Add "ABI_compatibility" regions to wait_event_names.txt

The current design behind the automatic generation of the C code and
documentation related to wait events introduced in fa88928470b5 does not
offer a way to attach new wait events without breaking ABI
compatibility, as all the events are forcibly reordered for each section
in the input file wait_event_names.txt.  Adding new wait events to
stable branches is something that has happened in the past, 0b6517a3b79a
being a recent example of that with VERSION_FILE_SYNC, so we need a way
to generate any C code for wait events while maintaining compatibility
on stable branches already released.

This commit solves this issue by adding a new region called
"ABI_compatibility" (keyword could be updated to something else if
someone had a better idea) to each section of wait_event_names.txt, so
as one can add new wait events to stable branches in
wait_event_names.txt while keeping the code ABI-compatible.
"ABI_compatibility" has no impact on the documentation generated: all
the wait events of one section are still alphabetically ordered.  LWLock
and Lock sections generate their C code elsewhere, so they do not need
an "ABI_compatibility" region.

For example, let's imagine a wait_event_names.txt like that:
Section: ClassName - Foo
FOO_1	"Waiting in Foo 1"
FOO_2	"Waiting in Foo 2"
ABI_compatibility:
NEW_FOO_1	"Waiting in New Foo 1"
NEW_BAR_1	"Waiting in New Bar 1"

This results in the following enum, where the events in the ABI region
are listed last with the same ordering as in wait_event_names.txt:
typedef enum
{
    WAIT_EVENT_FOO_1,
    WAIT_EVENT_FOO_2,
    WAIT_EVENT_NEW_FOO_1,
    WAIT_EVENT_NEW_BAR_1
} WaitEventFoo;

New wait events added in stable branches should be added at the end of
each ABI_compatibility region, and ABI_compatibility should remain empty
on HEAD and unreleased stable branches.

This design has been suggested by Noah Misch and me.

Reported-by: Noah Misch
Author: Bertrand Drouvot
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/20240317183114.16@rfd.leadboat.com

2 files changed, 45 insertions, 1 deletions

diff --git a/src/backend/utils/activity/generate-wait_event_types.pl b/src/backend/utils/activity/generate-wait_event_types.pl
index f1adf0e8e74..42f36f405bd 100644
--- a/src/backend/utils/activity/generate-wait_event_types.pl
+++ b/src/backend/utils/activity/generate-wait_event_types.pl
@@ -38,7 +38,9 @@ die "Not possible to specify --docs and --code simultaneously"
 
 open my $wait_event_names, '<', $ARGV[0] or die;
 
+my @abi_compatibility_lines;
 my @lines;
+my $abi_compatibility = 0;
 my $section_name;
 my $note;
 my $note_name;
@@ -59,10 +61,27 @@ while (<$wait_event_names>)
 	{
 		$section_name = $_;
 		$section_name =~ s/^.*- //;
+		$abi_compatibility = 0;
 		next;
 	}
 
-	push(@lines, $section_name . "\t" . $_);
+	# ABI_compatibility region, preserving ABI compatibility of the code
+	# generated.  Any wait events listed in this part of the file will
+	# not be sorted during the code generation.
+	if (/^ABI_compatibility:$/)
+	{
+		$abi_compatibility = 1;
+		next;
+	}
+
+	if ($gen_code && $abi_compatibility)
+	{
+		push(@abi_compatibility_lines, $section_name . "\t" . $_);
+	}
+	else
+	{
+		push(@lines, $section_name . "\t" . $_);
+	}
 }
 
 # Sort the lines based on the second column.
@@ -70,6 +89,13 @@ while (<$wait_event_names>)
 my @lines_sorted =
   sort { uc((split(/\t/, $a))[1]) cmp uc((split(/\t/, $b))[1]) } @lines;
 
