bug#60311: json-available-p: make dynamically correct for Windows

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

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The implementation of `json-available-p`,

  (and (fboundp 'json-serialize)
       (condition-case nil
           (json-serialize t)
         (:success t)
         (json-unavailable nil))))

probably isn't quite right on Windows: `json-serialize` is pure so it will be called at compile time and the result, "true", used in the code (actually not even that since the result is never used). Thus, if libjansson could not be loaded during actual Emacs use (as opposed to when Emacs was built), this would never be detected and json-available-p would still return t.

The patch below should take care of this. Would someone please help testing it on Windows?


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diff --git a/lisp/subr.el b/lisp/subr.el
index fff4c88ccf..ce5ac14f5f 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -6905,11 +6905,11 @@ internal--format-docstring-line
 
 (defun json-available-p ()
   "Return non-nil if Emacs has libjansson support."
-  (and (fboundp 'json-serialize)
-       (condition-case nil
-           (json-serialize t)
-         (:success t)
-         (json-unavailable nil))))
+  (declare (side-effect-free error-free))
+  (and (eval-when-compile (fboundp 'json-serialize))
+       ;; If `json--available-p' is present, we need to call it at run-time.
+       (or (not (eval-when-compile (fboundp 'json--available-p)))
+           (json--available-p))))
 
 (defun ensure-list (object)
   "Return OBJECT as a list.
diff --git a/src/json.c b/src/json.c
index cdcc11358e..d2105bc27b 100644
--- a/src/json.c
+++ b/src/json.c
@@ -555,6 +555,38 @@ json_parse_args (ptrdiff_t nargs,
   }
 }
 
+#ifdef WINDOWSNT
+static bool
+json_available_p (void)
+{
+  if (json_initialized)
+    return true;
+  json_initialized = init_json_functions ();
+  Lisp_Object status = json_initialized ? Qt : Qnil;
+  Vlibrary_cache = Fcons (Fcons (Qjson, status), Vlibrary_cache);
+  return json_initialized;
+}
+#endif
+
+static void
+ensure_json_available (void)
+{
+#ifdef WINDOWSNT
+  if (!json_available_p ())
+    Fsignal (Qjson_unavailable,
+	     list1 (build_unibyte_string ("jansson library not found")));
+#endif
+}
+
+#ifdef WINDOWSNT
+DEFUN ("json--available-p", Fjson__available_p, Sjson__available_p, 0, 0, NULL,
+       doc: /* Whether libjansson is available (internal).  */)
+  (void)
+{
+  return json_available_p () ? Qt : Qnil;
+}
+#endif
+
 DEFUN ("json-serialize", Fjson_serialize, Sjson_serialize, 1, MANY,
        NULL,
        doc: /* Return the JSON representation of OBJECT as a string.
@@ -585,19 +617,7 @@ DEFUN ("json-serialize", Fjson_serialize, Sjson_serialize, 1, MANY,
      (ptrdiff_t nargs, Lisp_Object *args)
 {
   specpdl_ref count = SPECPDL_INDEX ();
-
-#ifdef WINDOWSNT
-  if (!json_initialized)
-    {
-      Lisp_Object status;
-      json_initialized = init_json_functions ();
-      status = json_initialized ? Qt : Qnil;
-      Vlibrary_cache = Fcons (Fcons (Qjson, status), Vlibrary_cache);
-    }
-  if (!json_initialized)
-    Fsignal (Qjson_unavailable,
-	     list1 (build_unibyte_string ("jansson library not found")));
-#endif
+  ensure_json_available ();
 
   struct json_configuration conf =
     {json_object_hashtable, json_array_array, QCnull, QCfalse};
@@ -694,19 +714,7 @@ DEFUN ("json-insert", Fjson_insert, Sjson_insert, 1, MANY,
      (ptrdiff_t nargs, Lisp_Object *args)
 {
   specpdl_ref count = SPECPDL_INDEX ();
-
-#ifdef WINDOWSNT
-  if (!json_initialized)
-    {
-      Lisp_Object status;
-      json_initialized = init_json_functions ();
-      status = json_initialized ? Qt : Qnil;
-      Vlibrary_cache = Fcons (Fcons (Qjson, status), Vlibrary_cache);
-    }
-  if (!json_initialized)
-    Fsignal (Qjson_unavailable,
-	     list1 (build_unibyte_string ("jansson library not found")));
-#endif
+  ensure_json_available ();
 
   struct json_configuration conf =
     {json_object_hashtable, json_array_array, QCnull, QCfalse};
@@ -951,19 +959,7 @@ DEFUN ("json-parse-string", Fjson_parse_string, Sjson_parse_string, 1, MANY,
   (ptrdiff_t nargs, Lisp_Object *args)
 {
   specpdl_ref count = SPECPDL_INDEX ();
-
-#ifdef WINDOWSNT
-  if (!json_initialized)
-    {
-      Lisp_Object status;
-      json_initialized = init_json_functions ();
-      status = json_initialized ? Qt : Qnil;
-      Vlibrary_cache = Fcons (Fcons (Qjson, status), Vlibrary_cache);
-    }
-  if (!json_initialized)
-    Fsignal (Qjson_unavailable,
-	     list1 (build_unibyte_string ("jansson library not found")));
-#endif
+  ensure_json_available ();
 
   Lisp_Object string = args[0];
   CHECK_STRING (string);
@@ -1048,19 +1044,7 @@ DEFUN ("json-parse-buffer", Fjson_parse_buffer, Sjson_parse_buffer,
      (ptrdiff_t nargs, Lisp_Object *args)
 {
   specpdl_ref count = SPECPDL_INDEX ();
-
-#ifdef WINDOWSNT
-  if (!json_initialized)
-    {
-      Lisp_Object status;
-      json_initialized = init_json_functions ();
-      status = json_initialized ? Qt : Qnil;
-      Vlibrary_cache = Fcons (Fcons (Qjson, status), Vlibrary_cache);
-    }
-  if (!json_initialized)
-    Fsignal (Qjson_unavailable,
-	     list1 (build_unibyte_string ("jansson library not found")));
-#endif
+  ensure_json_available ();
 
   struct json_configuration conf =
     {json_object_hashtable, json_array_array, QCnull, QCfalse};
@@ -1137,6 +1121,9 @@ syms_of_json (void)
   DEFSYM (Qplist, "plist");
   DEFSYM (Qarray, "array");
 
+#ifdef WINDOWSNT
+  defsubr (&Sjson__available_p);
+#endif
   defsubr (&Sjson_serialize);
   defsubr (&Sjson_insert);
   defsubr (&Sjson_parse_string);