filebuf: configurable hash type

`git_filebuf` hashes contents as its written; allow for SHA1 or SHA256
as that hash. Currently, most callers simply use SHA1 as they do not yet
know about SHA256 themselves.

src/libgit2/config_file.c

@@ -1116,7 +1116,12 @@ static int write_on_eof(
 /*
  * This is pretty much the parsing, except we write out anything we don't have
  */
-static int config_file_write(config_file_backend *cfg, const char *orig_key, const char *key, const git_regexp *preg, const char *value)
+static int config_file_write(
+	config_file_backend *cfg,
+	const char *orig_key,
+	const char *key,
+	const git_regexp *preg,
+	const char *value)
 
 {
 	char *orig_section = NULL, *section = NULL, *orig_name, *name, *ldot;
@@ -1124,15 +1129,18 @@ static int config_file_write(config_file_backend *cfg, const char *orig_key, con
 	git_config_parser parser = GIT_CONFIG_PARSER_INIT;
 	git_filebuf file = GIT_FILEBUF_INIT;
 	struct write_data write_data;
-	int error;
+	int filebuf_hash, error;
+
+	filebuf_hash = git_filebuf_hash_flags(git_oid_algorithm(GIT_OID_SHA1));
+	GIT_ASSERT(filebuf_hash);
 
 	memset(&write_data, 0, sizeof(write_data));
 
 	if (cfg->locked) {
 		error = git_str_puts(&contents, git_str_cstr(&cfg->locked_content) == NULL ? "" : git_str_cstr(&cfg->locked_content));
 	} else {
-		if ((error = git_filebuf_open(&file, cfg->file.path, GIT_FILEBUF_HASH_CONTENTS,
-					      GIT_CONFIG_FILE_MODE)) < 0)
+		if ((error = git_filebuf_open(&file, cfg->file.path,
+				filebuf_hash, GIT_CONFIG_FILE_MODE)) < 0)
 			goto done;
 
 		/* We need to read in our own config file */

src/libgit2/index.c

@@ -3668,19 +3668,23 @@ int git_indexwriter_init(
 	git_indexwriter *writer,
 	git_index *index)
 {
-	int error;
+	int filebuf_hash, error;
 
 	GIT_REFCOUNT_INC(index);
 
 	writer->index = index;
 
+	filebuf_hash = git_filebuf_hash_flags(git_oid_algorithm(GIT_OID_SHA1));
+	GIT_ASSERT(filebuf_hash);
+
 	if (!index->index_file_path)
 		return create_index_error(-1,
 			"failed to write index: The index is in-memory only");
 
-	if ((error = git_filebuf_open(
-		&writer->file, index->index_file_path, GIT_FILEBUF_HASH_CONTENTS, GIT_INDEX_FILE_MODE)) < 0) {
-
+	if ((error = git_filebuf_open(&writer->file,
+			index->index_file_path,
+			git_filebuf_hash_flags(filebuf_hash),
+			GIT_INDEX_FILE_MODE)) < 0) {
 		if (error == GIT_ELOCKED)
 			git_error_set(GIT_ERROR_INDEX, "the index is locked; this might be due to a concurrent or crashed process");
 

src/libgit2/indexer.c

@@ -1232,6 +1232,7 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 	git_filebuf index_file = {0};
 	void *packfile_trailer;
 	size_t checksum_size;
+	int filebuf_hash;
 	bool mismatch;
 
 	if (!idx->parsed_header) {
@@ -1240,6 +1241,7 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 	}
 
 	checksum_size = git_hash_size(indexer_hash_algorithm(idx));
+	filebuf_hash = git_filebuf_hash_flags(indexer_hash_algorithm(idx));
 	GIT_ASSERT(checksum_size);
 
 	/* Test for this before resolve_deltas(), as it plays with idx->off */
@@ -1314,8 +1316,7 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 		return -1;
 
 	if (git_filebuf_open(&index_file, filename.ptr,
-		GIT_FILEBUF_HASH_CONTENTS |
-		(idx->do_fsync ? GIT_FILEBUF_FSYNC : 0),
+		filebuf_hash | (idx->do_fsync ? GIT_FILEBUF_FSYNC : 0),
 		idx->mode) < 0)
 		goto on_error;
 

src/util/filebuf.c

@@ -302,11 +302,16 @@ int git_filebuf_open_withsize(git_filebuf *file, const char *path, int flags, mo
 	}
 
 	/* If we are hashing on-write, allocate a new hash context */
-	if (flags & GIT_FILEBUF_HASH_CONTENTS) {
+	if (flags & GIT_FILEBUF_HASH_SHA1) {
 		file->compute_digest = 1;
 
 		if (git_hash_ctx_init(&file->digest, GIT_HASH_ALGORITHM_SHA1) < 0)
 			goto cleanup;
+	} else if (flags & GIT_FILEBUF_HASH_SHA256) {
+		file->compute_digest = 1;
+
+		if (git_hash_ctx_init(&file->digest, GIT_HASH_ALGORITHM_SHA256) < 0)
+			goto cleanup;
 	}
 
 	compression = flags >> GIT_FILEBUF_DEFLATE_SHIFT;

src/util/filebuf.h

@@ -17,13 +17,14 @@
 #	define GIT_FILEBUF_THREADS
 #endif
 
-#define GIT_FILEBUF_HASH_CONTENTS		(1 << 0)
-#define GIT_FILEBUF_APPEND				(1 << 2)
+#define GIT_FILEBUF_HASH_SHA1           (1 << 0)
+#define GIT_FILEBUF_HASH_SHA256         (1 << 1)
+#define GIT_FILEBUF_APPEND              (1 << 2)
 #define GIT_FILEBUF_CREATE_LEADING_DIRS	(1 << 3)
-#define GIT_FILEBUF_TEMPORARY			(1 << 4)
-#define GIT_FILEBUF_DO_NOT_BUFFER		(1 << 5)
-#define GIT_FILEBUF_FSYNC				(1 << 6)
-#define GIT_FILEBUF_DEFLATE_SHIFT		(7)
+#define GIT_FILEBUF_TEMPORARY           (1 << 4)
+#define GIT_FILEBUF_DO_NOT_BUFFER       (1 << 5)
+#define GIT_FILEBUF_FSYNC               (1 << 6)
+#define GIT_FILEBUF_DEFLATE_SHIFT       (7)
 
 #define GIT_FILELOCK_EXTENSION ".lock\0"
 #define GIT_FILELOCK_EXTLENGTH 6
@@ -91,4 +92,16 @@ int git_filebuf_hash(unsigned char *out, git_filebuf *file);
 int git_filebuf_flush(git_filebuf *file);
 int git_filebuf_stats(time_t *mtime, size_t *size, git_filebuf *file);
 
+GIT_INLINE(int) git_filebuf_hash_flags(git_hash_algorithm_t algorithm)
+{
+	switch (algorithm) {
+	case GIT_HASH_ALGORITHM_SHA1:
+		return GIT_FILEBUF_HASH_SHA1;
+	case GIT_HASH_ALGORITHM_SHA256:
+		return GIT_FILEBUF_HASH_SHA256;
+	default:
+		return 0;
+	}
+}
+
 #endif