From 86f6080503a5e986e7daf771d5d7970f36cc2fe6 Mon Sep 17 00:00:00 2001 From: Michael Paquier Date: Thu, 18 Sep 2025 09:48:44 +0900 Subject: [PATCH v2] Document no-padding rule for PgStat_HashKey This removes a couple of memset(0) calls, as we don't want padding in it. While on it, add a cheap assertion checking that no padding is introduced in the structure, by checking that the size of PgStat_HashKey matches with the sum of the field sizes. --- src/include/utils/pgstat_internal.h | 17 ++++++++++++++++- src/backend/utils/activity/pgstat.c | 5 +---- src/backend/utils/activity/pgstat_shmem.c | 10 ++-------- 3 files changed, 19 insertions(+), 13 deletions(-) diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h index 6cf00008f633..bf75ebcef31c 100644 --- a/src/include/utils/pgstat_internal.h +++ b/src/include/utils/pgstat_internal.h @@ -48,7 +48,13 @@ * PgStatShared_Common as the first element. */ -/* struct for shared statistics hash entry key. */ +/* + * Struct for shared statistics hash entry key. + * + * NB: We assume that this struct contains no padding. Also, 8 bytes + * allocated for the object ID are good enough to ensure the uniqueness + * of the hash key, hence the addition of new fields is not recommended. + */ typedef struct PgStat_HashKey { PgStat_Kind kind; /* statistics entry kind */ @@ -57,6 +63,15 @@ typedef struct PgStat_HashKey * identifier. */ } PgStat_HashKey; +/* + * PgStat_HashKey should not have any padding. Checking that the structure + * size matches with the sum of each field is a check simple enough to + * enforce this policy. + */ +StaticAssertDecl((sizeof(PgStat_Kind) + sizeof(uint64) + sizeof(Oid)) == + sizeof(PgStat_HashKey), + "PgStat_HashKey should have no padding"); + /* * Shared statistics hash entry. Doesn't itself contain any stats, but points * to them (with ->body). That allows the stats entries themselves to be of diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c index f8e91484e36b..73c2ced3f4e2 100644 --- a/src/backend/utils/activity/pgstat.c +++ b/src/backend/utils/activity/pgstat.c @@ -932,7 +932,7 @@ pgstat_clear_snapshot(void) void * pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, uint64 objid) { - PgStat_HashKey key; + PgStat_HashKey key = {0}; PgStat_EntryRef *entry_ref; void *stats_data; const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind); @@ -943,9 +943,6 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, uint64 objid) pgstat_prep_snapshot(); - /* clear padding */ - memset(&key, 0, sizeof(struct PgStat_HashKey)); - key.kind = kind; key.dboid = dboid; key.objid = objid; diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c index 9dc3212f7dd0..ca36fd247f64 100644 --- a/src/backend/utils/activity/pgstat_shmem.c +++ b/src/backend/utils/activity/pgstat_shmem.c @@ -456,14 +456,11 @@ PgStat_EntryRef * pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, uint64 objid, bool create, bool *created_entry) { - PgStat_HashKey key; + PgStat_HashKey key = {0}; PgStatShared_HashEntry *shhashent; PgStatShared_Common *shheader = NULL; PgStat_EntryRef *entry_ref; - /* clear padding */ - memset(&key, 0, sizeof(struct PgStat_HashKey)); - key.kind = kind; key.dboid = dboid; key.objid = objid; @@ -988,13 +985,10 @@ pgstat_drop_database_and_contents(Oid dboid) bool pgstat_drop_entry(PgStat_Kind kind, Oid dboid, uint64 objid) { - PgStat_HashKey key; + PgStat_HashKey key = {0}; PgStatShared_HashEntry *shent; bool freed = true; - /* clear padding */ - memset(&key, 0, sizeof(struct PgStat_HashKey)); - key.kind = kind; key.dboid = dboid; key.objid = objid; -- 2.51.0