t: add an interoperability test harness

The current test suite is good at letting you test a
particular version of Git. But it's not very good at letting
you test _two_ versions and seeing how they interact (e.g.,
one cloning from the other).

This commit adds a test harness that will build two
arbitrary versions of git and make it easy to call them from
inside your tests. See the README and the example script for
details.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2254,6 +2254,9 @@ endif
 ifdef GIT_PERF_MAKE_OPTS
 	@echo GIT_PERF_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_PERF_MAKE_OPTS)))'\' >>$@+
 endif
+ifdef GIT_INTEROP_MAKE_OPTS
+	@echo GIT_INTEROP_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_INTEROP_MAKE_OPTS)))'\' >>$@+
+endif
 ifdef TEST_GIT_INDEX_VERSION
 	@echo TEST_GIT_INDEX_VERSION=\''$(subst ','\'',$(subst ','\'',$(TEST_GIT_INDEX_VERSION)))'\' >>$@+
 endif

t/interop/.gitignore

@@ -0,0 +1,4 @@
+/trash directory*/
+/test-results/
+/.prove/
+/build/

t/interop/Makefile

@@ -0,0 +1,16 @@
+-include ../../config.mak
+export GIT_TEST_OPTIONS
+
+SHELL_PATH ?= $(SHELL)
+SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
+T = $(sort $(wildcard i[0-9][0-9][0-9][0-9]-*.sh))
+
+all: $(T)
+
+$(T):
+	@echo "*** $@ ***"; '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
+
+clean:
+	rm -rf build "trash directory".* test-results
+
+.PHONY: all clean $(T)

t/interop/README

@@ -0,0 +1,85 @@
+Git version interoperability tests
+==================================
+
+This directory has interoperability tests for git. Each script is
+similar to the normal test scripts found in t/, but with the added twist
+that two special versions of git, "git.a" and "git.b", are available in
+the PATH. Individual tests can then check the interaction between the
+two versions.
+
+When you add a feature that handles backwards compatibility between git
+versions, it's encouraged to add a test here to make sure it behaves as
+you expect.
+
+
+Running Tests
+-------------
+
+The easiest way to run tests is to say "make".  This runs all
+the tests against their default versions.
+
+You can run a single test like:
+
+    $ ./i0000-basic.sh
+    ok 1 - bare git is forbidden
+    ok 2 - git.a version (v1.6.6.3)
+    ok 3 - git.b version (v2.11.1)
+    # passed all 3 test(s)
+    1..3
+
+Each test contains default versions to run against. You may override
+these by setting `GIT_TEST_VERSION_A` and `GIT_TEST_VERSION_B` in the
+environment. Note that not all combinations will give sensible outcomes
+for all tests (e.g., a test checking for a specific old/new interaction
+may want something "old" enough" and something "new" enough; see
+individual tests for details).
+
+Version names should be resolvable as revisions in the current
+repository. They will be exported and built as needed using the
+config.mak files found at the root of your working tree.
+
+The exception is the special version "." which uses the currently-built
+contents of your working tree.
+
+You can set the following variables (in the environment or in your config.mak):
+
+    GIT_INTEROP_MAKE_OPTS
+	Options to pass to `make` when building a git version (e.g.,
+	`-j8`).
+
+You can also pass any command-line options taken by ordinary git tests (e.g.,
+"-v").
+
+
+Naming Tests
+------------
+
+The interop test files are named like:
+
+	iNNNN-short-description.sh
+
+where N is a decimal digit.  The same conventions for choosing NNNN as
+for normal tests apply.
+
+
+Writing Tests
+-------------
+
+An interop test script starts like a normal script, declaring a few
+variables and then including interop-lib.sh (which includes test-lib.sh).
+Besides test_description, you should also set the $VERSION_A and $VERSION_B
+variables to give the default versions to test against. See t0000-basic.sh for
+an example.
+
+You can then use test_expect_success as usual, with a few differences:
+
+  1. The special commands "git.a" and "git.b" correspond to the
+     two versions.
+
+  2. You cannot call a bare "git". This is to prevent accidents where
+     you meant "git.a" or "git.b".
+
+  3. The trash directory is _not_ a git repository by default. You
+     should create one with the appropriate version of git.
+
+At the end of the script, call test_done as usual.

