init_privs ip ON ip.objoid = cl.oid AND ip.objsubid = 0 AND ip.classoid = 'pg_class'::regclass
-WHERE
-  nspname IN ('_timescaledb_catalog', '_timescaledb_config')
-  OR (
-    relname IN ('hypertable_chunk_local_size', 'compressed_chunk_stats', 'bgw_job_stat', 'bgw_policy_chunk_stats')
-    AND nspname = '_timescaledb_internal'
-  )
-;
+-- This file is always prepended to all upgrade scripts.
 
 -- ERROR if trying to update the extension on PG16 using Multi-Node
 DO $$

sql/updates/pre-version-change.sql

@@ -0,0 +1,56 @@
+-- This file and its contents are licensed under the Apache License 2.0.
+-- Please see the included NOTICE for copyright information and
+-- LICENSE-APACHE for a copy of the license.
+
+-- This file is always prepended to all upgrade and downgrade scripts.
+-- This file must avoid referencing extension objects directly as that
+-- would limit the things we can alter in extension update/downgrade
+-- itself.
+SET LOCAL search_path TO pg_catalog, pg_temp;
+
+-- Disable parallel execution for the duration of the update process.
+-- This avoids version mismatch errors that would have beeen triggered by the
+-- parallel workers in ts_extension_check_version().
+SET LOCAL max_parallel_workers = 0;
+
+-- Triggers should be disabled during upgrades to avoid having them
+-- invoke functions that might load an old version of the shared
+-- library before those functions have been updated.
+DROP EVENT TRIGGER IF EXISTS timescaledb_ddl_command_end;
+DROP EVENT TRIGGER IF EXISTS timescaledb_ddl_sql_drop;
+
+-- Since we want to call the new version of restart_background_workers we
+-- create a function that points to that version. The proper restart_background_workers
+-- may either be in _timescaledb_internal or in _timescaledb_functions
+-- depending on the version we are upgrading from and we can't make
+-- the move in this location as the new schema might not have been set up.
+CREATE FUNCTION _timescaledb_internal._tmp_restart_background_workers()
+RETURNS BOOL
+AS '@LOADER_PATHNAME@', 'ts_bgw_db_workers_restart'
+LANGUAGE C VOLATILE;
+SELECT _timescaledb_internal._tmp_restart_background_workers();
+DROP FUNCTION _timescaledb_internal._tmp_restart_background_workers();
+
+-- Table for ACL and initprivs of tables.
+CREATE TABLE _timescaledb_internal.saved_privs(
+       tmpnsp name,
+       tmpname name,
+       tmpacl aclitem[],
+       tmpini aclitem[],
+       UNIQUE (tmpnsp, tmpname));
+
+-- We save away both the ACL and the initprivs for all tables and
+-- views in the extension (but not for chunks and internal objects) so
+-- that we can restore them to the proper state after the update.
+INSERT INTO _timescaledb_internal.saved_privs
+SELECT nspname, relname, relacl, initprivs
+  FROM pg_class cl JOIN pg_namespace ns ON ns.oid = relnamespace
+                   JOIN pg_init_privs ip ON ip.objoid = cl.oid AND ip.objsubid = 0 AND ip.classoid = 'pg_class'::regclass
+WHERE
+  nspname IN ('_timescaledb_catalog', '_timescaledb_config')
+  OR (
+    relname IN ('hypertable_chunk_local_size', 'compressed_chunk_stats', 'bgw_job_stat', 'bgw_policy_chunk_stats')
+    AND nspname = '_timescaledb_internal'
+  )
+;
+