tests: Add tests for automatic use of pager

Git’s automatic pagination support has some subtleties.  Add some
tests to make sure we don’t break:

 - when git will use a pager by default;
 - the effect of the --paginate and --no-pager options;
 - the effect of pagination on use of color;
 - how the choice of pager is configured.

This does not yet test:

 - use of pager by scripted commands (git svn and git am);
 - effect of the pager.* configuration variables;
 - setting of the LESS variable.

Some features involve checking whether stdout is a terminal, so many
of these tests are skipped unless output is passed through to the
terminal (i.e., unless $GIT_TEST_OPTS includes --verbose).

The immediate purpose for these tests was to avoid making things worse
after the breakage from my jn/editor-pager series (see commit 376f39,
2009-11-20).  Thanks to Sebastian Celis <sebastian@sebastiancelis.com>
for the report.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7006-pager.sh

@@ -0,0 +1,163 @@
+#!/bin/sh
+
+test_description='Test automatic use of a pager.'
+
+. ./test-lib.sh
+
+rm -f stdout_is_tty
+test_expect_success 'is stdout a terminal?' '
+	if test -t 1
+	then
+		: > stdout_is_tty
+	fi
+'
+
+if test -e stdout_is_tty
+then
+	test_set_prereq TTY
+else
+	say stdout is not a terminal, so skipping some tests.
+fi
+
+unset GIT_PAGER GIT_PAGER_IN_USE
+git config --unset core.pager
+PAGER='cat > paginated.out'
+export PAGER
+
+test_expect_success 'setup' '
+	test_commit initial
+'
+
+rm -f paginated.out
+test_expect_success TTY 'some commands use a pager' '
+	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 -e paginated.out
+'
+
+rm -f paginated.out
+test_expect_success 'no pager when stdout is a pipe' '
+	git log | cat &&
+	! test -e paginated.out
+'
+
+rm -f paginated.out
+test_expect_success 'no pager when stdout is a regular file' '
+	git log > file &&
+	! test -e paginated.out
+'
+
+rm -f paginated.out
+test_expect_success TTY 'git --paginate rev-list uses a pager' '
+	git --paginate rev-list HEAD  &&
+	test -e paginated.out
+'
+
+rm -f file paginated.out
+test_expect_success 'no pager even with --paginate when stdout is a pipe' '
+	git --paginate log | cat &&
+	! test -e paginated.out
+'
+
+rm -f paginated.out
+test_expect_success TTY 'no pager with --no-pager' '
+	git --no-pager log &&
+	! test -e paginated.out
+'
+
+# A colored commit log will begin with an appropriate ANSI escape
+# for the first color; the text "commit" comes later.
+colorful() {
+	read firstline < $1
+	! expr "$firstline" : "^[a-zA-Z]" >/dev/null
+}
+
+rm -f colorful.log colorless.log
+test_expect_success 'tests can detect color' '
+	git log --no-color > colorless.log &&
+	git log --color > colorful.log &&
+	! colorful colorless.log &&
+	colorful colorful.log
+'
+
+rm -f colorless.log
+git config color.ui auto
+test_expect_success 'no color when stdout is a regular file' '
+	git log > colorless.log &&
+	! colorful colorless.log
+'
+
+rm -f paginated.out
+git config color.ui auto
+test_expect_success TTY 'color when writing to a pager' '
+	TERM=vt100 git log &&
+	colorful paginated.out
+'
+
+rm -f colorful.log
+git config color.ui auto
+test_expect_success 'color when writing to a file intended for a pager' '
+	TERM=vt100 GIT_PAGER_IN_USE=true git log > colorful.log &&
+	colorful colorful.log
+'
+
+unset PAGER GIT_PAGER
+git config --unset core.pager
+test_expect_success 'determine default pager' '
+	less=$(git var GIT_PAGER) &&
+	test -n "$less"
+'
+
+if expr "$less" : '^[a-z]*$' > /dev/null && test_have_prereq TTY
+then
+	test_set_prereq SIMPLEPAGER
+fi
+
+unset PAGER GIT_PAGER
+git config --unset core.pager
+rm -f default_pager_used
+test_expect_success SIMPLEPAGER 'default pager is used by default' '
+	cat > $less <<-EOF &&
+	#!$SHELL_PATH
+	: > default_pager_used
+	EOF
+	chmod +x $less &&
+	PATH=.:$PATH git log &&
+	test -e default_pager_used
+'
+
+unset GIT_PAGER
+git config --unset core.pager
+rm -f PAGER_used
+test_expect_success TTY 'PAGER overrides default pager' '
+	PAGER=": > PAGER_used" &&
+	export PAGER &&
+	git log &&
+	test -e PAGER_used
+'
+
+unset GIT_PAGER
+rm -f core.pager_used
+test_expect_success TTY 'core.pager overrides PAGER' '
+	PAGER=: &&
+	export PAGER &&
+	git config core.pager ": > core.pager_used" &&
+	git log &&
+	test -e core.pager_used
+'
+
+rm -f GIT_PAGER_used
+test_expect_success TTY 'GIT_PAGER overrides core.pager' '
+	git config core.pager : &&
+	GIT_PAGER=": > GIT_PAGER_used" &&
+	export GIT_PAGER &&
+	git log &&
+	test -e GIT_PAGER_used
+'
+
+test_done