+# If we are generating code, concat @lines_sorted and then
+# @abi_compatibility_lines.
+if ($gen_code)
+{
+	push(@lines_sorted, @abi_compatibility_lines);
+}
+
 # Read the sorted lines and populate the hash table
 foreach my $line (@lines_sorted)
 {
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
index 0d288d6b3d8..39a1c40d76c 100644
--- a/src/backend/utils/activity/wait_event_names.txt
+++ b/src/backend/utils/activity/wait_event_names.txt
@@ -26,6 +26,13 @@
 # When adding a new wait event, make sure it is placed in the appropriate
 # ClassName section.
 #
+# Wait events added in stable branches should be appended to the lists in
+# the "ABI_compatibility:" region of their related ClassName section to
+# preserve ABI compatibility of the C code generated from this file's data,
+# respecting the order of any wait event already listed there.  The
+# "ABI_compatibility:" regions should remain empty on the master branch and
+# on unreleased branches.
+#
 # WaitEventLWLock and WaitEventLock have their own C code for their wait event
 # enums and function names.  Hence, for these, only the SGML documentation is
 # generated.
@@ -61,6 +68,7 @@ WAL_SENDER_MAIN	"Waiting in main loop of WAL sender process."
 WAL_SUMMARIZER_WAL	"Waiting in WAL summarizer for more WAL to be generated."
 WAL_WRITER_MAIN	"Waiting in main loop of WAL writer process."
 
+ABI_compatibility:
 
 #
 # Wait Events - Client
@@ -83,6 +91,7 @@ WAIT_FOR_WAL_REPLAY	"Waiting for a replay of the particular WAL position on the
 WAL_SENDER_WAIT_FOR_WAL	"Waiting for WAL to be flushed in WAL sender process."
 WAL_SENDER_WRITE_DATA	"Waiting for any activity when processing replies from WAL receiver in WAL sender process."
 
+ABI_compatibility:
 
 #
 # Wait Events - IPC
@@ -150,6 +159,7 @@ WAL_RECEIVER_WAIT_START	"Waiting for startup process to send initial data for st
 WAL_SUMMARY_READY	"Waiting for a new WAL summary to be generated."
 XACT_GROUP_UPDATE	"Waiting for the group leader to update transaction status at end of a parallel operation."
 
+ABI_compatibility:
 
 #
 # Wait Events - Timeout
@@ -170,6 +180,7 @@ VACUUM_DELAY	"Waiting in a cost-based vacuum delay point."
 VACUUM_TRUNCATE	"Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed."
 WAL_SUMMARIZER_ERROR	"Waiting after a WAL summarizer error."
 
+ABI_compatibility:
 
 #
 # Wait Events - IO
@@ -257,6 +268,7 @@ WAL_SYNC	"Waiting for a WAL file to reach durable storage."
 WAL_SYNC_METHOD_ASSIGN	"Waiting for data to reach durable storage while assigning a new WAL sync method."
 WAL_WRITE	"Waiting for a write to a WAL file."
 
+ABI_compatibility:
 
 #
 # Wait Events - Buffer Pin
@@ -266,6 +278,7 @@ Section: ClassName - WaitEventBufferPin
 
 BUFFER_PIN	"Waiting to acquire an exclusive pin on a buffer."
 
+ABI_compatibility:
 
 #
 # Wait Events - Extension
@@ -275,6 +288,8 @@ Section: ClassName - WaitEventExtension
 
 Extension	"Waiting in an extension."
 
+ABI_compatibility:
+
 #
 # Wait Events - LWLock
 #
@@ -379,6 +394,7 @@ SubtransSLRU	"Waiting to access the sub-transaction SLRU cache."
 XactSLRU	"Waiting to access the transaction status SLRU cache."
 ParallelVacuumDSA	"Waiting for parallel vacuum dynamic shared memory allocation."
 
+# No "ABI_compatibility" region here as WaitEventLWLock has its own C code.
 
 #
 # Wait Events - Lock
@@ -401,3 +417,5 @@ object	"Waiting to acquire a lock on a non-relation database object."
 userlock	"Waiting to acquire a user lock."
 advisory	"Waiting to acquire an advisory user lock."
 applytransaction	"Waiting to acquire a lock on a remote transaction being applied by a logical replication subscriber."
+
+# No "ABI_compatibility" region here as WaitEventLock has its own C code.