gstdio: make g_close() async-signal-safe under certain conditions

g_close() does something useful. It is not trivial to get EINTR handling of
close() right, in a portable manner. g_close() abstracts this.

We should allow glib users to use the function even in async-signal-safe
contexts, at least if the user heeds the caveat about GError and take care
not to fail assertions.

Backport 2.74: Modified to drop documentation changes to g_close() which
document its new async-signal-safe guarantees. They are not public
guarantees until 2.76. Also modified to include moving the code to
ignore `EINTR` from commit d5dc7d266f.

glib/gstdio.c

@@ -1749,8 +1749,9 @@ g_utime (const gchar    *filename,
  * @fd: A file descriptor
  * @error: a #GError
  *
- * This wraps the close() call; in case of error, %errno will be
+ * This wraps the close() call. In case of error, %errno will be
  * preserved, but the error will also be stored as a #GError in @error.
+ * In case of success, %errno is undefined.
  *
  * Besides using #GError, there is another major reason to prefer this
  * function over the call provided by the system; on Unix, it will
@@ -1766,24 +1767,38 @@ g_close (gint       fd,
          GError   **error)
 {
   int res;
+
+  /* Important: if @error is NULL, we must not do anything that is
+   * not async-signal-safe.
+   */
   res = close (fd);
+
+  if (res == -1)
+    {
+      int errsv = errno;
+
+      if (errsv == EINTR)
+        {
           /* Just ignore EINTR for now; a retry loop is the wrong thing to do
            * on Linux at least.  Anyone who wants to add a conditional check
            * for e.g. HP-UX is welcome to do so later...
            *
+           * https://lwn.net/Articles/576478/
            * http://lkml.indiana.edu/hypermail/linux/kernel/0509.1/0877.html
            * https://bugzilla.gnome.org/show_bug.cgi?id=682819
            * http://utcc.utoronto.ca/~cks/space/blog/unix/CloseEINTR
            * https://sites.google.com/site/michaelsafyan/software-engineering/checkforeintrwheninvokingclosethinkagain
            */
-  if (G_UNLIKELY (res == -1 && errno == EINTR))
           return TRUE;
-  else if (res == -1)
+        }
+
+      if (error)
         {
-      int errsv = errno;
           g_set_error_literal (error, G_FILE_ERROR,
                                g_file_error_from_errno (errsv),
                                g_strerror (errsv));
+        }
+
       errno = errsv;
       return FALSE;
     }