csum-file: introduce sha1file_checkpoint

It is useful to be able to rewind a check-summed file to a certain
previous state after writing data into it using sha1write() API. The
fast-import command does this after streaming a blob data to the packfile
being generated and then noticing that the same blob has already been
written, and it does this with a private code truncate_pack() that is
commented as "Yes, this is a layering violation".

Introduce two API functions, sha1file_checkpoint(), that allows the caller
to save a state of a sha1file, and then later revert it to the saved state.
Use it to reimplement truncate_pack().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/csum-file.c b/csum-file.c
index fc97d6e045..53f5375b6c 100644
--- a/csum-file.c
+++ b/csum-file.c
@@ -158,6 +158,26 @@ struct sha1file *sha1fd_throughput(int fd, const char *name, struct progress *tp
 	return f;
 }
 
+void sha1file_checkpoint(struct sha1file *f, struct sha1file_checkpoint *checkpoint)
+{
+	sha1flush(f);
+	checkpoint->offset = f->total;
+	checkpoint->ctx = f->ctx;
+}
+
+int sha1file_truncate(struct sha1file *f, struct sha1file_checkpoint *checkpoint)
+{
+	off_t offset = checkpoint->offset;
+
+	if (ftruncate(f->fd, offset) ||
+	    lseek(f->fd, offset, SEEK_SET) != offset)
+		return -1;
+	f->total = offset;
+	f->ctx = checkpoint->ctx;
+	f->offset = 0; /* sha1flush() was called in checkpoint */
+	return 0;
+}
+
 void crc32_begin(struct sha1file *f)
 {
 	f->crc32 = crc32(0, NULL, 0);