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, 14 insertions, 9 deletions

diff --git a/daemon/xattr.c b/daemon/xattr.c
index 24457488..92d0cf18 100644
--- a/daemon/xattr.c
+++ b/daemon/xattr.c
@@ -511,6 +511,11 @@ do_lgetxattr (const char *path, const char *name, size_t *size_r)
 }
 
 #else /* no xattr.h */
+
+/* Note that the wrapper code (daemon/stubs.c) ensures that the
+ * functions below are never called because
+ * optgroup_linuxxattrs_available returns false.
+ */
 int
 optgroup_linuxxattrs_available (void)
 {
@@ -520,55 +525,55 @@ optgroup_linuxxattrs_available (void)
 guestfs_int_xattr_list *
 do_getxattrs (const char *path)
 {
-  NOT_AVAILABLE (NULL);
+  abort ();
 }
 
 guestfs_int_xattr_list *
 do_lgetxattrs (const char *path)
 {
-  NOT_AVAILABLE (NULL);
+  abort ();
 }
 
 int
 do_setxattr (const char *xattr, const char *val, int vallen, const char *path)
 {
-  NOT_AVAILABLE (-1);
+  abort ();
 }
 
 int
 do_lsetxattr (const char *xattr, const char *val, int vallen, const char *path)
 {
-  NOT_AVAILABLE (-1);
+  abort ();
 }
 
 int
 do_removexattr (const char *xattr, const char *path)
 {
-  NOT_AVAILABLE (-1);
+  abort ();
 }
 
 int
 do_lremovexattr (const char *xattr, const char *path)
 {
-  NOT_AVAILABLE (-1);
+  abort ();
 }
 
 guestfs_int_xattr_list *
 do_lxattrlist (const char *path, char *const *names)
 {
-  NOT_AVAILABLE (NULL);
+  abort ();
 }
 
 char *
 do_getxattr (const char *path, const char *name, size_t *size_r)
 {
-  NOT_AVAILABLE (NULL);
+  abort ();
 }
 
 char *
 do_lgetxattr (const char *path, const char *name, size_t *size_r)
 {
-  NOT_AVAILABLE (NULL);
+  abort ();
 }
 
 #endif /* no xattr.h */