unistd-safer: allow preservation of cloexec status via flag

If cloexec is in use, allow the ability to preserve cloexec
flag across *_safer functions.

* lib/unistd-safer.h (dup_safer_flag, fd_safer_flag): New
prototypes.
* lib/dup-safer.c (dup_safer_flag): New function.
* lib/fd-safer.c (fd_safer_flag): Likewise.
* modules/cloexec (configure.ac): Set witness.

Signed-off-by: Eric Blake <ebb9@byu.net>

1 files changed, 29 insertions, 0 deletions

diff --git a/lib/fd-safer.c b/lib/fd-safer.c
index fb9900180e..d2e1309c84 100644
--- a/lib/fd-safer.c
+++ b/lib/fd-safer.c
@@ -48,3 +48,32 @@ fd_safer (int fd)
 
   return fd;
 }
+
+#if GNULIB_CLOEXEC
+
+/* Return FD, unless FD would be a copy of standard input, output, or
+   error; in that case, return a duplicate of FD, closing FD.  If FLAG
+   contains O_CLOEXEC, the returned FD will have close-on-exec
+   semantics.  On failure to duplicate, close FD, set errno, and
+   return -1.  Preserve errno if FD is negative, so that the caller
+   can always inspect errno when the returned value is negative.
+
+   This function is usefully wrapped around functions that return file
+   descriptors, e.g., fd_safer_flag (open ("file", O_RDONLY | flag), flag).  */
+
+int
+fd_safer_flag (int fd, int flag)
+{
+  if (STDIN_FILENO <= fd && fd <= STDERR_FILENO)
+    {
+      int f = dup_safer_flag (fd, flag);
+      int e = errno;
+      close (fd);
+      errno = e;
+      fd = f;
+    }
+
+  return fd;
+}
+
+#endif /* GNULIB_CLOEXEC */