test_terminal: redirect child process' stdin to a pty

When resuming, git-am detects if we are trying to feed it patches or not
by checking if stdin is a TTY.

However, the test library redirects stdin to /dev/null. This makes it
difficult, for instance, to test the behavior of "git am -3" when
resuming, as git-am will think we are trying to feed it patches and
error out.

Support this use case by extending test-terminal.perl to create a
pseudo-tty for the child process' standard input as well.

Note that due to the way the code is structured, the child's stdin
pseudo-tty will be closed when we finish reading from our stdin. This
means that in the common case, where our stdin is attached to /dev/null,
the child's stdin pseudo-tty will be closed immediately. Some operations
like isatty(), which git-am uses, require the file descriptor to be
open, and hence if the success of the command depends on such functions,
test_terminal's stdin should be redirected to a source with large amount
of data to ensure that the child's stdin is not closed, e.g.

	test_terminal git am --3way </dev/zero

Cc: Jonathan Nieder <[email protected]>
Cc: Jeff King <[email protected]>
Signed-off-by: Paul Tan <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pyokagan

t/test-terminal.perl

@@ -5,15 +5,17 @@
 use IO::Pty;
 use File::Copy;
 
-# Run @$argv in the background with stdio redirected to $out and $err.
+# Run @$argv in the background with stdio redirected to $in, $out and $err.
 sub start_child {
-	my ($argv, $out, $err) = @_;
+	my ($argv, $in, $out, $err) = @_;
 	my $pid = fork;
 	if (not defined $pid) {
 		die "fork failed: $!"
 	} elsif ($pid == 0) {
+		open STDIN, "<&", $in;
 		open STDOUT, ">&", $out;
 		open STDERR, ">&", $err;
+		close $in;
 		close $out;
 		exec(@$argv) or die "cannot exec '$argv->[0]': $!"
 	}
@@ -49,6 +51,17 @@ sub xsendfile {
 	copy($in, $out, 4096) or $!{EIO} or die "cannot copy from child: $!";
 }
 
+sub copy_stdin {
+	my ($in) = @_;
+	my $pid = fork;
+	if (!$pid) {
+		xsendfile($in, \*STDIN);
+		exit 0;
+	}
+	close($in);
+	return $pid;
+}
+
 sub copy_stdio {
 	my ($out, $err) = @_;
 	my $pid = fork;
@@ -67,14 +80,25 @@ sub copy_stdio {
 if ($#ARGV < 1) {
 	die "usage: test-terminal program args";
 }
+my $master_in = new IO::Pty;
 my $master_out = new IO::Pty;
 my $master_err = new IO::Pty;
+$master_in->set_raw();
 $master_out->set_raw();
 $master_err->set_raw();
+$master_in->slave->set_raw();
 $master_out->slave->set_raw();
 $master_err->slave->set_raw();
-my $pid = start_child(\@ARGV, $master_out->slave, $master_err->slave);
+my $pid = start_child(\@ARGV, $master_in->slave, $master_out->slave, $master_err->slave);
+close $master_in->slave;
 close $master_out->slave;
 close $master_err->slave;
+my $in_pid = copy_stdin($master_in);
 copy_stdio($master_out, $master_err);
-exit(finish_child($pid));
+my $ret = finish_child($pid);
+# If the child process terminates before our copy_stdin() process is able to
+# write all of its data to $master_in, the copy_stdin() process could stall.
+# Send SIGTERM to it to ensure it terminates.
+kill 'TERM', $in_pid;
+finish_child($in_pid);
+exit($ret);