chainlint.pl: don't flag broken &&-chain if $? handled explicitly

There are cases in which tests capture and check a command's exit code
explicitly without employing test_expect_code(). They do so by
intentionally breaking the &&-chain since it would be impossible to
capture "$?" in the failing case if the `status=$?` assignment was part
of the &&-chain. Since such constructs are manually checking the exit
code, their &&-chain breakage is legitimate and safe, thus should not be
flagged. Therefore, stop flagging &&-chain breakage in such cases.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/chainlint.pl

@@ -497,6 +497,12 @@ sub accumulate {
 	# did previous command end with "&&", "|", "|| return" or similar?
 	goto DONE if match_ending($tokens, \@safe_endings);
 
+	# if this command handles "$?" specially, then okay for previous
+	# command to be missing "&&"
+	for my $token (@$cmd) {
+		goto DONE if $token =~ /\$\?/;
+	}
+
 	# flag missing "&&" at end of previous command
 	my $n = find_non_nl($tokens);
 	splice(@$tokens, $n + 1, 0, '?!AMP?!') unless $n < 0;

t/chainlint/chain-break-status.expect

@@ -0,0 +1,9 @@
+OUT=$(( ( large_git ; echo $? 1 >& 3 ) | : ) 3 >& 1) &&
+test_match_signal 13 "$OUT" &&
+
+{ test-tool sigchain > actual ; ret=$? ; } &&
+{
+	test_match_signal 15 "$ret" ||
+	test "$ret" = 3
+} &&
+test_cmp expect actual

t/chainlint/chain-break-status.test

@@ -0,0 +1,11 @@
+# LINT: broken &&-chain okay if next command handles "$?" explicitly
+OUT=$( ((large_git; echo $? 1>&3) | :) 3>&1 ) &&
+test_match_signal 13 "$OUT" &&
+
+# LINT: broken &&-chain okay if next command handles "$?" explicitly
+{ test-tool sigchain >actual; ret=$?; } &&
+{
+	test_match_signal 15 "$ret" ||
+	test "$ret" = 3
+} &&
+test_cmp expect actual