exif: do not blanket-delete all custom metadata on Xmp read

Through the call chain dt_image_import ->
_image_import_internal -> dt_exif_xmp_read -> _exif_decode_xmp_data, the
function _exif_decode_xmp_data would be called with exif_read == FALSE,
causing it to delete all metadata which has previously been read in
_exif_decode_exif_data (see the parent commit).

The intent was likely to be able to update descriptions etc. which had
been written through other programs in the Xmp files in darktable. In
order to continue to support this use case, we do still remove
Xmp-related tags if we don't find them in the sidecar, but only those
(via a string match).

Author: horazont

src/common/exif.cc

@@ -610,7 +610,6 @@ static bool _exif_decode_xmp_data(dt_image_t *img,
     if(version == -1 || version > 0)
     {
       dt_pthread_mutex_lock(&darktable.metadata_threadsafe);
-      if(!exif_read) dt_metadata_clear(imgs, FALSE);
 
       for(GList *iter = dt_metadata_get_list(); iter; iter = iter->next)
       {
@@ -629,6 +628,12 @@ static bool _exif_decode_xmp_data(dt_image_t *img,
           dt_metadata_set_import(img->id, metadata->tagname, value);
           free(adr);
         }
+        else if(!exif_read && strstr(metadata->tagname, "Xmp.") == metadata->tagname)
+        {
+          // Only remove Xmp. metadata fields, do not touch metadata from other
+          // sources (e.g. Exif.*).
+          dt_metadata_unset(img->id, metadata->tagname, FALSE);
+        }
       }
       dt_pthread_mutex_unlock(&darktable.metadata_threadsafe);
     }

src/common/metadata.c

@@ -838,6 +838,34 @@ void dt_metadata_set_list_id(const GList *img,
   }
 }
 
+void dt_metadata_unset(const dt_imgid_t imgid, const char *key, const gboolean undo_on)
+{
+  if(!key || !dt_is_valid_imgid(imgid)) return;
+
+  int keyid = dt_metadata_get_keyid(key);
+  if(keyid == -1) return;
+
+  GList *imgs = NULL;
+  imgs = g_list_prepend(imgs, GINT_TO_POINTER(imgid));
+  GList *undo = NULL;
+  if(undo_on) dt_undo_start_group(darktable.undo, DT_UNDO_METADATA);
+
+  const gchar *ckey = g_strdup_printf("%d", keyid);
+  GList *metadata = NULL;
+  metadata = g_list_append(metadata, (gpointer)ckey);
+  metadata = g_list_append(metadata, NULL);
+
+  _metadata_execute(imgs, metadata, &undo, undo_on, DT_MA_REMOVE);
+
+  g_list_free_full(metadata, g_free);
+  g_list_free(imgs);
+  if(undo_on)
+  {
+    dt_undo_record(darktable.undo, NULL, DT_UNDO_METADATA, undo, _pop_undo, _metadata_undo_data_free);
+    dt_undo_end_group(darktable.undo);
+  }
+}
+
 gboolean dt_metadata_already_imported(const char *filename, const char *datetime)
 {
   if(!filename || !datetime)

src/common/metadata.h

@@ -98,6 +98,8 @@ void dt_metadata_set_list(const GList *imgs, GList *key_value, const gboolean un
     if clear_on TRUE the image metadata are cleared before attaching the new ones*/
 void dt_metadata_set_list_id(const GList *img, const GList *metadata, const gboolean clear_on,
                              const gboolean undo_on);
+/** Unset a specific metadata key for a specific image. Noop if the key isn't set. */
+void dt_metadata_unset(const dt_imgid_t imgid, const char *key, const gboolean undo_on);
 /** Get metadata (named keys) for a specific image, or all selected for an invalid imgid
     For keys which return a string, the caller has to make sure that it
     is freed after usage. With mutex lock. */