Improve client error messages for immediate-stop situations.

Up to now, if the DBA issued "pg_ctl stop -m immediate", the message
sent to clients was the same as for a crash-and-restart situation.
This is confusing, not least because the message claims that the
database will soon be up again, something we have no business
predicting.

Improve things so that we can generate distinct messages for the two
cases (and also recognize an ad-hoc SIGQUIT, should somebody try that).
To do that, add a field to pmsignal.c's shared memory data structure
that the postmaster sets just before broadcasting SIGQUIT to its
children.  No interlocking seems to be necessary; the intervening
signal-sending and signal-receipt should sufficiently serialize accesses
to the field.  Hence, this isn't any riskier than the existing usages
of pmsignal.c.

We might in future extend this idea to improve other
postmaster-to-children signal scenarios, although none of them
currently seem to be as badly overloaded as SIGQUIT.

Discussion: https://postgr.es/m/559291.1608587013@sss.pgh.pa.us

1 files changed, 13 insertions, 1 deletions

diff --git a/src/include/storage/pmsignal.h b/src/include/storage/pmsignal.h
index 56c5ec44814..b6aa158160a 100644
--- a/src/include/storage/pmsignal.h
+++ b/src/include/storage/pmsignal.h
@@ -1,7 +1,7 @@
 /*-------------------------------------------------------------------------
  *
  * pmsignal.h
- *	  routines for signaling the postmaster from its child processes
+ *	  routines for signaling between the postmaster and its child processes
  *
  *
  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
@@ -45,6 +45,16 @@ typedef enum
 	NUM_PMSIGNALS				/* Must be last value of enum! */
 } PMSignalReason;
 
+/*
+ * Reasons why the postmaster would send SIGQUIT to its children.
+ */
+typedef enum
+{
+	PMQUIT_NOT_SENT = 0,		/* postmaster hasn't sent SIGQUIT */
+	PMQUIT_FOR_CRASH,			/* some other backend bought the farm */
+	PMQUIT_FOR_STOP				/* immediate stop was commanded */
+} QuitSignalReason;
+
 /* PMSignalData is an opaque struct, details known only within pmsignal.c */
 typedef struct PMSignalData PMSignalData;
 
@@ -55,6 +65,8 @@ extern Size PMSignalShmemSize(void);
 extern void PMSignalShmemInit(void);
 extern void SendPostmasterSignal(PMSignalReason reason);
 extern bool CheckPostmasterSignal(PMSignalReason reason);
+extern void SetQuitSignalReason(QuitSignalReason reason);
+extern QuitSignalReason GetQuitSignalReason(void);
 extern int	AssignPostmasterChildSlot(void);
 extern bool ReleasePostmasterChildSlot(int slot);
 extern bool IsPostmasterChildWalSender(int slot);