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Commit6cc6619

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Improve memory handling across SQL-callable backup functions
Since pg_backup_start() and pg_backup_stop() exist, the tablespace mapdata and the backup state data (backup_label string until7d70809) havebeen allocated in the TopMemoryContext. This approach would causememory leaks in the session calling these functions if failures happenbefore pg_backup_stop() ends, leaking more memory on repeated failures.Both things need little memory so that would not be really noticeablefor most users, except perhaps connection poolers with long-livedconnections able to trigger backup failures with these functions.This commit improves the logic in this area by not allocating anymorethe backup-related data that needs to travel across the SQL-callablebackup functions in TopMemoryContext, by using instead a dedicatedmemory context child of TopMemoryContext. The memory context is createdin pg_backup_start() and deleted when finishing pg_backup_stop(). Inthe event of an in-flight failure, this memory context gets reset in thefollow-up pg_backup_start() call, so as we are sure that only one runworth of data is leaked at any time. Some cleanup was already done forthe backup data on a follow-up call of pg_backup_start(), but using amemory context makes the whole simpler.BASE_BACKUP commands are executed in isolation, relying on the memorycontext created for replication commands, hence these do not need suchan extra logic.Author: Bharath RupireddyReviewed-by: Robert Haas, Alvaro Herrera, Cary Huang, Michael PaquierDiscussion:https://postgr.es/m/CALj2ACXqvfKF2B0beQ=aJMdWnpNohmBPsRg=EDQj_6y1t2O8mQ@mail.gmail.com
1 parent1f0c4fa commit6cc6619

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1 file changed

+22
-19
lines changed

1 file changed

+22
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lines changed

‎src/backend/access/transam/xlogfuncs.c

Lines changed: 22 additions & 19 deletions
Original file line numberDiff line numberDiff line change
@@ -45,6 +45,9 @@
4545
staticBackupState*backup_state=NULL;
4646
staticStringInfotablespace_map=NULL;
4747

48+
/* Session-level context for the SQL-callable backup functions */
49+
staticMemoryContextbackupcontext=NULL;
50+
4851
/*
4952
* pg_backup_start: set up for taking an on-line backup dump
5053
*
@@ -72,27 +75,26 @@ pg_backup_start(PG_FUNCTION_ARGS)
7275

7376
/*
7477
* backup_state and tablespace_map need to be long-lived as they are used
75-
* in pg_backup_stop().
78+
* in pg_backup_stop(). These are allocated in a dedicated memory context
79+
* child of TopMemoryContext, deleted at the end of pg_backup_stop(). If
80+
* an error happens before ending the backup, memory would be leaked in
81+
* this context until pg_backup_start() is called again.
7682
*/
77-
oldcontext=MemoryContextSwitchTo(TopMemoryContext);
78-
79-
/* Allocate backup state or reset it, if it comes from a previous run */
80-
if (backup_state==NULL)
81-
backup_state= (BackupState*)palloc0(sizeof(BackupState));
83+
if (backupcontext==NULL)
84+
{
85+
backupcontext=AllocSetContextCreate(TopMemoryContext,
86+
"on-line backup context",
87+
ALLOCSET_START_SMALL_SIZES);
88+
}
8289
else
83-
MemSet(backup_state,0,sizeof(BackupState));
84-
85-
/*
86-
* tablespace_map may have been created in a previous backup, so take this
87-
* occasion to clean it.
88-
*/
89-
if (tablespace_map!=NULL)
9090
{
91-
pfree(tablespace_map->data);
92-
pfree(tablespace_map);
91+
backup_state=NULL;
9392
tablespace_map=NULL;
93+
MemoryContextReset(backupcontext);
9494
}
9595

96+
oldcontext=MemoryContextSwitchTo(backupcontext);
97+
backup_state= (BackupState*)palloc0(sizeof(BackupState));
9698
tablespace_map=makeStringInfo();
9799
MemoryContextSwitchTo(oldcontext);
98100

@@ -157,12 +159,13 @@ pg_backup_stop(PG_FUNCTION_ARGS)
157159
values[2]=CStringGetTextDatum(tablespace_map->data);
158160

159161
/* Deallocate backup-related variables */
160-
pfree(backup_state);
162+
pfree(backup_label);
163+
164+
/* Clean up the session-level state and its memory context */
161165
backup_state=NULL;
162-
pfree(tablespace_map->data);
163-
pfree(tablespace_map);
164166
tablespace_map=NULL;
165-
pfree(backup_label);
167+
MemoryContextDelete(backupcontext);
168+
backupcontext=NULL;
166169

167170
/* Returns the record as Datum */
168171
PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc,values,nulls)));

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