t7006-pager: if stdout is not a terminal, make a new one

Testing pagination requires (fake or real) access to a terminal so we
can see whether the pagination automatically kicks in, which makes it
hard to get good coverage when running tests without --verbose.  There
are a number of ways to work around that:

 - Replace all isatty calls with calls to a custom xisatty wrapper
   that usually checks for a terminal but can be overridden for tests.
   This would be workable, but it would require implementing xisatty
   separately in three languages (C, shell, and perl) and making sure
   that any code that is to be tested always uses the wrapper.

 - Redirect stdout to /dev/tty.  This would be problematic because
   there might be no terminal available, and even if a terminal is
   available, it might not be appropriate to spew output to it.

 - Create a new pseudo-terminal on the fly and capture its output.

This patch implements the third approach.

The new test-terminal.perl helper uses IO::Pty from Expect.pm to create
a terminal and executes the program specified by its arguments with
that terminal as stdout.  If the IO::Pty module is missing or not
working on a system, the test script will maintain its old behavior
(skipping most of its tests unless GIT_TEST_OPTS includes --verbose).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7006-pager.sh

@@ -5,18 +5,31 @@ test_description='Test automatic use of a pager.'
 . ./test-lib.sh
 
 rm -f stdout_is_tty
-test_expect_success 'is stdout a terminal?' '
+test_expect_success 'set up terminal for tests' '
 	if test -t 1
 	then
 		: > stdout_is_tty
+	elif
+		test_have_prereq PERL &&
+		"$PERL_PATH" "$TEST_DIRECTORY"/t7006/test-terminal.perl \
+			sh -c "test -t 1"
+	then
+		: > test_terminal_works
 	fi
 '
 
 if test -e stdout_is_tty
 then
+	test_terminal() { "$@"; }
+	test_set_prereq TTY
+elif test -e test_terminal_works
+then
+	test_terminal() {
+		"$PERL_PATH" "$TEST_DIRECTORY"/t7006/test-terminal.perl "$@"
+	}
 	test_set_prereq TTY
 else
-	say stdout is not a terminal, so skipping some tests.
+	say no usable terminal, so skipping some tests
 fi
 
 unset GIT_PAGER GIT_PAGER_IN_USE
@@ -30,13 +43,13 @@ test_expect_success 'setup' '
 
 rm -f paginated.out
 test_expect_success TTY 'some commands use a pager' '
-	git log &&
+	test_terminal git log &&
 	test -e paginated.out
 '
 
 rm -f paginated.out
 test_expect_success TTY 'some commands do not use a pager' '
-	git rev-list HEAD &&
+	test_terminal git rev-list HEAD &&
 	! test -e paginated.out
 '
 
@@ -54,7 +67,7 @@ test_expect_success 'no pager when stdout is a regular file' '
 
 rm -f paginated.out
 test_expect_success TTY 'git --paginate rev-list uses a pager' '
-	git --paginate rev-list HEAD  &&
+	test_terminal git --paginate rev-list HEAD &&
 	test -e paginated.out
 '
 
@@ -66,7 +79,7 @@ test_expect_success 'no pager even with --paginate when stdout is a pipe' '
 
 rm -f paginated.out
 test_expect_success TTY 'no pager with --no-pager' '
-	git --no-pager log &&
+	test_terminal git --no-pager log &&
 	! test -e paginated.out
 '
 
@@ -95,7 +108,7 @@ test_expect_success 'no color when stdout is a regular file' '
 rm -f paginated.out
 git config color.ui auto
 test_expect_success TTY 'color when writing to a pager' '
-	TERM=vt100 git log &&
+	TERM=vt100 test_terminal git log &&
 	colorful paginated.out
 '
 
@@ -127,7 +140,7 @@ test_expect_success SIMPLEPAGER 'default pager is used by default' '
 	: > default_pager_used
 	EOF
 	chmod +x $less &&
-	PATH=.:$PATH git log &&
+	PATH=.:$PATH test_terminal git log &&
 	test -e default_pager_used
 '
 
@@ -137,7 +150,7 @@ rm -f PAGER_used
 test_expect_success TTY 'PAGER overrides default pager' '
 	PAGER=": > PAGER_used" &&
 	export PAGER &&
-	git log &&
+	test_terminal git log &&
 	test -e PAGER_used
 '
 
@@ -147,7 +160,7 @@ test_expect_success TTY 'core.pager overrides PAGER' '
 	PAGER=: &&
 	export PAGER &&
 	git config core.pager ": > core.pager_used" &&
-	git log &&
+	test_terminal git log &&
 	test -e core.pager_used
 '
 
@@ -156,7 +169,7 @@ test_expect_success TTY 'GIT_PAGER overrides core.pager' '
 	git config core.pager : &&
 	GIT_PAGER=": > GIT_PAGER_used" &&
 	export GIT_PAGER &&
-	git log &&
+	test_terminal git log &&
 	test -e GIT_PAGER_used
 '
 

t/t7006/test-terminal.perl

@@ -0,0 +1,58 @@
+#!/usr/bin/perl
+use strict;
+use warnings;
+use IO::Pty;
+use File::Copy;
+
+# Run @$argv in the background with stdout redirected to $out.
+sub start_child {
+	my ($argv, $out) = @_;
+	my $pid = fork;
+	if (not defined $pid) {
+		die "fork failed: $!"
+	} elsif ($pid == 0) {
+		open STDOUT, ">&", $out;
+		close $out;
+		exec(@$argv) or die "cannot exec '$argv->[0]': $!"
+	}
+	return $pid;
+}
+
+# Wait for $pid to finish.
+sub finish_child {
+	# Simplified from wait_or_whine() in run-command.c.
+	my ($pid) = @_;
+
+	my $waiting = waitpid($pid, 0);
+	if ($waiting < 0) {
+		die "waitpid failed: $!";
+	} elsif ($? & 127) {
+		my $code = $? & 127;
+		warn "died of signal $code";
+		return $code - 128;
+	} else {
+		return $? >> 8;
+	}
+}
+
+sub xsendfile {
+	my ($out, $in) = @_;
+
+	# Note: the real sendfile() cannot read from a terminal.
+
+	# It is unspecified by POSIX whether reads
+	# from a disconnected terminal will return
+	# EIO (as in AIX 4.x, IRIX, and Linux) or
+	# end-of-file.  Either is fine.
+	copy($in, $out, 4096) or $!{EIO} or die "cannot copy from child: $!";
+}
+
+if ($#ARGV < 1) {
+	die "usage: test-terminal program args";
+}
+my $master = new IO::Pty;
+my $slave = $master->slave;
+my $pid = start_child(\@ARGV, $slave);
+close $slave;
+xsendfile(\*STDOUT, $master);
+exit(finish_child($pid));