From c871a1d17b8433d98df59b03da5538f10c4ae52c Mon Sep 17 00:00:00 2001
From: Junio C Hamano <gitster@pobox.com>
Date: Thu, 5 Jan 2012 10:54:14 -0800
Subject: commit --amend -S: strip existing gpgsig headers

Any existing commit signature was made against the contents of the old
commit, including its committer date that is about to change, and will
become invalid by amending it.

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

diff --git a/builtin/commit.c b/builtin/commit.c
index fa41ec8..970a836 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1494,7 +1494,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	}
 
 	if (amend) {
-		extra = read_commit_extra_headers(current_head);
+		const char *exclude_gpgsig[2] = { "gpgsig", NULL };
+		extra = read_commit_extra_headers(current_head, exclude_gpgsig);
 	} else {
 		struct commit_extra_header **tail = &extra;
 		append_merge_tag_headers(parents, &tail);
diff --git a/commit.c b/commit.c
index 27c7226..2162a7c 100644
--- a/commit.c
+++ b/commit.c
@@ -981,14 +981,15 @@ static void add_extra_header(struct strbuf *buffer,
 		strbuf_addch(buffer, '\n');
 }
 
-struct commit_extra_header *read_commit_extra_headers(struct commit *commit)
+struct commit_extra_header *read_commit_extra_headers(struct commit *commit,
+						      const char **exclude)
 {
 	struct commit_extra_header *extra = NULL;
 	unsigned long size;
 	enum object_type type;
 	char *buffer = read_sha1_file(commit->object.sha1, &type, &size);
 	if (buffer && type == OBJ_COMMIT)
-		extra = read_commit_extra_header_lines(buffer, size);
+		extra = read_commit_extra_header_lines(buffer, size, exclude);
 	free(buffer);
 	return extra;
 }
@@ -1002,7 +1003,23 @@ static inline int standard_header_field(const char *field, size_t len)
 		(len == 8 && !memcmp(field, "encoding ", 9)));
 }
 
-struct commit_extra_header *read_commit_extra_header_lines(const char *buffer, size_t size)
+static int excluded_header_field(const char *field, size_t len, const char **exclude)
+{
+	if (!exclude)
+		return 0;
+
+	while (*exclude) {
+		size_t xlen = strlen(*exclude);
+		if (len == xlen &&
+		    !memcmp(field, *exclude, xlen) && field[xlen] == ' ')
+			return 1;
+		exclude++;
+	}
+	return 0;
+}
+
+struct commit_extra_header *read_commit_extra_header_lines(const char *buffer, size_t size,
+							   const char **exclude)
 {
 	struct commit_extra_header *extra = NULL, **tail = &extra, *it = NULL;
 	const char *line, *next, *eof, *eob;
@@ -1028,7 +1045,8 @@ struct commit_extra_header *read_commit_extra_header_lines(const char *buffer, s
 		if (next <= eof)
 			eof = next;
 
-		if (standard_header_field(line, eof - line))
+		if (standard_header_field(line, eof - line) ||
+		    excluded_header_field(line, eof - line, exclude))
 			continue;
 
 		it = xcalloc(1, sizeof(*it));
diff --git a/commit.h b/commit.h
index 6107648..123aea3 100644
--- a/commit.h
+++ b/commit.h
@@ -200,8 +200,8 @@ extern int commit_tree_extended(const char *msg, unsigned char *tree,
 				const char *author, const char *sign_commit,
 				struct commit_extra_header *);
 
-extern struct commit_extra_header *read_commit_extra_headers(struct commit *);
-extern struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len);
+extern struct commit_extra_header *read_commit_extra_headers(struct commit *, const char **);
+extern struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 
 extern void free_commit_extra_headers(struct commit_extra_header *extra);
 
diff --git a/t/t7510-signed-commit.sh b/t/t7510-signed-commit.sh
index 30401ce..1d3c56f 100755
--- a/t/t7510-signed-commit.sh
+++ b/t/t7510-signed-commit.sh
@@ -24,7 +24,8 @@ test_expect_success GPG 'create signed commits' '
 	echo 4 >file && test_tick && git commit -a -m "fourth unsigned" &&
 	git tag fourth-unsigned &&
 
-	test_tick && git commit --amend -S -m "fourth signed"
+	test_tick && git commit --amend -S -m "fourth signed" &&
+	git tag fourth-signed
 '
 
 test_expect_success GPG 'show signatures' '
@@ -68,4 +69,12 @@ test_expect_success GPG 'detect fudged signature with NUL' '
 	! grep "Good signature from" actual2
 '
 
+test_expect_success GPG 'amending already signed commit' '
+	git checkout fourth-signed^0 &&
+	git commit --amend -S --no-edit &&
+	git show -s --show-signature HEAD >actual &&
+	grep "Good signature from" actual &&
+	! grep "BAD signature from" actual
+'
+
 test_done
-- 
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