[PATCH 1/3] pack-write: lift and expose index-pack report parsing

[Justin Tobler <jltobler@gmail.com>]

Once git-index-pack(1) finishes writing a packfile, it reports the
hexadecimal name of that packfile on stdout prefixed with either "pack"
or "keep" depending on whether a ".keep" file was also created alongside
it. Callers that are required to manage the lifecycle of ".keep" files
parse this report via `index_pack_lockfile()`, which provides the path
to the temporary ".keep" file.

In a subsequent commit, the "files" ODB transaction begins staging
packfiles in a transaction without eagerly creating a ".keep" file and
must instead read git-index-pack(1) "pack" output to get the packfile
name. Extract the generic parsing logic for the git-index-pack(1) output
into `read_index_pack_report()` in preparation accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 pack-write.c | 69 +++++++++++++++++++++++++++++++++++++++-------------
 pack.h       | 10 ++++++++
 2 files changed, 62 insertions(+), 17 deletions(-)

diff --git a/pack-write.c b/pack-write.c
index 85674e4b72..5f5bb894a5 100644
--- a/pack-write.c
+++ b/pack-write.c
@@ -469,33 +469,68 @@ void fixup_pack_header_footer(const struct git_hash_algo *hash_algo,
 	fsync_component_or_die(FSYNC_COMPONENT_PACK, pack_fd, pack_name);
 }
 
-char *index_pack_lockfile(struct odb_source *source, int ip_out,
-			  int *is_well_formed)
+int read_index_pack_report(struct repository *repo, int ip_out,
+			   struct strbuf *pack_name, int *is_keep)
 {
+	const int len = repo->hash_algo->hexsz + 6;
 	char packname[GIT_MAX_HEXSZ + 6];
-	const int len = source->odb->repo->hash_algo->hexsz + 6;
+	const char *name;
+	int keep;
 
 	/*
-	 * The first thing we expect from index-pack's output
-	 * is "pack\t%40s\n" or "keep\t%40s\n" (46 bytes) where
-	 * %40s is the newly created pack SHA1 name.  In the "keep"
-	 * case, we need it to remove the corresponding .keep file
-	 * later on.  If we don't get that then tough luck with it.
+	 * The first thing we expect from index-pack's output is
+	 * "pack\t<name>\n" or "keep\t<name>\n", where <name> is the hexadecimal
+	 * name of the packfile it has just written. The line is therefore five
+	 * bytes of prefix, `hexsz` bytes of name, and a trailing newline. A
+	 * "keep" prefix tells us that index-pack has also created the
+	 * corresponding ".keep" file.
 	 */
-	if (read_in_full(ip_out, packname, len) == len && packname[len-1] == '\n') {
-		const char *name;
+	if (read_in_full(ip_out, packname, len) != len || packname[len-1] != '\n')
+		return -1;
+
+	packname[len-1] = 0;
+	if (skip_prefix(packname, "pack\t", &name))
+		keep = 0;
+	else if (skip_prefix(packname, "keep\t", &name))
+		keep = 1;
+	else
+		return -1;
+
+	strbuf_addstr(pack_name, name);
+	if (is_keep)
+		*is_keep = keep;
+
+	return 0;
+}
+
+char *index_pack_lockfile(struct odb_source *source, int ip_out,
+			  int *is_well_formed)
+{
+	struct strbuf pack_name = STRBUF_INIT;
+	char *lockfile = NULL;
+	int is_keep = 0;
 
+	if (read_index_pack_report(source->odb->repo, ip_out, &pack_name, &is_keep)) {
 		if (is_well_formed)
-			*is_well_formed = 1;
-		packname[len-1] = 0;
-		if (skip_prefix(packname, "keep\t", &name))
-			return xstrfmt("%s/pack/pack-%s.keep",
-				       source->path, name);
+			*is_well_formed = 0;
 		return NULL;
 	}
+
 	if (is_well_formed)
-		*is_well_formed = 0;
-	return NULL;
+		*is_well_formed = 1;
+
+	/*
+	 * If there is no ".keep" file, then there is no lockfile to return.
+	 */
+	if (!is_keep) {
+		strbuf_release(&pack_name);
+		return NULL;
+	}
+
+	lockfile = xstrfmt("%s/pack/pack-%s.keep", source->path, pack_name.buf);
+
+	strbuf_release(&pack_name);
+	return lockfile;
 }
 
 /*
diff --git a/pack.h b/pack.h
index ada506b5c5..ddf2d52934 100644
--- a/pack.h
+++ b/pack.h
@@ -106,6 +106,16 @@ off_t write_pack_header(struct hashfile *f, uint32_t);
 void fixup_pack_header_footer(const struct git_hash_algo *, int,
 			      unsigned char *, const char *, uint32_t,
 			      unsigned char *, off_t);
+
+/*
+ * Read the report that git-index-pack(1) writes to its standard output after
+ * the packfile is written. The hexadecimal name of the packfile is appended to
+ * `pack_name` and, if `is_keep` is non-NULL, set it to indicate whether a
+ * ".keep" file was written. Return 0 on success and a negative error code on
+ * failure.
+ */
+int read_index_pack_report(struct repository *repo, int ip_out,
+			   struct strbuf *pack_name, int *is_keep);
 char *index_pack_lockfile(struct odb_source *source, int fd,
 			  int *is_well_formed);
 
-- 
2.55.0.424.g13c7afec21