git-changebar: Add support for libgit2 1.4

The buffer API changed a lot in libgit2 1.4, so compatibility is a bit
nastier than one could hope for.

Fixes #1164.

git-changebar/src/gcb-plugin.c

@@ -219,30 +219,19 @@ static const struct {
 };
 
 
-/* workaround https://github.com/libgit2/libgit2/pull/3187 */
-static int
-gcb_git_buf_grow (git_buf  *buf,
-                  size_t    target_size)
-{
-  if (buf->asize == 0) {
-    if (target_size == 0) {
-      target_size = buf->size;
-    }
-    if ((target_size & 7) == 0) {
-      target_size++;
-    }
-  }
-  return git_buf_grow (buf, target_size);
-}
-#define git_buf_grow gcb_git_buf_grow
-
 static void
 buf_zero (git_buf *buf)
 {
   if (buf) {
     buf->ptr = NULL;
     buf->size = 0;
+#if ! CHECK_LIBGIT2_VERSION(1, 4)
     buf->asize = 0;
+#else
+    /* we don't really need this field, but the documentation states that all
+     * fields should be set to 0, so fill it as well */
+    buf->reserved = 0;
+#endif
   }
 }
 
@@ -256,6 +245,52 @@ clear_cached_blob_contents (void)
   G_blob_contents_tag = 0;
 }
 
+/* similar to old git_blob_filtered_content() but makes sure the caller owns
+ * the data in the output buffer -- and uses a boolean return */
+static gboolean
+get_blob_contents (git_buf     *out,
+                   git_blob    *blob,
+                   const char  *as_path,
+                   int          check_for_binary_data)
+{
+/* libgit2 1.4 changed buffer API quite a bit */
+#if ! CHECK_LIBGIT2_VERSION(1, 4)
+  gboolean success = TRUE;
+
+  if (git_blob_filtered_content (out, blob, as_path,
+                                 check_for_binary_data) != 0)
+    return FALSE;
+
+  /* Workaround for https://github.com/libgit2/libgit2/pull/3187
+   * We want to own the buffer, which git_buf_grow(buf, 0) was supposed to do,
+   * but there is a corner case where it doesn't do what it should and
+   * truncates the buffer contents, so we fix this manually. */
+  if (out->asize == 0) {
+    size_t target_size = out->size;
+    if ((target_size & 7) == 0) {
+      target_size++;
+    }
+    success = (git_buf_grow (out, target_size) == 0);
+  }
+
+  return success;
+#else /* libgit2 >= 1.4 */
+  /* Here we can assume we will always get a buffer we own (at least as of
+   * 2022-06-05 it is the case), so there's no need for a pendent to the
+   * previous git_buf_grow() shenanigans.
+   * This code path does the same as the older git_blob_filtered_content()
+   * but with non-deprecated API */
+  git_blob_filter_options opts = GIT_BLOB_FILTER_OPTIONS_INIT;
+
+  if (check_for_binary_data)
+    opts.flags |= GIT_BLOB_FILTER_CHECK_FOR_BINARY;
+  else
+    opts.flags &= ~GIT_BLOB_FILTER_CHECK_FOR_BINARY;
+
+  return git_blob_filter(out, blob, as_path, &opts) == 0;
+#endif
+}
+
 /* get the file blob for @relpath at HEAD */
 static gboolean
 repo_get_file_blob_contents (git_repository  *repo,
@@ -279,11 +314,8 @@ repo_get_file_blob_contents (git_repository  *repo,
           git_blob *blob;
 
           if (git_blob_lookup (&blob, repo, git_tree_entry_id (entry)) == 0) {
-            if (git_blob_filtered_content (contents, blob, relpath,
-                                           check_for_binary_data) == 0 &&
-                git_buf_grow (contents, 0) == 0) {
-              success = TRUE;
-            }
+            success = get_blob_contents (contents, blob, relpath,
+                                         check_for_binary_data);
             git_blob_free (blob);
           }
           git_tree_entry_free (entry);