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Use actual backend IDs in pg_stat_get_backend_idset() and friends.

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Up to now, the ID values returned by pg_stat_get_backend_idset() and
used by pg_stat_get_backend_activity() and allied functions were just
indexes into a local array of sessions seen by the last stats refresh.
This is problematic for a few reasons.  The "ID" of a session can vary
over its existence, which is surprising.  Also, while these numbers
often match the "backend ID" used for purposes like temp schema
assignment, that isn't reliably true.  We can fairly cheaply switch
things around to make these numbers actually be the sessions' backend
IDs.  The added test case illustrates that with this definition, the
temp schema used by a given session can be obtained given its PID.

While here, delete some dead code that guarded against getting
a NULL return from pgstat_fetch_stat_local_beentry().  That can't
happen as long as the caller is careful to pass an in-range array
index, as all the callers are.  (This code may not have been dead
when written, but it surely is now.)

Nathan Bossart

Discussion: https://postgr.es/m/20220815205811.GA250990@nathanxps13
This commit is contained in:
Tom Lane
2022-09-29 12:14:39 -04:00
parent d5e3fe682a
commit d7e39d72ca
6 changed files with 107 additions and 51 deletions
Download Patch File Download Diff File Expand all files Collapse all files

55
src/backend/utils/activity/backend_status.c
View File

@@ -846,6 +846,13 @@ pgstat_read_current_status(void)
/* Only valid entries get included into the local array */
if (localentry->backendStatus.st_procpid > 0)
{
/*
* The BackendStatusArray index is exactly the BackendId of the
* source backend. Note that this means localBackendStatusTable
* is in order by backend_id. pgstat_fetch_stat_beentry() depends
* on that.
*/
localentry->backend_id = i;
BackendIdGetTransactionIds(i,
&localentry->backend_xid,
&localentry->backend_xmin);
@@ -1045,26 +1052,57 @@ pgstat_get_my_query_id(void)
return MyBEEntry->st_query_id;
}
/* ----------
* cmp_lbestatus
*
* Comparison function for bsearch() on an array of LocalPgBackendStatus.
* The backend_id field is used to compare the arguments.
* ----------
*/
static int
cmp_lbestatus(const void *a, const void *b)
{
const LocalPgBackendStatus *lbestatus1 = (const LocalPgBackendStatus *) a;
const LocalPgBackendStatus *lbestatus2 = (const LocalPgBackendStatus *) b;
return lbestatus1->backend_id - lbestatus2->backend_id;
}
/* ----------
* pgstat_fetch_stat_beentry() -
*
* Support function for the SQL-callable pgstat* functions. Returns
* our local copy of the current-activity entry for one backend.
* our local copy of the current-activity entry for one backend,
* or NULL if the given beid doesn't identify any known session.
*
* The beid argument is the BackendId of the desired session
* (note that this is unlike pgstat_fetch_stat_local_beentry()).
*
* NB: caller is responsible for a check if the user is permitted to see
* this info (especially the querystring).
* ----------
*/
PgBackendStatus *
pgstat_fetch_stat_beentry(int beid)
pgstat_fetch_stat_beentry(BackendId beid)
{
LocalPgBackendStatus key;
LocalPgBackendStatus *ret;
pgstat_read_current_status();
if (beid < 1 || beid > localNumBackends)
return NULL;
/*
* Since the localBackendStatusTable is in order by backend_id, we can use
* bsearch() to search it efficiently.
*/
key.backend_id = beid;
ret = (LocalPgBackendStatus *) bsearch(&key, localBackendStatusTable,
localNumBackends,
sizeof(LocalPgBackendStatus),
cmp_lbestatus);
if (ret)
return &ret->backendStatus;
return &localBackendStatusTable[beid - 1].backendStatus;
return NULL;
}
@@ -1074,6 +1112,10 @@ pgstat_fetch_stat_beentry(int beid)
* Like pgstat_fetch_stat_beentry() but with locally computed additions (like
* xid and xmin values of the backend)
*
* The beid argument is a 1-based index in the localBackendStatusTable
* (note that this is unlike pgstat_fetch_stat_beentry()).
* Returns NULL if the argument is out of range (no current caller does that).
*
* NB: caller is responsible for a check if the user is permitted to see
* this info (especially the querystring).
* ----------
@@ -1094,7 +1136,8 @@ pgstat_fetch_stat_local_beentry(int beid)
* pgstat_fetch_stat_numbackends() -
*
* Support function for the SQL-callable pgstat* functions. Returns
* the maximum current backend id.
* the number of sessions known in the localBackendStatusTable, i.e.
* the maximum 1-based index to pass to pgstat_fetch_stat_local_beentry().
* ----------
*/
int

