app, pdb, plug-ins: improve error feedback for gimp_temp_file().

It is not a fix yet for #14681 but would provide nicer and earlier error
when a call to gimp_temp_file() fails, which may happen in some cases.
E.g. if the temp directory doesn't exist and we fail to create it.

As a test, if I delete /tmp/gimp/3.2/ and give root ownership to
/tmp/gimp/, now the error when opening our gimp-splash.xcf.xz is:

GIMP-Error: Opening '/path/to/gimp/gimp-data/images/gimp-splash.xcf.xz' failed: Error creating directory /tmp/gimp/3.2: Permission denied

app/core/gimp.c

@@ -1273,8 +1273,9 @@ gimp_image_opened (Gimp  *gimp,
 }
 
 GFile *
-gimp_get_temp_file (Gimp        *gimp,
+gimp_get_temp_file (Gimp         *gimp,
-                    const gchar *extension)
+                    const gchar  *extension,
+                    GError      **error)
 {
   static gint  id = 0;
   static gint  pid;
@@ -1293,13 +1294,24 @@ gimp_get_temp_file (Gimp        *gimp,
     basename = g_strdup_printf ("gimp-temp-%d%d", pid, id++);
 
   dir = gimp_file_new_for_config_path (GIMP_GEGL_CONFIG (gimp->config)->temp_path,
-                                       NULL);
+                                       error);
+  if (dir == NULL)
+    {
+      g_free (basename);
+      return NULL;
+    }
+
   if (! g_file_query_exists (dir, NULL))
     {
       /* Try to make the temp directory if it doesn't exist.
        * Ignore any error.
        */
-      g_file_make_directory_with_parents (dir, NULL, NULL);
+      if (! g_file_make_directory_with_parents (dir, NULL, error))
+        {
+          g_free (basename);
+          g_object_unref (dir);
+          return NULL;
+        }
     }
   file = g_file_get_child (dir, basename);
   g_free (basename);

app/core/gimp.h

@@ -256,7 +256,8 @@ void           gimp_image_opened           (Gimp                *gimp,
                                             GFile               *file);
 
 GFile        * gimp_get_temp_file          (Gimp                *gimp,
-                                            const gchar         *extension);
+                                            const gchar         *extension,
+                                            GError             **error);
 
 GimpDataFactory *
                gimp_get_data_factory       (Gimp                *gimp,

app/file/file-remote.c

@@ -258,13 +258,13 @@ file_remote_get_temp_file (Gimp  *gimp,
       const gchar *ext = strchr (basename, '.');
 
       if (ext && strlen (ext))
-        temp_file = gimp_get_temp_file (gimp, ext + 1);
+        temp_file = gimp_get_temp_file (gimp, ext + 1, NULL);
 
       g_free (basename);
     }
 
   if (! temp_file)
-    temp_file = gimp_get_temp_file (gimp, "xxx");
+    temp_file = gimp_get_temp_file (gimp, "xxx", NULL);
 
   return temp_file;
 }

app/pdb/gimp-cmds.c

@@ -219,7 +219,8 @@ temp_file_invoker (GimpProcedure         *procedure,
 
   if (success)
     {
-      file = gimp_get_temp_file (gimp, extension);
+      file    = gimp_get_temp_file (gimp, extension, error);
+      success = (file != NULL);
     }
 
   return_vals = gimp_procedure_get_return_values (procedure, success,

pdb/groups/gimp.pdb

@@ -220,7 +220,8 @@ HELP
     %invoke = (
         code => <<'CODE'
 {
-  file = gimp_get_temp_file (gimp, extension);
+  file    = gimp_get_temp_file (gimp, extension, error);
+  success = (file != NULL);
 }
 CODE
     );

plug-ins/common/file-compressor.c

@@ -486,6 +486,12 @@ export_image (const CompressorEntry  *compressor,
   /* get a temp name with the right extension and save into it. */
 
   tmp_file = gimp_temp_file (ext + 1);
+  if (tmp_file == NULL)
+    {
+      g_set_error (error, G_FILE_ERROR, G_FILE_ERROR_FAILED,
+                   "%s", gimp_pdb_get_last_error (gimp_get_pdb ()));
+      return gimp_pdb_get_last_status (gimp_get_pdb ());
+    }
 
   if (! (gimp_file_save (run_mode, image, tmp_file, options) &&
          valid_file (tmp_file)))
@@ -544,6 +550,13 @@ load_image (const CompressorEntry  *compressor,
 
   /* find a temp name */
   tmp_file = gimp_temp_file (ext + 1);
+  if (tmp_file == NULL)
+    {
+      *status = gimp_pdb_get_last_status (gimp_get_pdb ());
+      g_set_error (error, G_FILE_ERROR, G_FILE_ERROR_FAILED,
+                   "%s", gimp_pdb_get_last_error (gimp_get_pdb ()));
+      return NULL;
+    }
 
   if (! compressor->load_fn (file, tmp_file, error))
     {