Fix "database is ready" race condition

Hi,

is there a good reason to print the "database system is ready" message
in StartupXLOG() in xact.c? It has a) nothing to do with xlog and b)
opens a small race condition: the message gets printed, while it still
take some CPU cycles until the postmaster really gets the SIGCHLD signal
and sets StartupPID = 0. If you (or rather: an automated test program)
try to connect within this timespan, you get a "database is starting up"
error, which clearly contradicts the "is ready" message.

I admit this is not a real issue in the common case and only matters in
automated testing or some such. But in case this does not break
anything...  (ereport is used in the reaper, so I guess it's fine to use
that in signal handlers). I'm not sure if the message is needed at all
in BS_XLOG_BOOTSTRAP mode. Probably it should better say something
different.

Patch attached.

Regards

Markus
============================================================
*** src/backend/access/transam/xlog.c    2191ee8ca338d74f666b4d3509cc4361c44b4353
--- src/backend/access/transam/xlog.c    e77a26a26ec46d4479563ed7ff5885ea9c21135a
*************** StartupXLOG(void)
*** 5168,5176 ****
      /* Reload shared-memory state for prepared transactions */
      RecoverPreparedTransactions();

-     ereport(LOG,
-             (errmsg("database system is ready")));
-
      /* Shut down readFile facility, free space */
      if (readFile >= 0)
      {
--- 5168,5173 ----
============================================================
*** src/backend/bootstrap/bootstrap.c    55fd17241f51b6f23131a0d36d5ce583aa7a3488
--- src/backend/bootstrap/bootstrap.c    8a54e88b06acad46c83320ca8fe13caa75ad77b9
*************** BootstrapMain(int argc, char *argv[])
*** 418,423 ****
--- 418,425 ----
              bootstrap_signals();
              BootStrapXLOG();
              StartupXLOG();
+             ereport(LOG,
+                     (errmsg("database system is ready")));
              break;

          case BS_XLOG_STARTUP:
============================================================
*** src/backend/postmaster/postmaster.c    561d13618e62e95a32b42b2e9305a638edacf24f
--- src/backend/postmaster/postmaster.c    5a567893b0ed78d312a19e7054127dc5b6b69df3
*************** reaper(SIGNAL_ARGS)
*** 2040,2045 ****
--- 2040,2048 ----
          if (StartupPID != 0 && pid == StartupPID)
          {
              StartupPID = 0;
+             ereport(LOG,
+                     (errmsg("database system is ready")));
+
              /* Note: FATAL exit of startup is treated as catastrophic */
              if (!EXIT_STATUS_0(exitstatus))
              {