miext: Move SyncShm FDs out of the way of clients

Applications may end up allocating a bunch of shmfence objects, each
of which uses a file descriptor, which must be kept open lest some
other client ask for a copy of it later on.

Lacking an API that can turn a memory mapping back into a file
descriptor, about the best we can do is push the file descriptors out
of the way of other X clients so that we don't run out of the ability
to accept new connections.

This uses fcntl F_GETFD to push the FD up above MAXCLIENTS.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/os/utils.c b/os/utils.c
index fb20da755..608ee6ab0 100644
--- a/os/utils.c
+++ b/os/utils.c
@@ -2071,3 +2071,27 @@ FormatUInt64Hex(uint64_t num, char *string)
 
     string[len] = '\0';
 }
+
+/* Move a file descriptor out of the way of our select mask; this
+ * is useful for file descriptors which will never appear in the
+ * select mask to avoid reducing the number of clients that can
+ * connect to the server
+ */
+int
+os_move_fd(int fd)
+{
+    int newfd;
+
+#ifdef F_DUPFD_CLOEXEC
+    newfd = fcntl(fd, F_DUPFD_CLOEXEC, MAXCLIENTS);
+#else
+    newfd = fcntl(fd, F_DUPFD, MAXCLIENTS);
+#endif
+    if (newfd < 0)
+        return fd;
+#ifndef F_DUPFD_CLOEXEC
+    fcntl(newfd, F_SETFD, FD_CLOEXEC);
+#endif
+    close(fd);
+    return newfd;
+}