[PATCH v2 06/17] refs.c: add new function copy_reflog_into_strbuf

[Ronnie Sahlberg <sahlberg@google.com>]

Add a new function copy_reflog_into_strbuf() that is a backend specific
optimized way to read the whole reflog as is into a strbuf so we can
copy it elsewhere instead of having to iterate over all the entries using
the iterators.

For the current type of refs backend that use files we simple read_in_full
the whole file into a strbuf. When we add other types of backends for refs
we will need to implement similarly optimized versions of this functions for
them.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
---
 refs.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/refs.c b/refs.c
index 3d13624..1c968f5 100644
--- a/refs.c
+++ b/refs.c
@@ -2826,6 +2826,37 @@ static int rename_ref_available(const char *oldname, const char *newname)
 static int write_ref_sha1(struct ref_lock *lock, const unsigned char *sha1,
 			  const char *logmsg);
 
+/*
+ * This is an optimized function to read the whole reflog as a blob
+ * into a strbuf. It is used during ref_rename so that we can use an
+ * efficient method to read the whole log and later write it back to a
+ * different file.
+ */
+static int copy_reflog_into_strbuf(const char *refname, struct strbuf *buf)
+{
+	struct stat st;
+	int fd;
+
+	if (lstat(git_path("logs/%s", refname), &st) == -1)
+		return 1;
+	if ((fd = open(git_path("logs/%s", refname), O_RDONLY)) == -1) {
+		error("failed to open reflog %s, %s",
+		      refname, strerror(errno));
+		return 1;
+	}
+	strbuf_init(buf, st.st_size);
+	strbuf_setlen(buf, st.st_size);
+	if (read_in_full(fd, buf->buf, st.st_size) != st.st_size) {
+		close(fd);
+		error("failed to read reflog %s, %s",
+		      refname, strerror(errno));
+		return 1;
+	}
+	close(fd);
+
+	return 0;
+}
+
 int rename_ref(const char *oldrefname, const char *newrefname, const char *logmsg)
 {
 	unsigned char sha1[20], orig_sha1[20];
-- 
2.1.0.rc2.206.gedb03e5