Fix longstanding problems in VACUUM caused by untimely interruptions

In VACUUM FULL, an interrupt after the initial transaction has been recorded
as committed can cause postmaster to restart with the following error message:
PANIC: cannot abort transaction NNNN, it was already committed
This problem has been reported many times.

In lazy VACUUM, an interrupt after the table has been truncated by
lazy_truncate_heap causes other backends' relcache to still point to the
removed pages; this can cause future INSERT and UPDATE queries to error out
with the following error message:
could not read block XX of relation 1663/NNN/MMMM: read only 0 of 8192 bytes
The window to this race condition is extremely narrow, but it has been seen in
the wild involving a cancelled autovacuum process.

The solution for both problems is to inhibit interrupts in both operations
until after the respective transactions have been committed.  It's not a
complete solution, because the transaction could theoretically be aborted by
some other error, but at least fixes the most common causes of both problems.

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index e3ec8772ce2291ad655819223936591a59dad749..b83299172dddc6e90335899437dd9b5616219a25 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.364.2.2 2008/09/11 14:01:35 alvherre Exp $
+ *   $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.364.2.3 2009/11/10 18:00:44 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -210,10 +210,10 @@ static List *get_rel_oids(List *relids, const RangeVar *vacrel,
 static void vac_truncate_clog(TransactionId frozenXID);
 static void vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
                       bool for_wraparound);
-static void full_vacuum_rel(Relation onerel, VacuumStmt *vacstmt);
+static bool full_vacuum_rel(Relation onerel, VacuumStmt *vacstmt);
 static void scan_heap(VRelStats *vacrelstats, Relation onerel,
          VacPageList vacuum_pages, VacPageList fraged_pages);
-static void repair_frag(VRelStats *vacrelstats, Relation onerel,
+static bool repair_frag(VRelStats *vacrelstats, Relation onerel,
            VacPageList vacuum_pages, VacPageList fraged_pages,
            int nindexes, Relation *Irel);
 static void move_chain_tuple(Relation rel,
@@ -976,6 +976,7 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
    Oid         toast_relid;
    Oid         save_userid;
    bool        save_secdefcxt;
+   bool        heldoff;
 
    /* Begin a transaction for vacuuming this relation */
    StartTransactionCommand();
@@ -1121,9 +1122,9 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
     * Do the actual work --- either FULL or "lazy" vacuum
     */
    if (vacstmt->full)
-       full_vacuum_rel(onerel, vacstmt);
+       heldoff = full_vacuum_rel(onerel, vacstmt);
    else
-       lazy_vacuum_rel(onerel, vacstmt, vac_strategy);
+       heldoff = lazy_vacuum_rel(onerel, vacstmt, vac_strategy);
 
    /* Restore userid */
    SetUserIdAndContext(save_userid, save_secdefcxt);
@@ -1136,6 +1137,10 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
     */
    CommitTransactionCommand();
 
+   /* now we can allow interrupts again, if disabled */
+   if (heldoff)
+       RESUME_INTERRUPTS();
+
    /*
     * If the relation has a secondary toast rel, vacuum that too while we
     * still hold the session lock on the master table.  Note however that
@@ -1169,8 +1174,11 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
  *
  *     At entry, we have already established a transaction and opened
  *     and locked the relation.
+ *
+ *     The return value indicates whether this function has held off
+ *     interrupts -- caller must RESUME_INTERRUPTS() after commit if true.
  */
-static void
+static bool
 full_vacuum_rel(Relation onerel, VacuumStmt *vacstmt)
 {
    VacPageListData vacuum_pages;       /* List of pages to vacuum and/or
@@ -1181,6 +1189,7 @@ full_vacuum_rel(Relation onerel, VacuumStmt *vacstmt)
    int         nindexes,
                i;
    VRelStats  *vacrelstats;
+   bool        heldoff = false;
 
    vacuum_set_xid_limits(vacstmt->freeze_min_age, onerel->rd_rel->relisshared,
                          &OldestXmin, &FreezeLimit);
@@ -1231,7 +1240,7 @@ full_vacuum_rel(Relation onerel, VacuumStmt *vacstmt)
    if (fraged_pages.num_pages > 0)
    {
        /* Try to shrink heap */
-       repair_frag(vacrelstats, onerel, &vacuum_pages, &fraged_pages,
+       heldoff = repair_frag(vacrelstats, onerel, &vacuum_pages, &fraged_pages,
                    nindexes, Irel);
        vac_close_indexes(nindexes, Irel, NoLock);
    }
@@ -1256,6 +1265,8 @@ full_vacuum_rel(Relation onerel, VacuumStmt *vacstmt)
    /* report results to the stats collector, too */
    pgstat_report_vacuum(RelationGetRelid(onerel), onerel->rd_rel->relisshared,
                         vacstmt->analyze, vacrelstats->rel_tuples);
+
+   return heldoff;
 }
 
