Feedback-ID: i0fe9450f:Fastmail
Date: Wed, 17 Dec 2025 17:11:19 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Peter Eisentraut <peter@eisentraut.org>
Cc: Sami Imseih <samimseih@gmail.com>, Chao Li <li.evan.chao@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: [Proposal] Adding callback support for custom statistics kinds
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On Wed, Dec 17, 2025 at 08:03:36AM +0100, Peter Eisentraut wrote:
> So it seems to me that either the callbacks API needs some adjustments, or
> this particular implementation of the callback function is incorrect.

Hmm, you are right that this is not aligned.  This can be improved
with one change for each callback:
- It is OK with from_serialized_data() to manipulate the header data,
because we want to fill a portion of the shmem data with extra data
read from disk (the module wants to add a reference to a DSA stored in
the shmem entry, read from the second file).  So we should discard the
const marker from the callback definition.
- The const usage is OK for to_serialized_data(): it is better to
encourage a policy where the header data cannot be manipulated.  So
the const needs to be kept in the definition, but I also think that we
should change the module implementation so as the cast to
PgStatShared_CustomVarEntry is a const.

These changes result in the attached.  Sami, what do you think?
--
Michael

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diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat=
_internal.h
index 5c1ce4d3d6af..01db3b701bdf 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -335,7 +335,7 @@ typedef struct PgStat_KindInfo
 									   const PgStatShared_Common *header,
 									   FILE *statfile);
 	bool		(*from_serialized_data) (const PgStat_HashKey *key,
-										 const PgStatShared_Common *header,
+										 PgStatShared_Common *header,
 										 FILE *statfile);
=20
 	/*
diff --git a/src/test/modules/test_custom_stats/test_custom_var_stats.c b/s=
rc/test/modules/test_custom_stats/test_custom_var_stats.c
index c71922dc4a8f..3dea38e3fe83 100644
--- a/src/test/modules/test_custom_stats/test_custom_var_stats.c
+++ b/src/test/modules/test_custom_stats/test_custom_var_stats.c
@@ -92,7 +92,7 @@ static void test_custom_stats_var_to_serialized_data(cons=
t PgStat_HashKey *key,
=20
 /* Deserialization callback: read auxiliary entry data */
 static bool test_custom_stats_var_from_serialized_data(const PgStat_HashKe=
y *key,
-													   const PgStatShared_Common *header,
+													   PgStatShared_Common *header,
 													   FILE *statfile);
=20
 /* Finish callback: end of statistics file operations */
@@ -196,9 +196,10 @@ test_custom_stats_var_to_serialized_data(const PgStat_=
HashKey *key,
 {
 	char	   *description;
 	size_t		len;
-	PgStatShared_CustomVarEntry *entry =3D (PgStatShared_CustomVarEntry *) he=
ader;
 	bool		found;
 	uint32		magic_number =3D TEST_CUSTOM_VAR_MAGIC_NUMBER;
+	const PgStatShared_CustomVarEntry *entry =3D
+		(const PgStatShared_CustomVarEntry *) header;
=20
 	/*
 	 * First mark the main file with a magic number, keeping a trace that some
@@ -276,7 +277,7 @@ test_custom_stats_var_to_serialized_data(const PgStat_H=
ashKey *key,
  */
 static bool
 test_custom_stats_var_from_serialized_data(const PgStat_HashKey *key,
-										   const PgStatShared_Common *header,
+										   PgStatShared_Common *header,
 										   FILE *statfile)
 {
 	PgStatShared_CustomVarEntry *entry;

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