From eec462367ee2b41e02c6e29135c857ad6f2da66a Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 26 Aug 2019 11:14:33 +0900
Subject: [PATCH] Fix error handling of vacuumdb when running out of fds
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When trying to use a high number of jobs, vacuumdb has only checked for
a maximum number of jobs used, causing confusing failures when running
out of file descriptors when the jobs open connections to Postgres.
This commit changes the error handling so as we do not check anymore for
a maximum number of allowed jobs when parsing the option value with
FD_SETSIZE, but check instead if a file descriptor is within the
supported range when opening the connections for the jobs so as this is
detected at the earliest time possible.

Also, improve the error message to give a hint about the number of jobs
recommended, using a wording given by the reviewers of the patch.

Reported-by: Andres Freund
Author: Michael Paquier
Reviewed-by: Andres Freund, Álvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/20190818001858.ho3ev4z57fqhs7a5@alap3.anarazel.de
Backpatch-through: 9.5
---
 src/bin/scripts/vacuumdb.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/bin/scripts/vacuumdb.c b/src/bin/scripts/vacuumdb.c
index f888bf73bc..4ac765244a 100644
--- a/src/bin/scripts/vacuumdb.c
+++ b/src/bin/scripts/vacuumdb.c
@@ -200,12 +200,6 @@ main(int argc, char *argv[])
 							progname);
 					exit(1);
 				}
-				if (concurrentCons > FD_SETSIZE - 1)
-				{
-					fprintf(stderr, _("%s: too many parallel jobs requested (maximum: %d)\n"),
-							progname, FD_SETSIZE - 1);
-					exit(1);
-				}
 				break;
 			case 2:
 				maintenance_db = pg_strdup(optarg);
@@ -443,6 +437,20 @@ vacuum_one_database(const char *dbname, vacuumingOptions *vacopts,
 		{
 			conn = connectDatabase(dbname, host, port, username, prompt_password,
 								   progname, echo, false, true);
+
+			/*
+			 * Fail and exit immediately if trying to use a socket in an
+			 * unsupported range.  POSIX requires open(2) to use the lowest
+			 * unused file descriptor and the hint given relies on that.
+			 */
+			if (PQsocket(conn) >= FD_SETSIZE)
+			{
+				fprintf(stderr,
+						_("%s: too many jobs for this platform -- try %d"),
+						progname, i);
+				exit(1);
+			}
+
 			init_slot(slots + i, conn);
 		}
 	}
-- 
2.23.0