 
@@ -1805,8 +1816,11 @@ scan_heap(VRelStats *vacrelstats, Relation onerel,
  *     for them after committing (in hack-manner - without losing locks
  *     and freeing memory!) current transaction. It truncates relation
  *     if some end-blocks are gone away.
+ *
+ *     The return value indicates whether this function has held off
+ *     interrupts -- caller must RESUME_INTERRUPTS() after commit if true.
  */
-static void
+static bool
 repair_frag(VRelStats *vacrelstats, Relation onerel,
            VacPageList vacuum_pages, VacPageList fraged_pages,
            int nindexes, Relation *Irel)
@@ -1831,6 +1845,7 @@ repair_frag(VRelStats *vacrelstats, Relation onerel,
    int         keep_tuples = 0;
    int         keep_indexed_tuples = 0;
    PGRUsage    ru0;
+   bool        heldoff = false;
 
    pg_rusage_init(&ru0);
 
@@ -2635,8 +2650,12 @@ repair_frag(VRelStats *vacrelstats, Relation onerel,
         *
         * XXX This desperately needs to be revisited.  Any failure after this
         * point will result in a PANIC "cannot abort transaction nnn, it was
-        * already committed"!
+        * already committed"!  As a precaution, we prevent cancel interrupts
+        * after this point to mitigate this problem; caller is responsible for
+        * re-enabling them after committing the transaction.
         */
+       HOLD_INTERRUPTS();
+       heldoff = true;
        ForceSyncCommit();
        (void) RecordTransactionCommit();
    }
@@ -2832,6 +2851,8 @@ repair_frag(VRelStats *vacrelstats, Relation onerel,
        pfree(vacrelstats->vtlinks);
 
    ExecContext_Finish(&ec);
+
+   return heldoff;
 }
 
 /*

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index d9224dc28e5a751f22b1ac9c9fca22ecc965550b..574a14ad814672c400f8dcac1f2f317d38198e92 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -38,7 +38,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.103.2.2 2009/01/06 14:55:44 heikki Exp $
+ *   $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.103.2.3 2009/11/10 18:00:44 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -147,8 +147,11 @@ static int vac_cmp_page_spaces(const void *left, const void *right);
  *
  *     At entry, we have already established a transaction and opened
  *     and locked the relation.
+ *
+ *     The return value indicates whether this function has held off
+ *     interrupts -- caller must RESUME_INTERRUPTS() after commit if true.
  */
-void
+bool
 lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
                BufferAccessStrategy bstrategy)
 {
@@ -158,6 +161,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
    BlockNumber possibly_freeable;
    PGRUsage    ru0;
    TimestampTz starttime = 0;
+   bool        heldoff = false;
 
    pg_rusage_init(&ru0);
 
@@ -198,12 +202,22 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
     *
     * Don't even think about it unless we have a shot at releasing a goodly
     * number of pages.  Otherwise, the time taken isn't worth it.
+    *
+    * Note that after we've truncated the heap, it's too late to abort the
+    * transaction; doing so would lose the sinval messages needed to tell
+    * the other backends about the table being shrunk.  We prevent interrupts
+    * in that case; caller is responsible for re-enabling them after
+    * committing the transaction.
     */
    possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
    if (possibly_freeable > 0 &&
        (possibly_freeable >= REL_TRUNCATE_MINIMUM ||
         possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION))
+   {
+       HOLD_INTERRUPTS();
+       heldoff = true;
        lazy_truncate_heap(onerel, vacrelstats);
+   }
 
    /* Update shared free space map with final free space info */
    lazy_update_fsm(onerel, vacrelstats);
@@ -248,6 +262,8 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
                        vacrelstats->tuples_deleted, vacrelstats->rel_tuples,
                            pg_rusage_show(&ru0))));
    }
+
+   return heldoff;
 }
 
 

diff --git a/src/include/commands/vacuum.h b/src/include/commands/vacuum.h
index 4beb1f8c785d7d9fca8081fb26c41002977698df..f263c81a00888ed5ce85fd8a550ee7928c804fc5 100644 (file)
--- a/src/include/commands/vacuum.h
+++ b/src/include/commands/vacuum.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/commands/vacuum.h,v 1.75.2.1 2008/03/14 17:26:01 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/commands/vacuum.h,v 1.75.2.2 2009/11/10 18:00:44 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -131,7 +131,7 @@ extern bool vac_is_partial_index(Relation indrel);
 extern void vacuum_delay_point(void);
 
 /* in commands/vacuumlazy.c */
-extern void lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
+extern bool lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
                BufferAccessStrategy bstrategy);
 
 /* in commands/analyze.c */