[PATCH 5/5] fast-export: do automatic reencoding of commit messages only if requested

[Elijah Newren <newren@gmail.com>]

Automatic re-encoding of commit messages (and dropping of the encoding
header) hurts attempts to do reversible history rewrites (e.g. sha1sum
<-> sha256sum transitions, some subtree rewrites), and seems
inconsistent with the general principle followed elsewhere in
fast-export of requiring explicit user requests to modify the output
(e.g. --signed-tags=strip, --tag-of-filtered-object=rewrite).  Add a
--reencode flag that the user can use to specify, and like other
fast-export flags, default it to 'abort'.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 builtin/fast-export.c  | 35 ++++++++++++++++++++++++++++++++---
 t/t9350-fast-export.sh | 31 ++++++++++++++++++++++++++++---
 2 files changed, 60 insertions(+), 6 deletions(-)

diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 66331fa401..43cc52331c 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -33,6 +33,7 @@ static const char *fast_export_usage[] = {
 static int progress;
 static enum { SIGNED_TAG_ABORT, VERBATIM, WARN, WARN_STRIP, STRIP } signed_tag_mode = SIGNED_TAG_ABORT;
 static enum { TAG_FILTERING_ABORT, DROP, REWRITE } tag_of_filtered_mode = TAG_FILTERING_ABORT;
+static enum { REENCODE_ABORT, REENCODE_PLEASE, REENCODE_NEVER } reencode_mode = REENCODE_ABORT;
 static int fake_missing_tagger;
 static int use_done_feature;
 static int no_data;
@@ -77,6 +78,20 @@ static int parse_opt_tag_of_filtered_mode(const struct option *opt,
 	return 0;
 }
 
+static int parse_opt_reencode_mode(const struct option *opt,
+				   const char *arg, int unset)
+{
+	if (unset || !strcmp(arg, "abort"))
+		reencode_mode = REENCODE_ABORT;
+	else if (!strcmp(arg, "yes"))
+		reencode_mode = REENCODE_PLEASE;
+	else if (!strcmp(arg, "no"))
+		reencode_mode = REENCODE_NEVER;
+	else
+		return error("Unknown reencoding mode: %s", arg);
+	return 0;
+}
+
 static struct decoration idnums;
 static uint32_t last_idnum;
 
@@ -633,10 +648,21 @@ static void handle_commit(struct commit *commit, struct rev_info *rev,
 	}
 
 	mark_next_object(&commit->object);
-	if (anonymize)
+	if (anonymize) {
 		reencoded = anonymize_commit_message(message);
-	else if (!is_encoding_utf8(encoding))
-		reencoded = reencode_string(message, "UTF-8", encoding);
+	} else if (encoding) {
+		switch(reencode_mode) {
+		case REENCODE_PLEASE:
+			reencoded = reencode_string(message, "UTF-8", encoding);
+			break;
+		case REENCODE_NEVER:
+			break;
+		case REENCODE_ABORT:
+			die("Encountered commit-specific encoding %s in commit "
+			    "%s; use --reencode=<mode> to handle it",
+			    encoding, oid_to_hex(&commit->object.oid));
+		}
+	}
 	if (!commit->parents)
 		printf("reset %s\n", refname);
 	printf("commit %s\nmark :%"PRIu32"\n", refname, last_idnum);
@@ -1091,6 +1117,9 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 		OPT_CALLBACK(0, "tag-of-filtered-object", &tag_of_filtered_mode, N_("mode"),
 			     N_("select handling of tags that tag filtered objects"),
 			     parse_opt_tag_of_filtered_mode),
+		OPT_CALLBACK(0, "reencode", &reencode_mode, N_("mode"),
+			     N_("select handling of commit messages in an alternate encoding"),
+			     parse_opt_reencode_mode),
 		OPT_STRING(0, "export-marks", &export_filename, N_("file"),
 			     N_("Dump marks to this file")),
 		OPT_STRING(0, "import-marks", &import_filename, N_("file"),
diff --git a/t/t9350-fast-export.sh b/t/t9350-fast-export.sh
index 975c8c4014..4774926bb6 100755
--- a/t/t9350-fast-export.sh
+++ b/t/t9350-fast-export.sh
@@ -94,7 +94,7 @@ test_expect_success 'fast-export --show-original-ids | git fast-import' '
 	test $MUSS = $(git rev-parse --verify refs/tags/muss)
 '
 
-test_expect_success 'iso-8859-7' '
+test_expect_success 'reencoding iso-8859-7' '
 
 	test_when_finished "git reset --hard HEAD~1" &&
 	test_when_finished "git config --unset i18n.commitencoding" &&
@@ -102,7 +102,7 @@ test_expect_success 'iso-8859-7' '
 	test_tick &&
 	echo rosten >file &&
 	git commit -s -m "$(printf "Pi: \360")" file &&
-	git fast-export wer^..wer >iso-8859-7.fi &&
+	git fast-export --reencode=yes wer^..wer >iso-8859-7.fi &&
 	sed "s/wer/i18n/" iso-8859-7.fi |
 		(cd new &&
 		 git fast-import &&
@@ -110,6 +110,31 @@ test_expect_success 'iso-8859-7' '
 		 grep $(printf "\317\200") actual)
 '
 
+test_expect_success 'aborting on iso-8859-7' '
+
+	test_when_finished "git reset --hard HEAD~1" &&
+	test_when_finished "git config --unset i18n.commitencoding" &&
+	git config i18n.commitencoding iso-8859-7 &&
+	echo rosten >file &&
+	git commit -s -m "$(printf "Pi: \360")" file &&
+	test_must_fail git fast-export --reencode=abort wer^..wer >iso-8859-7.fi
+'
+
+test_expect_success 'preserving iso-8859-7' '
+
+	test_when_finished "git reset --hard HEAD~1" &&
+	test_when_finished "git config --unset i18n.commitencoding" &&
+	git config i18n.commitencoding iso-8859-7 &&
+	echo rosten >file &&
+	git commit -s -m "$(printf "Pi: \360")" file &&
+	git fast-export --reencode=no wer^..wer >iso-8859-7.fi &&
+	sed "s/wer/i18n-no-recoding/" iso-8859-7.fi |
+		(cd new &&
+		 git fast-import &&
+		 git cat-file commit i18n-no-recoding >actual &&
+		 grep $(printf "\360") actual)
+'
+
 test_expect_success 'encoding preserved if reencoding fails' '
 
 	test_when_finished "git reset --hard HEAD~1" &&
@@ -117,7 +142,7 @@ test_expect_success 'encoding preserved if reencoding fails' '
 	git config i18n.commitencoding iso-8859-7 &&
 	echo rosten >file &&
 	git commit -s -m "$(printf "Pi: \360; Invalid: \377")" file &&
-	git fast-export wer^..wer >iso-8859-7.fi &&
+	git fast-export --reencode=yes wer^..wer >iso-8859-7.fi &&
 	sed "s/wer/i18n-invalid/" iso-8859-7.fi |
 		(cd new &&
 		 git fast-import &&
-- 
2.21.0.779.g2f4b9c5032