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Document and check that PgStat_HashKey has no padding

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This change is a tighter rework of 7d85d87f4d, which tried to improve
the code so as it would work should PgStat_HashKey gain new fields that
create padding bytes.  However, the previous change is proving to not be
enough as some code paths of pgstats do not pass PgStat_HashKey by
reference (valgrind would warn when padding is added to the structure,
through a new field).

Per discussion, let's document and check that PgStat_HashKey has no
padding rather than try to complicate the code of pgstats so as it is
able to work around that.

This removes a couple of memset(0) calls that should not be required.
While on it, this commit adds a static assertion checking that no
padding is introduced in the structure, by checking that the size of
PgStat_HashKey matches with the sum of the size of all its fields.

The object ID part of the hash key is already 8 bytes, which should be
plenty enough already.  A comment is added to discourage the addition of
new fields.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0t9omat+HVSakJXwTMWvhpYFcAZb41RPWKwrKFUgmAFBQ@mail.gmail.com
This commit is contained in:
Michael Paquier
2025-09-19 09:54:05 +09:00
parent 16607718c0
commit 3cd3a039da
3 changed files with 19 additions and 13 deletions
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5
src/backend/utils/activity/pgstat.c
View File

@@ -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;

10
src/backend/utils/activity/pgstat_shmem.c
View File

@@ -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;

17
src/include/utils/pgstat_internal.h
View File

@@ -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