Don't count background workers against a user's connection limit.

Doing so doesn't seem to be within the purpose of the per user
connection limits, and has particularly unfortunate effects in
conjunction with parallel queries.

Backpatch to 9.6 where parallel queries were introduced.

David Rowley, reviewed by Robert Haas and Albe Laurenz.

Author: adunstan

doc/src/sgml/ref/create_database.sgml

@@ -258,7 +258,8 @@ CREATE DATABASE <replaceable class="PARAMETER">name</replaceable>
    The <literal>CONNECTION LIMIT</> option is only enforced approximately;
    if two new sessions start at about the same time when just one
    connection <quote>slot</> remains for the database, it is possible that
-   both will fail.  Also, the limit is not enforced against superusers.
+   both will fail.  Also, the limit is not enforced against superusers or
+   background worker processes.
   </para>
  </refsect1>
 

doc/src/sgml/ref/create_role.sgml

@@ -198,7 +198,10 @@ CREATE ROLE <replaceable class="PARAMETER">name</replaceable> [ [ WITH ] <replac
       <listitem>
        <para>
         If role can log in, this specifies how many concurrent connections
-        the role can make.  -1 (the default) means no limit.
+        the role can make.  -1 (the default) means no limit. Note that only
+        normal connections are counted towards this limit. Neither prepared
+        transactions nor background worker connections are counted towards
+        this limit.
        </para>
       </listitem>
      </varlistentry>

src/backend/access/transam/twophase.c

@@ -420,6 +420,7 @@ MarkAsPreparing(TransactionId xid, const char *gid,
 	proc->backendId = InvalidBackendId;
 	proc->databaseId = databaseid;
 	proc->roleId = owner;
+	proc->isBackgroundWorker = false;
 	proc->lwWaiting = false;
 	proc->lwWaitMode = 0;
 	proc->waitLock = NULL;

src/backend/storage/ipc/procarray.c

@@ -2744,6 +2744,38 @@ CountDBBackends(Oid databaseid)
 	return count;
 }
 
+/*
+ * CountDBConnections --- counts database backends ignoring any background
+ *		worker processes
+ */
+int
+CountDBConnections(Oid databaseid)
+{
+	ProcArrayStruct *arrayP = procArray;
+	int			count = 0;
+	int			index;
+
+	LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+	for (index = 0; index < arrayP->numProcs; index++)
+	{
+		int			pgprocno = arrayP->pgprocnos[index];
+		volatile PGPROC *proc = &allProcs[pgprocno];
+
+		if (proc->pid == 0)
+			continue;			/* do not count prepared xacts */
+		if (proc->isBackgroundWorker)
+			continue;			/* do not count background workers */
+		if (!OidIsValid(databaseid) ||
+			proc->databaseId == databaseid)
+			count++;
+	}
+
+	LWLockRelease(ProcArrayLock);
+
+	return count;
+}
+
 /*
  * CancelDBBackends --- cancel backends that are using specified database
  */
@@ -2803,6 +2835,8 @@ CountUserBackends(Oid roleid)
 
 		if (proc->pid == 0)
 			continue;			/* do not count prepared xacts */
+		if (proc->isBackgroundWorker)
+			continue;			/* do not count background workers */
 		if (proc->roleId == roleid)
 			count++;
 	}

src/backend/storage/lmgr/proc.c

@@ -370,6 +370,7 @@ InitProcess(void)
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
 	MyProc->roleId = InvalidOid;
+	MyProc->isBackgroundWorker = IsBackgroundWorker;
 	MyPgXact->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;
 	/* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
@@ -542,6 +543,7 @@ InitAuxiliaryProcess(void)
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
 	MyProc->roleId = InvalidOid;
+	MyProc->isBackgroundWorker = IsBackgroundWorker;
 	MyPgXact->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;
 	MyProc->lwWaiting = false;

src/backend/utils/init/postinit.c

@@ -350,7 +350,7 @@ CheckMyDatabase(const char *name, bool am_superuser)
 		 */
 		if (dbform->datconnlimit >= 0 &&
 			!am_superuser &&
-			CountDBBackends(MyDatabaseId) > dbform->datconnlimit)
+			CountDBConnections(MyDatabaseId) > dbform->datconnlimit)
 			ereport(FATAL,
 					(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 					 errmsg("too many connections for database \"%s\"",

src/include/storage/proc.h

@@ -103,6 +103,8 @@ struct PGPROC
 	Oid			databaseId;		/* OID of database this backend is using */
 	Oid			roleId;			/* OID of role using this backend */
 
+	bool		isBackgroundWorker; /* true if background worker. */
+
 	/*
 	 * While in hot standby mode, shows that a conflict signal has been sent
 	 * for the current transaction. Set/cleared while holding ProcArrayLock,

src/include/storage/procarray.h

@@ -73,6 +73,7 @@ extern pid_t CancelVirtualTransaction(VirtualTransactionId vxid, ProcSignalReaso
 
 extern bool MinimumActiveBackends(int min);
 extern int	CountDBBackends(Oid databaseid);
+extern int	CountDBConnections(Oid databaseid);
 extern void CancelDBBackends(Oid databaseid, ProcSignalReason sigmode, bool conflictPending);
 extern int	CountUserBackends(Oid roleid);
 extern bool CountOtherDBBackends(Oid databaseId,