Fix permanent memory leak in autovacuum launcher

get_database_list was uselessly allocating its output data, along some
created along the way, in a permanent memory context.  This didn't
matter when autovacuum was a single, short-lived process, but now that
the launcher is permanent, it shows up as a permanent leak.

To fix, make get_database list allocate its output data in the caller's
context, which is in charge of freeing it when appropriate; and the
memory leaked by heap_beginscan et al is allocated in a throwaway
transaction context.

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index e7176448a0e4f9929e5e0ada20126d93ff1ff56f..a617b882a34a286fbcee0c6526cc09fdb3bd9628 100644 (file)
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -1754,6 +1754,9 @@ autovac_balance_cost(void)
  * get_database_list
  *             Return a list of all databases found in pg_database.
  *
+ * The list and associated data is allocated in the caller's memory context,
+ * which is in charge of ensuring that it's properly cleaned up afterwards.
+ *
  * Note: this is the only function in which the autovacuum launcher uses a
  * transaction.  Although we aren't attached to any particular database and
  * therefore can't access most catalogs, we do have enough infrastructure
@@ -1766,6 +1769,10 @@ get_database_list(void)
        Relation        rel;
        HeapScanDesc scan;
        HeapTuple       tup;
+       MemoryContext   resultcxt;
+
+       /* This is the context that we will allocate our output data in */
+       resultcxt = CurrentMemoryContext;
 
        /*
         * Start a transaction so we can access pg_database, and get a snapshot.
@@ -1777,9 +1784,6 @@ get_database_list(void)
        StartTransactionCommand();
        (void) GetTransactionSnapshot();
 
-       /* Allocate our results in AutovacMemCxt, not transaction context */
-       MemoryContextSwitchTo(AutovacMemCxt);
-
        rel = heap_open(DatabaseRelationId, AccessShareLock);
        scan = heap_beginscan(rel, SnapshotNow, 0, NULL);
 
@@ -1787,6 +1791,15 @@ get_database_list(void)
        {
                Form_pg_database pgdatabase = (Form_pg_database) GETSTRUCT(tup);
                avw_dbase  *avdb;
+               MemoryContext   oldcxt;
+
+               /*
+                * Allocate our results in the caller's context, not the transaction's.
+                * We do this inside the loop, and restore the original context at the
+                * end, so that leaky things like heap_getnext() are not called in a
+                * potentially long-lived context.
+                */
+               oldcxt = MemoryContextSwitchTo(resultcxt);
 
                avdb = (avw_dbase *) palloc(sizeof(avw_dbase));
 
@@ -1797,6 +1810,7 @@ get_database_list(void)
                avdb->adw_entry = NULL;
 
                dblist = lappend(dblist, avdb);
+               MemoryContextSwitchTo(oldcxt);
        }
 
        heap_endscan(scan);