Clarify the error message when unavailable functions are called (RHBZ#679737).

Callers are supposed to use the availability API to check for
functions that may not be available in particular builds of
libguestfs.  If they don't do this, currently they tend to get obscure
error messages, eg:

  libguestfs: error: zerofree: /dev/vda1: zerofree: No such file or directory

This commit changes the error message to explain what callers ought to
be doing instead:

  libguestfs: error: zerofree: feature 'zerofree' is not available in this
  build of libguestfs.  Read 'AVAILABILITY' in the guestfs(3) man page for
  how to check for the availability of features.

This patch makes the stubs check for availability.  The stub code
changes to:

  static void
  zerofree_stub (XDR *xdr_in)
  {
  [...]

    /* The caller should have checked before calling this. */
    if (! optgroup_zerofree_available ()) {
      reply_with_error ("feature '%s' is not available in this\n"
                        "build of libguestfs.  Read 'AVAILABILITY' in the guestfs(3) man page for\n"
                        "how to check for the availability of features.",
                        "zerofree");
      goto done;
    }
  [...]

1 files changed, 50 insertions, 31 deletions

diff --git a/daemon/inotify.c b/daemon/inotify.c
index add1f146..df6b2e8d 100644
--- a/daemon/inotify.c
+++ b/daemon/inotify.c
@@ -56,13 +56,6 @@ optgroup_inotify_available (void)
 {
   return 1;
 }
-#else /* !HAVE_SYS_INOTIFY_H */
-int
-optgroup_inotify_available (void)
-{
-  return 0;
-}
-#endif
 
 /* Because inotify_init does NEED_ROOT, NEED_INOTIFY implies NEED_ROOT. */
 #define NEED_INOTIFY(errcode)						\
@@ -78,7 +71,6 @@ optgroup_inotify_available (void)
 int
 do_inotify_init (int max_events)
 {
-#ifdef HAVE_SYS_INOTIFY_H
   FILE *fp;
 
   NEED_ROOT (, return -1);
@@ -129,15 +121,11 @@ do_inotify_init (int max_events)
 #endif
 
   return 0;
-#else
-  NOT_AVAILABLE (-1);
-#endif
 }
 
 int
 do_inotify_close (void)
 {
-#ifdef HAVE_SYS_INOTIFY_H
   NEED_INOTIFY (-1);
 
   if (inotify_fd == -1) {
@@ -154,15 +142,11 @@ do_inotify_close (void)
   inotify_posn = 0;
 
   return 0;
-#else
-  NOT_AVAILABLE (-1);
-#endif
 }
 
 int64_t
 do_inotify_add_watch (const char *path, int mask)
 {
-#ifdef HAVE_SYS_INOTIFY_H
   int64_t r;
   char *buf;
 
@@ -182,15 +166,11 @@ do_inotify_add_watch (const char *path, int mask)
   }
 
   return r;
-#else
-  NOT_AVAILABLE (-1);
-#endif
 }
 
 int
 do_inotify_rm_watch (int wd)
 {
-#ifdef HAVE_SYS_INOTIFY_H
   NEED_INOTIFY (-1);
 
   if (inotify_rm_watch (inotify_fd, wd) == -1) {
@@ -199,15 +179,11 @@ do_inotify_rm_watch (int wd)
   }
 
   return 0;
-#else
-  NOT_AVAILABLE (-1);
-#endif
 }
 
 guestfs_int_inotify_event_list *
 do_inotify_read (void)
 {
-#ifdef HAVE_SYS_INOTIFY_H
   int space;
   guestfs_int_inotify_event_list *ret;
 
@@ -313,15 +289,11 @@ do_inotify_read (void)
   xdr_free ((xdrproc_t) xdr_guestfs_int_inotify_event_list, (char *) ret);
   free (ret);
   return NULL;
-#else
-  NOT_AVAILABLE (NULL);
-#endif
 }
 
 char **
 do_inotify_files (void)
 {
-#ifdef HAVE_SYS_INOTIFY_H
   char **ret = NULL;
   int size = 0, alloc = 0;
   unsigned int i;
@@ -404,7 +376,54 @@ do_inotify_files (void)
 
   unlink (tempfile);
   return NULL;
-#else
-  NOT_AVAILABLE (NULL);
-#endif
 }
+
+#else /* !HAVE_SYS_INOTIFY_H */
+
+/* Note that the wrapper code (daemon/stubs.c) ensures that the
+ * functions below are never called because optgroup_inotify_available
+ * returns false.
+ */
+int
+optgroup_inotify_available (void)
+{
+  return 0;
+}
+
+int
+do_inotify_init (int max_events)
+{
+  abort ();
+}
+
+int
+do_inotify_close (void)
+{
+  abort ();
+}
+
+int64_t
+do_inotify_add_watch (const char *path, int mask)
+{
+  abort ();
+}
+
+int
+do_inotify_rm_watch (int wd)
+{
+  abort ();
+}
+
+guestfs_int_inotify_event_list *
+do_inotify_read (void)
+{
+  abort ();
+}
+
+char **
+do_inotify_files (void)
+{
+  abort ();
+}
+
+#endif