Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    Don't spuriously report FD_SETSIZE exhaustion on Windows.
    
    Starting on 2023-08-03, this intermittently terminated a "pgbench -C"
    test in CI.  It could affect a high-client-count "pgbench" without "-C".
    While parallel reindexdb and vacuumdb reach the same problematic check,
    sufficient client count and/or connection turnover is less plausible for
    them.  Given the lack of examples from the buildfarm or from manual
    builds, reproducing this must entail rare operating system
    configurations.  Also correct the associated error message, which was
    wrong for non-Windows and did not comply with the style guide.
    Back-patch to v12, where the pgbench check first appeared.  While v11
    vacuumdb has the problematic check, reaching it with typical vacuumdb
    usage is implausible.
    
    Reviewed by FIXME.
    
    Discussion: https://postgr.es/m/CA+hUKG+JwvTNdcyJTriy9BbtzF1veSRQ=9M_ZKFn9_LqE7Kp7Q@mail.gmail.com

diff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.c
index e3919395..a1f566c 100644
--- a/src/bin/pgbench/pgbench.c
+++ b/src/bin/pgbench/pgbench.c
@@ -7837,14 +7837,22 @@ clear_socket_set(socket_set *sa)
 static void
 add_socket_to_set(socket_set *sa, int fd, int idx)
 {
+	/* See connect_slot() for background on this code. */
+#ifdef WIN32
+	if (sa->fds.fd_count + 1 >= FD_SETSIZE)
+	{
+		pg_log_error("too many concurrent database clients for this platform: %d",
+					 sa->fds.fd_count + 1);
+		exit(1);
+	}
+#else
 	if (fd < 0 || fd >= FD_SETSIZE)
 	{
-		/*
-		 * Doing a hard exit here is a bit grotty, but it doesn't seem worth
-		 * complicating the API to make it less grotty.
-		 */
-		pg_fatal("too many client connections for select()");
+		pg_log_error("socket file descriptor out of range: %d", fd);
+		pg_log_error_hint("Try fewer concurrent database clients.");
+		exit(1);
 	}
+#endif
 	FD_SET(fd, &sa->fds);
 	if (fd > sa->maxfd)
 		sa->maxfd = fd;
diff --git a/src/fe_utils/parallel_slot.c b/src/fe_utils/parallel_slot.c
index aca238b..2ee4b6d 100644
--- a/src/fe_utils/parallel_slot.c
+++ b/src/fe_utils/parallel_slot.c
@@ -295,8 +295,40 @@ connect_slot(ParallelSlotArray *sa, int slotno, const char *dbname)
 	slot->connection = connectDatabase(sa->cparams, sa->progname, sa->echo, false, true);
 	sa->cparams->override_dbname = old_override;
 
-	if (PQsocket(slot->connection) >= FD_SETSIZE)
-		pg_fatal("too many jobs for this platform");
+	/*
+	 * POSIX defines FD_SETSIZE as the highest file descriptor acceptable to
+	 * FD_SET() and allied macros.  Windows defines it as a ceiling on the
+	 * count of file descriptors in the set, not a ceiling on the value of
+	 * each file descriptor; see
+	 * https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-select
+	 * and
+	 * https://learn.microsoft.com/en-us/windows/win32/api/winsock/ns-winsock-fd_set.
+	 * We can't ignore that, because Windows starts file descriptors at a
+	 * higher value, delays reuse, and skips values.  With less than ten
+	 * concurrent file descriptors, opened and closed rapidly, one can reach
+	 * file descriptor 1024.
+	 *
+	 * Doing a hard exit here is a bit grotty, but it doesn't seem worth
+	 * complicating the API to make it less grotty.
+	 */
+#ifdef WIN32
+	if (slotno >= FD_SETSIZE)
+	{
+		pg_log_error("too many jobs for this platform: %d", slotno);
+		exit(1);
+	}
+#else
+	{
+		int			fd = PQsocket(slot->connection);
+
+		if (fd >= FD_SETSIZE)
+		{
+			pg_log_error("socket file descriptor out of range: %d", fd);
+			pg_log_error_hint("Try fewer jobs.");
+			exit(1);
+		}
+	}
+#endif
 
 	/* Setup the connection using the supplied command, if any. */
 	if (sa->initcmd)