Improve libpq's handling of OOM during error message construction.

Commit ffa2e467 changed libpq so that multiple error reports
occurring during one operation (a connection attempt or query)
are accumulated in conn->errorMessage, where before new ones
usually replaced any prior error.  At least in theory, that makes
us more vulnerable to running out of memory for the errorMessage
buffer.  If it did happen, the user would be left with just an
empty-string error report, which is pretty unhelpful.

We can improve this by relying on pqexpbuffer.c's existing "broken
buffer" convention to track whether we've hit OOM for the current
operation's error string, and then substituting a constant "out of
memory" string in the small number of places where the errorMessage
is read out.

While at it, apply the same method to similar OOM cases in
pqInternalNotice and pqGetErrorNotice3.

Back-patch to v14 where ffa2e467 came in.  In principle this could
go back further; but in view of the lack of field reports, the
hazard seems negligible in older branches.

Discussion: https://postgr.es/m/530153.1627425648@sss.pgh.pa.us

src/interfaces/libpq/fe-connect.c

@@ -6743,6 +6743,14 @@ PQerrorMessage(const PGconn *conn)
 	if (!conn)
 		return libpq_gettext("connection pointer is NULL\n");
 
+	/*
+	 * The errorMessage buffer might be marked "broken" due to having
+	 * previously failed to allocate enough memory for the message.  In that
+	 * case, tell the application we ran out of memory.
+	 */
+	if (PQExpBufferBroken(&conn->errorMessage))
+		return libpq_gettext("out of memory\n");
+
 	return conn->errorMessage.data;
 }
 

src/interfaces/libpq/fe-exec.c

@@ -191,7 +191,7 @@ PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status)
 				/* non-error cases */
 				break;
 			default:
-				pqSetResultError(result, conn->errorMessage.data);
+				pqSetResultError(result, &conn->errorMessage);
 				break;
 		}
 
@@ -662,14 +662,28 @@ pqResultStrdup(PGresult *res, const char *str)
  *		assign a new error message to a PGresult
  */
 void
-pqSetResultError(PGresult *res, const char *msg)
+pqSetResultError(PGresult *res, PQExpBuffer errorMessage)
 {
+	char	   *msg;
+
 	if (!res)
 		return;
-	if (msg && *msg)
-		res->errMsg = pqResultStrdup(res, msg);
+
+	/*
+	 * We handle two OOM scenarios here.  The errorMessage buffer might be
+	 * marked "broken" due to having previously failed to allocate enough
+	 * memory for the message, or it might be fine but pqResultStrdup fails
+	 * and returns NULL.  In either case, just make res->errMsg point directly
+	 * at a constant "out of memory" string.
+	 */
+	if (!PQExpBufferBroken(errorMessage))
+		msg = pqResultStrdup(res, errorMessage->data);
+	else
+		msg = NULL;
+	if (msg)
+		res->errMsg = msg;
 	else
-		res->errMsg = NULL;
+		res->errMsg = libpq_gettext("out of memory\n");
 }
 
 /*
@@ -852,19 +866,19 @@ pqInternalNotice(const PGNoticeHooks *hooks, const char *fmt,...)
 	/* XXX should provide a SQLSTATE too? */
 
 	/*
-	 * Result text is always just the primary message + newline. If we can't
-	 * allocate it, don't bother invoking the receiver.
+	 * Result text is always just the primary message + newline.  If we can't
+	 * allocate it, substitute "out of memory", as in pqSetResultError.
 	 */
 	res->errMsg = (char *) pqResultAlloc(res, strlen(msgBuf) + 2, false);
 	if (res->errMsg)
-	{
 		sprintf(res->errMsg, "%s\n", msgBuf);
+	else
+		res->errMsg = libpq_gettext("out of memory\n");
 
-		/*
-		 * Pass to receiver, then free it.
-		 */
-		res->noticeHooks.noticeRec(res->noticeHooks.noticeRecArg, res);
-	}
+	/*
+	 * Pass to receiver, then free it.
+	 */
+	res->noticeHooks.noticeRec(res->noticeHooks.noticeRecArg, res);
 	PQclear(res);
 }
 
@@ -2122,7 +2136,7 @@ PQgetResult(PGconn *conn)
 				appendPQExpBuffer(&conn->errorMessage,
 								  libpq_gettext("PGEventProc \"%s\" failed during PGEVT_RESULTCREATE event\n"),
 								  res->events[i].name);
-				pqSetResultError(res, conn->errorMessage.data);
+				pqSetResultError(res, &conn->errorMessage);
 				res->resultStatus = PGRES_FATAL_ERROR;
 				break;
 			}

src/interfaces/libpq/fe-protocol3.c

@@ -967,12 +967,12 @@ pqGetErrorNotice3(PGconn *conn, bool isError)
 	if (isError)
 	{
 		if (res)
-			res->errMsg = pqResultStrdup(res, workBuf.data);
+			pqSetResultError(res, &workBuf);
 		pqClearAsyncResult(conn);	/* redundant, but be safe */
 		conn->result = res;
 		if (PQExpBufferDataBroken(workBuf))
 			appendPQExpBufferStr(&conn->errorMessage,
-								 libpq_gettext("out of memory"));
+								 libpq_gettext("out of memory\n"));
 		else
 			appendPQExpBufferStr(&conn->errorMessage, workBuf.data);
 	}
@@ -981,8 +981,15 @@ pqGetErrorNotice3(PGconn *conn, bool isError)
 		/* if we couldn't allocate the result set, just discard the NOTICE */
 		if (res)
 		{
-			/* We can cheat a little here and not copy the message. */
-			res->errMsg = workBuf.data;
+			/*
+			 * We can cheat a little here and not copy the message.  But if we
+			 * were unlucky enough to run out of memory while filling workBuf,
+			 * insert "out of memory", as in pqSetResultError.
+			 */
+			if (PQExpBufferDataBroken(workBuf))
+				res->errMsg = libpq_gettext("out of memory\n");
+			else
+				res->errMsg = workBuf.data;
 			if (res->noticeHooks.noticeRec != NULL)
 				res->noticeHooks.noticeRec(res->noticeHooks.noticeRecArg, res);
 			PQclear(res);

src/interfaces/libpq/libpq-int.h

@@ -642,7 +642,7 @@ extern pgthreadlock_t pg_g_threadlock;
 
 /* === in fe-exec.c === */
 
-extern void pqSetResultError(PGresult *res, const char *msg);
+extern void pqSetResultError(PGresult *res, PQExpBuffer errorMessage);
 extern void *pqResultAlloc(PGresult *res, size_t nBytes, bool isBinary);
 extern char *pqResultStrdup(PGresult *res, const char *str);
 extern void pqClearAsyncResult(PGconn *conn);