I recently started getting the "sorry, too many clients already" error.
There are currently four places that can generate it, but fortunately
log_error_verbosity was set to verbose so I was able to see that in this
case the warning was generated by proc.c:InitProcess().

But it's still not much, because there are three different lists you can
run out of and get the same error message from InitProcess(). I was
lucky to be able to rule out two of them. max_connections is set to much
more than the sum of possible connections from all relevant pgBouncer
instances we have, so it's not hitting max_connections. Also, this is on
9.4 and we don't use any funny extensions, so it's probably not running
out of bgworkerFreeProcs either. autovacFreeProcs is what's left but
this is still just a guess and I'd prefer to actually know.

I've made a hack for myself (attached diff against 9.4) which adds a
DETAIL-level message telling me which proc list was saturated. It's not
committable in its current form because of a C99 feature and perhaps for
other reasons.

By the way, I've also noticed that the InitProcess() can segfault upon
hitting set_spins_per_delay(procglobal->spins_per_delay). This only
happens when I run REL9_4_STABLE under gdb, set a breakpoint on
InitProcess, see an autovacuum launcher hit the breakpoint and tell gdb
to continue. "p procglobal->spins_per_delay" says "Cannot access memory
at address 0xf01b2f90". Maybe this means nothing.
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index e608198..613b36a 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -279,6 +279,14 @@ InitProcess(void)
 	/* use volatile pointer to prevent code rearrangement */
 	volatile PROC_HDR *procglobal = ProcGlobal;
+#define LIST_TYPE_ENTRY(e) [(e)] = (#e)
+	enum listType { autovac, bgworker, backend } which;
+	char const * const listTypeNames[] = {
+		LIST_TYPE_ENTRY(autovac),
+		LIST_TYPE_ENTRY(bgworker),
+		LIST_TYPE_ENTRY(backend)
+	};
 	 * ProcGlobal should be set up already (if we are a backend, we inherit
@@ -309,11 +317,11 @@ InitProcess(void)
 	if (IsAnyAutoVacuumProcess())
-		MyProc = procglobal->autovacFreeProcs;
+		which = autovac, MyProc = procglobal->autovacFreeProcs;
 	else if (IsBackgroundWorker)
-		MyProc = procglobal->bgworkerFreeProcs;
+		which = bgworker, MyProc = procglobal->bgworkerFreeProcs;
-		MyProc = procglobal->freeProcs;
+		which = backend, MyProc = procglobal->freeProcs;
 	if (MyProc != NULL)
@@ -330,13 +338,13 @@ InitProcess(void)
 		 * If we reach here, all the PGPROCs are in use.  This is one of the
 		 * possible places to detect "too many backends", so give the standard
-		 * error message.  XXX do we need to give a different failure message
-		 * in the autovacuum case?
+		 * error message.
-				 errmsg("sorry, too many clients already")));
+				 errmsg("sorry, too many clients already"),
+				 errdetail("%s proc list saturated", listTypeNames[which])));
 	MyPgXact = &ProcGlobal->allPgXact[MyProc->pgprocno];
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