Fix rename detection for tree-to-tree diffs

The performance improvements I introduced for rename detection
were not able to run successfully for tree-to-tree diffs because
the blob size was not known early enough and so the file signature
always had to be calculated nonetheless.

This change separates loading blobs into memory from calculating
the signature.  I can't avoid having to load the large blobs into
memory, but by moving it forward, I'm able to avoid the signature
calculation if the blob won't come into play for renames.

1 files changed, 98 insertions, 47 deletions

diff --git a/src/diff_tform.c b/src/diff_tform.c
index cabcd1f80..08b0d5c38 100644
--- a/src/diff_tform.c
+++ b/src/diff_tform.c
@@ -408,54 +408,90 @@ GIT_INLINE(git_diff_file *) similarity_get_file(git_diff_list *diff, size_t idx)
 	return (idx & 1) ? &delta->new_file : &delta->old_file;
 }
 
-static int similarity_calc(
-	git_diff_list *diff,
-	const git_diff_find_options *opts,
-	size_t file_idx,
-	void **cache)
+typedef struct {
+	size_t idx;
+	git_iterator_type_t src;
+	git_repository *repo;
+	git_diff_file *file;
+	git_buf data;
+	git_blob *blob;
+	int loaded;
+} similarity_info;
+
+static void similarity_init(
+	similarity_info *info, git_diff_list *diff, size_t file_idx)
+{
+	info->idx  = file_idx;
+	info->src  = (file_idx & 1) ? diff->new_src : diff->old_src;
+	info->repo = diff->repo;
+	info->file = similarity_get_file(diff, file_idx);
+	info->blob = NULL;
+	info->loaded = 0;
+	git_buf_init(&info->data, 0);
+}
+
+static int similarity_load(similarity_info *info)
 {
 	int error = 0;
-	git_diff_file *file = similarity_get_file(diff, file_idx);
-	git_iterator_type_t src = (file_idx & 1) ? diff->new_src : diff->old_src;
+	git_diff_file *file = info->file;
 
-	if (src == GIT_ITERATOR_TYPE_WORKDIR) { /* compute hashsig from file */
-		git_buf path = GIT_BUF_INIT;
+	if (info->src == GIT_ITERATOR_TYPE_WORKDIR) {
+		error = git_buf_joinpath(
+			&info->data, git_repository_workdir(info->repo), file->path);
 
-		/* TODO: apply wd-to-odb filters to file data if necessary */
+		/* if path is not a regular file, just skip this item */
+		if (!error && !git_path_isfile(info->data.ptr))
+			git_buf_free(&info->data);
+	} else if (git_blob_lookup(&info->blob, info->repo, &file->oid) < 0) {
+		/* if lookup fails, just skip this item in similarity calc */
+		giterr_clear();
+	} else {
+		if (!file->size)
+			file->size = git_blob_rawsize(info->blob);
+		assert(file->size == git_blob_rawsize(info->blob));
 
-		if ((error = git_buf_joinpath(
-				 &path, git_repository_workdir(diff->repo), file->path)) < 0)
-			return error;
+		info->data.size = (size_t)(git__is_sizet(file->size) ? file->size : -1);
+		info->data.ptr  = (char *)git_blob_rawcontent(info->blob);
+	}
 
-		/* if path is not a regular file, just skip this item */
-		if (git_path_isfile(path.ptr))
-			error = opts->metric->file_signature(
-				&cache[file_idx], file, path.ptr, opts->metric->payload);
+	info->loaded = 1;
 
-		git_buf_free(&path);
-	} else { /* compute hashsig from blob buffer */
-		git_blob *blob = NULL;
-		git_off_t blobsize;
+	return error;
+}
 
-		/* TODO: add max size threshold a la diff? */
+static void similarity_unload(similarity_info *info)
+{
+	if (info->blob)
+		git_blob_free(info->blob);
+	else
+		git_buf_free(&info->data);
 
-		if (git_blob_lookup(&blob, diff->repo, &file->oid) < 0) {
-			/* if lookup fails, just skip this item in similarity calc */
-			giterr_clear();
-			return 0;
-		}
+	info->loaded = 0;
+}
 
-		blobsize = git_blob_rawsize(blob);
-		if (!file->size)
-			file->size = blobsize;
-		if (!git__is_sizet(blobsize)) /* ? what to do ? */
-			blobsize = (size_t)-1;
+static int similarity_calc(
+	similarity_info *info,
+	const git_diff_find_options *opts,
+	void **cache)
+{
+	int error = 0;
 
-		error = opts->metric->buffer_signature(
-			&cache[file_idx], file, git_blob_rawcontent(blob),
-			(size_t)blobsize, opts->metric->payload);
+	if (!info->loaded && (error = similarity_load(info)) < 0)
+		return error;
 
-		git_blob_free(blob);
+	if (!info->data.size)
+		return 0;
+
+	if (info->src == GIT_ITERATOR_TYPE_WORKDIR) {
+		/* TODO: apply wd-to-odb filters to file data if necessary */
+
+		error = opts->metric->file_signature(
+			&cache[info->idx], info->file,
+			info->data.ptr, opts->metric->payload);
+	} else {
+		error = opts->metric->buffer_signature(
+			&cache[info->idx], info->file,
+			info->data.ptr, info->data.size, opts->metric->payload);
 	}
 
 	return error;
@@ -478,6 +514,8 @@ static int similarity_measure(
 	git_diff_file *a_file = similarity_get_file(diff, a_idx);
 	git_diff_file *b_file = similarity_get_file(diff, b_idx);
 	bool exact_match = FLAG_SET(opts, GIT_DIFF_FIND_EXACT_MATCH_ONLY);
+	int error = 0;
+	similarity_info a_info, b_info;
 
 	*score = -1;
 
@@ -512,26 +550,39 @@ static int similarity_measure(
 		return 0;
 	}
 
+	similarity_init(&a_info, diff, a_idx);
+	similarity_init(&b_info, diff, b_idx);
+
+	if (!a_file->size && (error = similarity_load(&a_info)) < 0)
+		goto done;
+	if (!b_file->size && (error = similarity_load(&b_info)) < 0)
+		goto done;
+
 	/* check if file sizes are nowhere near each other */
 	if (a_file->size > 127 &&
 		b_file->size > 127 &&
 		(a_file->size > (b_file->size << 4) ||
 		 b_file->size > (a_file->size << 4)))
-		return 0;
+		goto done;
 
 	/* update signature cache if needed */
-	if (!cache[a_idx] && similarity_calc(diff, opts, a_idx, cache) < 0)
-		return -1;
-	if (!cache[b_idx] && similarity_calc(diff, opts, b_idx, cache) < 0)
-		return -1;
+	if (!cache[a_idx] && (error = similarity_calc(&a_info, opts, cache)) < 0)
+		goto done;
+	if (!cache[b_idx] && (error = similarity_calc(&b_info, opts, cache)) < 0)
+		goto done;
 
-	/* some metrics may not wish to process this file (too big / too small) */
-	if (!cache[a_idx] || !cache[b_idx])
-		return 0;
+	/* calculate similarity provided that the metric choose to process
+	 * both the a and b files (some may not if file is too big, etc).
+	 */
+	if (cache[a_idx] && cache[b_idx])
+		error = opts->metric->similarity(
+			score, cache[a_idx], cache[b_idx], opts->metric->payload);
 
-	/* compare signatures */
-	return opts->metric->similarity(
-		score, cache[a_idx], cache[b_idx], opts->metric->payload);
+done:
+	similarity_unload(&a_info);
+	similarity_unload(&b_info);
+
+	return error;
 }
 
 static int calc_self_similarity(