Fix const correctness in pgstat data serialization callbacks

michaelpq · samimseih · michaelpq · commit 167cb26718e3 · 2025-12-18T07:33:40.000+09:00
4ba012a defined the "header" (pointer to the stats data) of from_serialized_data() as a const, even though it is fine (and expected!) for the callback to modify the shared memory entry when loading the stats at startup. While on it, this commit updates the callback to_serialized_data() in the test module test_custom_stats to make the data extracted from the "header" parameter a const since it should never be modified: the stats are written to disk and no modifications are expected in the shared memory entry. This clarifies the API contract of these new callbacks. Reported-By: Peter Eisentraut <peter@eisentraut.org> Author: Michael Paquier <michael@paquier.xyz> Co-authored-by: Sami Imseih <samimseih@gmail.com> Discussion: https://postgr.es/m/d87a93b0-19c7-4db6-b9c0-d6827e7b2da1@eisentraut.org
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
@@ -329,13 +329,14 @@ typedef struct PgStat_KindInfo
 	 *
 	 * "statfile" is a pointer to the on-disk stats file, named
 	 * PGSTAT_STAT_PERMANENT_FILENAME.  "key" is the hash key of the entry
-	 * just written or read.  "header" is a pointer to the stats data.
+	 * just written or read.  "header" is a pointer to the stats data; it may
+	 * be modified only in from_serialized_data to reconstruct an entry.
 	 */
 	void		(*to_serialized_data) (const PgStat_HashKey *key,
 									   const PgStatShared_Common *header,
 									   FILE *statfile);
 	bool		(*from_serialized_data) (const PgStat_HashKey *key,
-										 const PgStatShared_Common *header,
+										 PgStatShared_Common *header,
 										 FILE *statfile);
 
 	/*
diff --git 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(const PgStat_HashKey *key,
 
 /* Deserialization callback: read auxiliary entry data */
 static bool test_custom_stats_var_from_serialized_data(const PgStat_HashKey *key,
-													   const PgStatShared_Common *header,
+													   PgStatShared_Common *header,
 													   FILE *statfile);
 
 /* Finish callback: end of statistics file operations */
@@ -196,7 +196,7 @@ test_custom_stats_var_to_serialized_data(const PgStat_HashKey *key,
 {
 	char	   *description;
 	size_t		len;
-	PgStatShared_CustomVarEntry *entry = (PgStatShared_CustomVarEntry *) header;
+	const PgStatShared_CustomVarEntry *entry = (const PgStatShared_CustomVarEntry *) header;
 	bool		found;
 	uint32		magic_number = TEST_CUSTOM_VAR_MAGIC_NUMBER;
 
@@ -276,7 +276,7 @@ test_custom_stats_var_to_serialized_data(const PgStat_HashKey *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;
