From 798e0f451661f81f4568dce4033cf1c9076f224f Mon Sep 17 00:00:00 2001
From: Jeff King <peff@peff.net>
Date: Sun, 19 Jan 2025 08:23:37 -0500
Subject: [PATCH] packfile: factor out --pack_header argument parsing

Both index-pack and unpack-objects accept a --pack_header argument. This
is an undocumented internal argument used by receive-pack and fetch to
pass along information about the header of the pack, which they've
already read from the incoming stream.

In preparation for a bugfix, let's factor the duplicated code into a
common helper.

The callers are still responsible for identifying the option. While this
could likewise be factored out, it is more flexible this way (e.g., if
they ever started using parse-options and wanted to handle both the
stuck and unstuck forms).

Likewise, the callers are responsible for reporting errors, though they
both just call die(). I've tweaked unpack-objects to match index-pack in
marking the error for translation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin/index-pack.c     | 14 +++-----------
 builtin/unpack-objects.c | 16 ++++------------
 packfile.c               | 17 +++++++++++++++++
 packfile.h               |  6 ++++++
 4 files changed, 30 insertions(+), 23 deletions(-)

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 763b01372a..bab42dfc2a 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1801,18 +1801,10 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 					nr_threads = 1;
 				}
 			} else if (starts_with(arg, "--pack_header=")) {
-				struct pack_header *hdr;
-				char *c;
-
-				hdr = (struct pack_header *)input_buffer;
-				hdr->hdr_signature = htonl(PACK_SIGNATURE);
-				hdr->hdr_version = htonl(strtoul(arg + 14, &c, 10));
-				if (*c != ',')
+				if (parse_pack_header_option(arg + 14,
+							     input_buffer,
+							     &input_len) < 0)
 					die(_("bad %s"), arg);
-				hdr->hdr_entries = htonl(strtoul(c + 1, &c, 10));
-				if (*c)
-					die(_("bad %s"), arg);
-				input_len = sizeof(*hdr);
 			} else if (!strcmp(arg, "-v")) {
 				verbose = 1;
 			} else if (!strcmp(arg, "--progress-title")) {
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 08fa2a7a74..3161447274 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -15,6 +15,7 @@
 #include "progress.h"
 #include "decorate.h"
 #include "fsck.h"
+#include "packfile.h"
 
 static int dry_run, quiet, recover, has_errors, strict;
 static const char unpack_usage[] = "git unpack-objects [-n] [-q] [-r] [--strict]";
@@ -639,18 +640,9 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix UNUSED)
 				continue;
 			}
 			if (starts_with(arg, "--pack_header=")) {
-				struct pack_header *hdr;
-				char *c;
-
-				hdr = (struct pack_header *)buffer;
-				hdr->hdr_signature = htonl(PACK_SIGNATURE);
-				hdr->hdr_version = htonl(strtoul(arg + 14, &c, 10));
-				if (*c != ',')
-					die("bad %s", arg);
-				hdr->hdr_entries = htonl(strtoul(c + 1, &c, 10));
-				if (*c)
-					die("bad %s", arg);
-				len = sizeof(*hdr);
+				if (parse_pack_header_option(arg + 14,
+							     buffer, &len) < 0)
+					die(_("bad %s"), arg);
 				continue;
 			}
 			if (skip_prefix(arg, "--max-input-size=", &arg)) {
diff --git a/packfile.c b/packfile.c
index 813584646f..e2bdadc7cb 100644
--- a/packfile.c
+++ b/packfile.c
@@ -2294,3 +2294,20 @@ int is_promisor_object(const struct object_id *oid)
 	}
 	return oidset_contains(&promisor_objects, oid);
 }
+
+int parse_pack_header_option(const char *in, unsigned char *out, unsigned int *len)
+{
+	struct pack_header *hdr;
+	char *c;
+
+	hdr = (struct pack_header *)out;
+	hdr->hdr_signature = htonl(PACK_SIGNATURE);
+	hdr->hdr_version = htonl(strtoul(in, &c, 10));
+	if (*c != ',')
+		return -1;
+	hdr->hdr_entries = htonl(strtoul(c + 1, &c, 10));
+	if (*c)
+		return -1;
+	*len = sizeof(*hdr);
+	return 0;
+}
diff --git a/packfile.h b/packfile.h
index eb18ec15db..41f38b4832 100644
--- a/packfile.h
+++ b/packfile.h
@@ -210,4 +210,10 @@ int is_promisor_object(const struct object_id *oid);
 int load_idx(const char *path, const unsigned int hashsz, void *idx_map,
 	     size_t idx_size, struct packed_git *p);
 
+/*
+ * Parse a --pack_header option as accepted by index-pack and unpack-objects,
+ * turning it into the matching bytes we'd find in a pack.
+ */
+int parse_pack_header_option(const char *in, unsigned char *out, unsigned int *len);
+
 #endif