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>
diff --git a/csum-file.c b/csum-file.c
index fc97d6e..53f5375 100644
--- a/csum-file.c
+++ b/csum-file.c
@@ -158,6 +158,26 @@
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);
