Fix error handling of vacuumdb and reindexdb when running out of fds

When trying to use a high number of jobs, vacuumdb (and more recently
reindexdb) 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/reindexdb.c

@@ -153,12 +153,6 @@ main(int argc, char *argv[])
 					pg_log_error("number of parallel jobs must be at least 1");
 					exit(1);
 				}
-				if (concurrentCons > ParallelSlotsMax())
-				{
-					pg_log_error("too many parallel jobs requested (maximum: %d)",
-								 ParallelSlotsMax());
-					exit(1);
-				}
 				break;
 			case 'v':
 				verbose = true;

src/bin/scripts/scripts_parallel.c

@@ -94,20 +94,6 @@ select_loop(int maxFd, fd_set *workerset, bool *aborting)
 	return i;
 }
 
-/*
- * ParallelSlotsMax
- *		Returns the maximum number of parallel slots supported.
- *
- * Note that this is included here as FD_SETSIZE is declared in sys/select.h
- * per POSIX.
- */
-int
-ParallelSlotsMax(void)
-{
-	/* leave some room for pre-existing fds */
-	return FD_SETSIZE - 10;
-}
-
 /*
  * ParallelSlotsGetIdle
  *		Return a connection slot that is ready to execute a command.
@@ -246,6 +232,18 @@ ParallelSlotsSetup(const char *dbname, const char *host, const char *port,
 		{
 			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)
+			{
+				pg_log_fatal("too many jobs for this platform -- try %d", i);
+				exit(1);
+			}
+
 			init_slot(slots + i, conn);
 		}
 	}

src/bin/scripts/scripts_parallel.h

@@ -21,8 +21,6 @@ typedef struct ParallelSlot
 	bool		isFree;			/* Is it known to be idle? */
 } ParallelSlot;
 
-extern int	ParallelSlotsMax(void);
-
 extern ParallelSlot *ParallelSlotsGetIdle(ParallelSlot *slots, int numslots);
 
 extern ParallelSlot *ParallelSlotsSetup(const char *dbname, const char *host,

src/bin/scripts/vacuumdb.c

@@ -181,12 +181,6 @@ main(int argc, char *argv[])
 					pg_log_error("number of parallel jobs must be at least 1");
 					exit(1);
 				}
-				if (concurrentCons > ParallelSlotsMax())
-				{
-					pg_log_error("too many parallel jobs requested (maximum: %d)",
-								 ParallelSlotsMax());
-					exit(1);
-				}
 				break;
 			case 2:
 				maintenance_db = pg_strdup(optarg);