t/interop/i0000-basic.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+# Note that this test only works on real version numbers,
+# as it depends on matching the output to "git version".
+VERSION_A=v1.6.6.3
+VERSION_B=v2.11.1
+
+test_description='sanity test interop library'
+. ./interop-lib.sh
+
+test_expect_success 'bare git is forbidden' '
+	test_must_fail git version
+'
+
+test_expect_success "git.a version ($VERSION_A)" '
+	echo git version ${VERSION_A#v} >expect &&
+	git.a version >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success "git.b version ($VERSION_B)" '
+	echo git version ${VERSION_B#v} >expect &&
+	git.b version >actual &&
+	test_cmp expect actual
+'
+
+test_done

t/interop/interop-lib.sh

@@ -0,0 +1,92 @@
+# Interoperability testing framework. Each script should source
+# this after setting default $VERSION_A and $VERSION_B variables.
+
+. ../../GIT-BUILD-OPTIONS
+INTEROP_ROOT=$(pwd)
+BUILD_ROOT=$INTEROP_ROOT/build
+
+build_version () {
+	if test -z "$1"
+	then
+		echo >&2 "error: test script did not set default versions"
+		return 1
+	fi
+
+	if test "$1" = "."
+	then
+		git rev-parse --show-toplevel
+		return 0
+	fi
+
+	sha1=$(git rev-parse "$1^{tree}") || return 1
+	dir=$BUILD_ROOT/$sha1
+
+	if test -e "$dir/.built"
+	then
+		echo "$dir"
+		return 0
+	fi
+
+	echo >&2 "==> Building $1..."
+
+	mkdir -p "$dir" || return 1
+
+	(cd "$(git rev-parse --show-cdup)" && git archive --format=tar "$sha1") |
+	(cd "$dir" && tar x) ||
+	return 1
+
+	for config in config.mak config.mak.autogen config.status
+	do
+		if test -e "$INTEROP_ROOT/../../$config"
+		then
+			cp "$INTEROP_ROOT/../../$config" "$dir/" || return 1
+		fi
+	done
+
+	(
+		cd "$dir" &&
+		make $GIT_INTEROP_MAKE_OPTS >&2 &&
+		touch .built
+	) || return 1
+
+	echo "$dir"
+}
+
+# Old versions of git don't have bin-wrappers, so let's give a rough emulation.
+wrap_git () {
+	write_script "$1" <<-EOF
+	GIT_EXEC_PATH="$2"
+	export GIT_EXEC_PATH
+	PATH="$2:\$PATH"
+	export GIT_EXEC_PATH
+	exec git "\$@"
+	EOF
+}
+
+generate_wrappers () {
+	mkdir -p .bin &&
+	wrap_git .bin/git.a "$DIR_A" &&
+	wrap_git .bin/git.b "$DIR_B" &&
+	write_script .bin/git <<-\EOF &&
+	echo >&2 fatal: test tried to run generic git
+	exit 1
+	EOF
+	PATH=$(pwd)/.bin:$PATH
+}
+
+VERSION_A=${GIT_TEST_VERSION_A:-$VERSION_A}
+VERSION_B=${GIT_TEST_VERSION_B:-$VERSION_B}
+
+if ! DIR_A=$(build_version "$VERSION_A") ||
+   ! DIR_B=$(build_version "$VERSION_B")
+then
+	echo >&2 "fatal: unable to build git versions"
+	exit 1
+fi
+
+TEST_DIRECTORY=$INTEROP_ROOT/..
+TEST_OUTPUT_DIRECTORY=$INTEROP_ROOT
+TEST_NO_CREATE_REPO=t
+. "$TEST_DIRECTORY"/test-lib.sh
+
+generate_wrappers || die "unable to set up interop test environment"