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.

8 files changed, 47 insertions, 3 deletions

diff --git a/doc/src/sgml/ref/create_database.sgml b/doc/src/sgml/ref/create_database.sgml
index cf33746c1e2..cf0d53b3014 100644
--- a/doc/src/sgml/ref/create_database.sgml
+++ b/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>
 
diff --git a/doc/src/sgml/ref/create_role.sgml b/doc/src/sgml/ref/create_role.sgml
index a3b8ed9ab4c..2ae576ede69 100644
--- a/doc/src/sgml/ref/create_role.sgml
+++ b/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>
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 5415604993a..ea7bfa0578b 100644
--- a/src/backend/access/transam/twophase.c
+++ b/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;
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 4adb286d5b5..651c561dfd5 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2752,6 +2752,38 @@ CountDBBackends(Oid databaseid)
 }
 
 /*
+ * 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
  */
 void
@@ -2810,6 +2842,8 @@ 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++;
 	}
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 9a758bd9160..cb6e1e3e5bc 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -372,6 +372,7 @@ 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 */
@@ -544,6 +545,7 @@ InitAuxiliaryProcess(void)
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
 	MyProc->roleId = InvalidOid;
+	MyProc->isBackgroundWorker = IsBackgroundWorker;
 	MyPgXact->delayChkpt = false;
 	MyPgXact->vacuumFlags = 0;
 	MyProc->lwWaiting = false;
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 824d5abf11f..cde4a798b62 100644
--- a/src/backend/utils/init/postinit.c
+++ b/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\"",
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 775c66a1971..c44d82b90c0 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -102,6 +102,8 @@ 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,
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index dd37c0cb070..ecc3c4a4f63 100644
--- a/src/include/storage/procarray.h
+++ b/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,