sequencer: detect author name errors in read_author_script()

As we parse the author-script file, we check for missing or duplicate
lines for GIT_AUTHOR_NAME, etc. But after reading the whole file, our
final error conditional checks "date_i" twice and "name_i" not at all.
This not only leads to us failing to abort, but we may do an
out-of-bounds read on the string_list array.

The bug goes back to 442c36bd08 (am: improve author-script error
reporting, 2018-10-31), though the code was soon after moved to this
spot by bcd33ec25f (add read_author_script() to libgit, 2018-10-31).
It was presumably just a typo in 442c36bd08.

We'll add test coverage for all the error cases here, though only the
GIT_AUTHOR_NAME ones fail (even in a vanilla build they segfault
consistently, but certainly with SANITIZE=address).

Reported-by: Michael V. Scovetta <michael.scovetta@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -872,7 +872,7 @@ int read_author_script(const char *path, char **name, char **email, char **date,
 		error(_("missing 'GIT_AUTHOR_EMAIL'"));
 	if (date_i == -2)
 		error(_("missing 'GIT_AUTHOR_DATE'"));
-	if (date_i < 0 || email_i < 0 || date_i < 0 || err)
+	if (name_i < 0 || email_i < 0 || date_i < 0 || err)
 		goto finish;
 	*name = kv.items[name_i].util;
 	*email = kv.items[email_i].util;

t/t3438-rebase-broken-files.sh

@@ -0,0 +1,59 @@
+#!/bin/sh
+
+test_description='rebase behavior when on-disk files are broken'
+. ./test-lib.sh
+
+test_expect_success 'set up conflicting branches' '
+	test_commit base file &&
+	git checkout -b branch1 &&
+	test_commit one file &&
+	git checkout -b branch2 HEAD^ &&
+	test_commit two file
+'
+
+create_conflict () {
+	test_when_finished "git rebase --abort" &&
+	git checkout -B tmp branch2 &&
+	test_must_fail git rebase branch1
+}
+
+check_resolve_fails () {
+	echo resolved >file &&
+	git add file &&
+	test_must_fail git rebase --continue
+}
+
+for item in NAME EMAIL DATE
+do
+	test_expect_success "detect missing GIT_AUTHOR_$item" '
+		create_conflict &&
+
+		grep -v $item .git/rebase-merge/author-script >tmp &&
+		mv tmp .git/rebase-merge/author-script &&
+
+		check_resolve_fails
+	'
+done
+
+for item in NAME EMAIL DATE
+do
+	test_expect_success "detect duplicate GIT_AUTHOR_$item" '
+		create_conflict &&
+
+		grep -i $item .git/rebase-merge/author-script >tmp &&
+		cat tmp >>.git/rebase-merge/author-script &&
+
+		check_resolve_fails
+	'
+done
+
+test_expect_success 'unknown key in author-script' '
+	create_conflict &&
+
+	echo "GIT_AUTHOR_BOGUS=${SQ}whatever${SQ}" \
+		>>.git/rebase-merge/author-script &&
+
+	check_resolve_fails
+'
+
+test_done