thodg/libgit2

diff --git a/src/merge.c b/src/merge.c
index 05a776e..afe69e5 100644
--- a/src/merge.c
+++ b/src/merge.c
@@ -68,6 +68,16 @@ struct merge_diff_df_data {
 	git_merge_diff *prev_conflict;
 };
 
+/*
+ * This acts as a negative cache entry marker. In case we've tried to calculate
+ * similarity metrics for a given blob already but `git_hashsig` determined
+ * that it's too small in order to have a meaningful hash signature, we will
+ * insert the address of this marker instead of `NULL`. Like this, we can
+ * easily check whether we have checked a gien entry already and skip doing the
+ * calculation again and again.
+ */
+static int cache_invalid_marker;
+
 /* Merge base computation */
 
 int merge_bases_many(git_commit_list **out, git_revwalk **walk_out, git_repository *repo, size_t length, const git_oid input_array[])
@@ -1027,6 +1037,9 @@ static int index_entry_similarity_calc(
 	git_object_size_t blobsize;
 	int error;
 
+	if (*out || *out == &cache_invalid_marker)
+		return 0;
+
 	*out = NULL;
 
 	if ((error = git_blob_lookup(&blob, repo, &entry->id)) < 0)
@@ -1047,6 +1060,8 @@ static int index_entry_similarity_calc(
 	error = opts->metric->buffer_signature(out, &diff_file,
 		git_blob_rawcontent(blob), (size_t)blobsize,
 		opts->metric->payload);
+	if (error == GIT_EBUFS)
+		*out = &cache_invalid_marker;
 
 	git_blob_free(blob);
 
@@ -1069,18 +1084,16 @@ static int index_entry_similarity_inexact(
 		return 0;
 
 	/* update signature cache if needed */
-	if (!cache[a_idx] && (error = index_entry_similarity_calc(&cache[a_idx], repo, a, opts)) < 0)
-		return error;
-	if (!cache[b_idx] && (error = index_entry_similarity_calc(&cache[b_idx], repo, b, opts)) < 0)
+	if ((error = index_entry_similarity_calc(&cache[a_idx], repo, a, opts)) < 0 ||
+	    (error = index_entry_similarity_calc(&cache[b_idx], repo, b, opts)) < 0)
 		return error;
 
 	/* some metrics may not wish to process this file (too big / too small) */
-	if (!cache[a_idx] || !cache[b_idx])
+	if (cache[a_idx] == &cache_invalid_marker || cache[b_idx] == &cache_invalid_marker)
 		return 0;
 
 	/* compare signatures */
-	if (opts->metric->similarity(
-		&score, cache[a_idx], cache[b_idx], opts->metric->payload) < 0)
+	if (opts->metric->similarity(&score, cache[a_idx], cache[b_idx], opts->metric->payload) < 0)
 		return -1;
 
 	/* clip score */
@@ -1550,7 +1563,7 @@ int git_merge_diff_list__find_renames(
 done:
 	if (cache != NULL) {
 		for (i = 0; i < cache_size; ++i) {
-			if (cache[i] != NULL)
+			if (cache[i] != NULL && cache[i] != &cache_invalid_marker)
 				opts->metric->free_signature(cache[i], opts->metric->payload);
 		}
 
diff --git a/tests/merge/trees/renames.c b/tests/merge/trees/renames.c
index e0b12af..c515aaf 100644
--- a/tests/merge/trees/renames.c
+++ b/tests/merge/trees/renames.c
@@ -274,3 +274,80 @@ void test_merge_trees_renames__submodules(void)
 	cl_assert(merge_test_index(index, merge_index_entries, 7));
 	git_index_free(index);
 }
+
+void test_merge_trees_renames__cache_recomputation(void)
+{
+	git_oid blob, binary, ancestor_oid, theirs_oid, ours_oid;
+	git_merge_options opts = GIT_MERGE_OPTIONS_INIT;
+	git_buf path = GIT_BUF_INIT;
+	git_treebuilder *builder;
+	git_tree *ancestor_tree, *their_tree, *our_tree;
+	git_index *index;
+	size_t blob_size;
+	void *data;
+	size_t i;
+
+	cl_git_pass(git_oid_fromstr(&blob, "a2d8d1824c68541cca94ffb90f79291eba495921"));
+
+	/*
+	 * Create a 50MB blob that consists of NUL bytes only. It is important
+	 * that this blob is of a special format, most importantly it cannot
+	 * contain more than four non-consecutive newlines or NUL bytes. This
+	 * is because of git_hashsig's inner workings where all files with less
+	 * than four "lines" are deemed to small.
+	 */
+	blob_size = 50 * 1024 * 1024;
+	cl_assert(data = git__calloc(blob_size, 1));
+	cl_git_pass(git_blob_create_from_buffer(&binary, repo, data, blob_size));
+
+	/*
+	 * Create the common ancestor, which has 1000 dummy blobs and the binary
+	 * blob. The dummy blobs serve as potential rename targets for the
+	 * dummy blob.
+	 */
+	cl_git_pass(git_treebuilder_new(&builder, repo, NULL));
+	for (i = 0; i < 1000; i++) {
+		cl_git_pass(git_buf_printf(&path, "%"PRIuMAX".txt", i));
+		cl_git_pass(git_treebuilder_insert(NULL, builder, path.ptr, &blob, GIT_FILEMODE_BLOB));
+		git_buf_clear(&path);
+	}
+	cl_git_pass(git_treebuilder_insert(NULL, builder, "original.bin", &binary, GIT_FILEMODE_BLOB));
+	cl_git_pass(git_treebuilder_write(&ancestor_oid, builder));
+
+	/* We now the binary blob in our tree. */
+	cl_git_pass(git_treebuilder_remove(builder, "original.bin"));
+	cl_git_pass(git_treebuilder_insert(NULL, builder, "renamed.bin", &binary, GIT_FILEMODE_BLOB));
+	cl_git_pass(git_treebuilder_write(&ours_oid, builder));
+
+	git_treebuilder_free(builder);
+
+	/* And move everything into a subdirectory in their tree. */
+	cl_git_pass(git_treebuilder_new(&builder, repo, NULL));
+	cl_git_pass(git_treebuilder_insert(NULL, builder, "subdir", &ancestor_oid, GIT_FILEMODE_TREE));
+	cl_git_pass(git_treebuilder_write(&theirs_oid, builder));
+
+	/*
+	 * Now merge ancestor, ours and theirs. As `git_hashsig` refuses to
+	 * create a hash signature for the 50MB binary file, we historically
+	 * didn't cache the hashsig computation for it. As a result, we now
+	 * started looking up the 50MB blob and scanning it at least 1000
+	 * times, which takes a long time.
+	 *
+	 * The number of 1000 blobs is chosen in such a way that it's
+	 * noticeable when the bug creeps in again, as it takes around 12
+	 * minutes on my machine to compute the following merge.
+	 */
+	opts.target_limit = 5000;
+	cl_git_pass(git_tree_lookup(&ancestor_tree, repo, &ancestor_oid));
+	cl_git_pass(git_tree_lookup(&their_tree, repo, &theirs_oid));
+	cl_git_pass(git_tree_lookup(&our_tree, repo, &ours_oid));
+	cl_git_pass(git_merge_trees(&index, repo, ancestor_tree, our_tree, their_tree, &opts));
+
+	git_treebuilder_free(builder);
+	git_buf_dispose(&path);
+	git_index_free(index);
+	git_tree_free(ancestor_tree);
+	git_tree_free(their_tree);
+	git_tree_free(our_tree);
+	git__free(data);
+}