[face] Add hb_face_create_or_fail_using()

Fixes https://github.com/harfbuzz/harfbuzz/issues/5117

Untested.

New API:
+hb_face_create_or_fail_using

docs/harfbuzz-sections.txt

@@ -368,6 +368,7 @@ hb_face_count
 hb_face_t
 hb_face_create
 hb_face_create_or_fail
+hb_face_create_or_fail_using
 hb_face_create_from_file_or_fail
 hb_face_create_from_file_or_fail_using
 hb_face_list_loaders

src/hb-face.cc

@@ -328,17 +328,29 @@ hb_face_create_from_file_or_fail (const char   *file_name,
 
 static struct supported_face_loaders_t {
 	char name[9];
-	hb_face_t * (*func) (const char *font_file, unsigned face_index);
+	hb_face_t * (*from_file) (const char *font_file, unsigned face_index);
+	hb_face_t * (*from_blob) (hb_blob_t *blob, unsigned face_index);
 } supported_face_loaders[] =
 {
+  {"ot",
 #ifndef HB_NO_OPEN
-  {"ot",	hb_face_create_from_file_or_fail},
+   hb_face_create_from_file_or_fail,
+#else
+   nullptr,
 #endif
+   hb_face_create_or_fail
+  },
 #ifdef HAVE_FREETYPE
-  {"ft",	hb_ft_face_create_from_file_or_fail},
+  {"ft",
+   hb_ft_face_create_from_file_or_fail,
+   hb_ft_face_create_from_blob_or_fail
+  },
 #endif
 #ifdef HAVE_CORETEXT
-  {"coretext",	hb_coretext_face_create_from_file_or_fail},
+  {"coretext",
+   hb_coretext_face_create_from_file_or_fail,
+   hb_coretext_face_create_from_blob_or_fail
+  },
 #endif
 };
 
@@ -368,7 +380,7 @@ static const char *get_default_loader_name ()
  * is used.
  *
  * For example, the FreeType ("ft") loader might be able to load
- * .woff and .woff2 files if FreeType is built with those features,
+ * WOFF and WOFF2 files if FreeType is built with those features,
  * whereas the OpenType ("ot") loader will not.
  *
  * Return value: (transfer full): The new face object, or `NULL` if
@@ -381,6 +393,7 @@ hb_face_create_from_file_or_fail_using (const char   *file_name,
 					unsigned int  index,
 					const char   *loader_name)
 {
+  // Duplicated in hb_face_create_or_fail_using
   bool retry = false;
   if (!loader_name || !*loader_name)
   {
@@ -392,8 +405,58 @@ hb_face_create_from_file_or_fail_using (const char   *file_name,
 retry:
   for (unsigned i = 0; i < ARRAY_LENGTH (supported_face_loaders); i++)
   {
-    if (!loader_name || !strcmp (supported_face_loaders[i].name, loader_name))
-      return supported_face_loaders[i].func (file_name, index);
+    if (!loader_name || (supported_face_loaders[i].from_file && !strcmp (supported_face_loaders[i].name, loader_name)))
+      return supported_face_loaders[i].from_file (file_name, index);
+  }
+
+  if (retry)
+  {
+    retry = false;
+    loader_name = nullptr;
+    goto retry;
+  }
+
+  return nullptr;
+}
+
+/**
+ * hb_face_create_or_fail_using:
+ * @blob: #hb_blob_t to work upon
+ * @index: The index of the face within @blob
+ * @loader_name: (nullable): The name of the loader to use, or `NULL`
+ *
+ * A thin wrapper around the face loader functions registered with HarfBuzz.
+ * If @loader_name is `NULL` or the empty string, the first available loader
+ * is used.
+ *
+ * For example, the FreeType ("ft") loader might be able to load
+ * WOFF and WOFF2 files if FreeType is built with those features,
+ * whereas the OpenType ("ot") loader will not.
+ *
+ * Return value: (transfer full): The new face object, or `NULL` if
+ * the loader fails to load the face.
+ *
+ * XSince: REPLACEME
+ **/
+hb_face_t *
+hb_face_create_or_fail_using (hb_blob_t    *blob,
+			      unsigned int  index,
+			      const char   *loader_name)
+{
+  // Duplicated in hb_face_create_from_file_or_fail_using
+  bool retry = false;
+  if (!loader_name || !*loader_name)
+  {
+    loader_name = get_default_loader_name ();
+    retry = true;
+  }
+  if (loader_name && !*loader_name) loader_name = nullptr;
+
+retry:
+  for (unsigned i = 0; i < ARRAY_LENGTH (supported_face_loaders); i++)
+  {
+    if (!loader_name || (supported_face_loaders[i].from_blob && !strcmp (supported_face_loaders[i].name, loader_name)))
+      return supported_face_loaders[i].from_blob (blob, index);
   }
 
   if (retry)

src/hb-face.h

@@ -63,6 +63,11 @@ HB_EXTERN hb_face_t *
 hb_face_create_or_fail (hb_blob_t    *blob,
 			unsigned int  index);
 
+HB_EXTERN hb_face_t *
+hb_face_create_or_fail_using (hb_blob_t    *blob,
+			      unsigned int  index,
+			      const char   *loader_name);
+
 HB_EXTERN hb_face_t *
 hb_face_create_from_file_or_fail (const char   *file_name,
 				  unsigned int  index);