Teach RequestCheckpoint() to wait and retry a few times if it can't signal

the bgwriter immediately.  This covers the case where the bgwriter is still
starting up, as seen in a recent buildfarm failure.  In future it might also
assist with clean recovery after a bgwriter termination and restart ---
right now the postmaster treats early bgwriter exit as a system crash,
but that might not always be so.

diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c
index 7d25811afd473681736842ea41392a1faa32a9f1..03f8f7e30f26f2fceffbffb1093dd1fee236d3b6 100644 (file)
--- a/src/backend/postmaster/bgwriter.c
+++ b/src/backend/postmaster/bgwriter.c
@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.53 2008/10/14 08:06:39 heikki Exp $
+ *   $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.54 2008/11/23 01:40:19 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -864,6 +864,7 @@ RequestCheckpoint(int flags)
 {
    /* use volatile pointer to prevent code rearrangement */
    volatile BgWriterShmemStruct *bgs = BgWriterShmem;
+   int         ntries;
    int         old_failed,
                old_started;
 
@@ -905,15 +906,38 @@ RequestCheckpoint(int flags)
    SpinLockRelease(&bgs->ckpt_lck);
 
    /*
-    * Send signal to request checkpoint.  When not waiting, we consider
-    * failure to send the signal to be nonfatal.
+    * Send signal to request checkpoint.  It's possible that the bgwriter
+    * hasn't started yet, or is in process of restarting, so we will retry
+    * a few times if needed.  Also, if not told to wait for the checkpoint
+    * to occur, we consider failure to send the signal to be nonfatal and
+    * merely LOG it.
     */
-   if (BgWriterShmem->bgwriter_pid == 0)
-       elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
-            "could not request checkpoint because bgwriter not running");
-   if (kill(BgWriterShmem->bgwriter_pid, SIGINT) != 0)
-       elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
-            "could not signal for checkpoint: %m");
+   for (ntries = 0; ; ntries++)
+   {
+       if (BgWriterShmem->bgwriter_pid == 0)
+       {
+           if (ntries >= 20)       /* max wait 2.0 sec */
+           {
+               elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
+                    "could not request checkpoint because bgwriter not running");
+               break;
+           }
+       }
+       else if (kill(BgWriterShmem->bgwriter_pid, SIGINT) != 0)
+       {
+           if (ntries >= 20)       /* max wait 2.0 sec */
+           {
+               elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
+                    "could not signal for checkpoint: %m");
+               break;
+           }
+       }
+       else
+           break;              /* signal sent successfully */
+
+       CHECK_FOR_INTERRUPTS();
+       pg_usleep(100000L);     /* wait 0.1 sec, then retry */
+   }
 
    /*
     * If requested, wait for completion.  We detect completion according to