48
src/backend/utils/adt/pgstatfuncs.c
View File

@@ -415,7 +415,6 @@ pg_stat_get_backend_idset(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
int *fctx;
int32 result;
/* stuff done only on the first call of the function */
if (SRF_IS_FIRSTCALL())
@@ -424,11 +423,10 @@ pg_stat_get_backend_idset(PG_FUNCTION_ARGS)
funcctx = SRF_FIRSTCALL_INIT();
fctx = MemoryContextAlloc(funcctx->multi_call_memory_ctx,
2 * sizeof(int));
sizeof(int));
funcctx->user_fctx = fctx;
fctx[0] = 0;
fctx[1] = pgstat_fetch_stat_numbackends();
}
/* stuff done on every call of the function */
@@ -436,12 +434,22 @@ pg_stat_get_backend_idset(PG_FUNCTION_ARGS)
fctx = funcctx->user_fctx;
fctx[0] += 1;
result = fctx[0];
if (result <= fctx[1])
/*
* We recheck pgstat_fetch_stat_numbackends() each time through, just in
* case the local status data has been refreshed since we started. It's
* plenty cheap enough if not. If a refresh does happen, we'll likely
* miss or duplicate some backend IDs, but we're content not to crash.
* (Refreshing midway through such a query would be problematic usage
* anyway, since the backend IDs we've already returned might no longer
* refer to extant sessions.)
*/
if (fctx[0] <= pgstat_fetch_stat_numbackends())
{
/* do when there is more left to send */
SRF_RETURN_NEXT(funcctx, Int32GetDatum(result));
LocalPgBackendStatus *local_beentry = pgstat_fetch_stat_local_beentry(fctx[0]);
SRF_RETURN_NEXT(funcctx, Int32GetDatum(local_beentry->backend_id));
}
else
{
@@ -493,17 +501,13 @@ pg_stat_get_progress_info(PG_FUNCTION_ARGS)
int i;
local_beentry = pgstat_fetch_stat_local_beentry(curr_backend);
if (!local_beentry)
continue;
beentry = &local_beentry->backendStatus;
/*
* Report values for only those backends which are running the given
* command.
*/
if (!beentry || beentry->st_progress_command != cmdtype)
if (beentry->st_progress_command != cmdtype)
continue;
/* Value available to all callers */
@@ -558,24 +562,6 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
/* Get the next one in the list */
local_beentry = pgstat_fetch_stat_local_beentry(curr_backend);
if (!local_beentry)
{
int i;
/* Ignore missing entries if looking for specific PID */
if (pid != -1)
continue;
for (i = 0; i < lengthof(nulls); i++)
nulls[i] = true;
nulls[5] = false;
values[5] = CStringGetTextDatum("<backend information not available>");
tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
continue;
}
beentry = &local_beentry->backendStatus;
/* If looking for specific PID, ignore all the others */
@@ -1180,9 +1166,9 @@ pg_stat_get_db_numbackends(PG_FUNCTION_ARGS)
result = 0;
for (beid = 1; beid <= tot_backends; beid++)
{
PgBackendStatus *beentry = pgstat_fetch_stat_beentry(beid);
LocalPgBackendStatus *local_beentry = pgstat_fetch_stat_local_beentry(beid);
if (beentry && beentry->st_databaseid == dbid)
if (local_beentry->backendStatus.st_databaseid == dbid)
result++;
}