pkg/bisect: add initial testing support for cause bisection

(note: incomplete change)

Refactor existing code as follows:
* Move reusable test utility functions from git_repo_test.go to
pkg/vcs/test_util.go and make them exported.
* Split Run() into Run()+runImpl().
* Change type of bisect.go:env.inst to `instance.BuilderTester`.
Change usage inside syz-testbuild/testbuild.go accordingly.
* Move most of linux.PreviousReleaseTags() into vcs/git.go as
git.previousReleaseTags().
* Allow build.CompilerIdentity to be mocked.

Introduce the following changes:
* instance.BuilderTester is an interface with methods
	BuildSyzkaller()
	BuildKernel()
	Test()
NewEnv() now returns this interface.
* type testEnv implements instance.BuilderTester.
* type testBuilder implements builder interface. Add a entry into table
inside pkg/build/build.go:getBuilder() to return testBuilder object.

1 files changed, 12 insertions, 6 deletions

diff --git a/pkg/instance/instance.go b/pkg/instance/instance.go
index cc1b147f1..b3e24aace 100644
--- a/pkg/instance/instance.go
+++ b/pkg/instance/instance.go
@@ -27,11 +27,17 @@ import (
 	"github.com/google/syzkaller/vm"
 )
 
-type Env struct {
+type BuilderTester interface {
+	BuildSyzkaller(string, string) error
+	BuildKernel(string, string, string, string, []byte) (string, error)
+	Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error)
+}
+
+type env struct {
 	cfg *mgrconfig.Config
 }
 
-func NewEnv(cfg *mgrconfig.Config) (*Env, error) {
+func NewEnv(cfg *mgrconfig.Config) (BuilderTester, error) {
 	if !vm.AllowsOvercommit(cfg.Type) {
 		return nil, fmt.Errorf("test instances are not supported for %v VMs", cfg.Type)
 	}
@@ -47,13 +53,13 @@ func NewEnv(cfg *mgrconfig.Config) (*Env, error) {
 	if err := osutil.MkdirAll(cfg.Workdir); err != nil {
 		return nil, fmt.Errorf("failed to create tmp dir: %v", err)
 	}
-	env := &Env{
+	env := &env{
 		cfg: cfg,
 	}
 	return env, nil
 }
 
-func (env *Env) BuildSyzkaller(repo, commit string) error {
+func (env *env) BuildSyzkaller(repo, commit string) error {
 	cfg := env.cfg
 	srcIndex := strings.LastIndex(cfg.Syzkaller, "/src/")
 	if srcIndex == -1 {
@@ -82,7 +88,7 @@ func (env *Env) BuildSyzkaller(repo, commit string) error {
 	return nil
 }
 
-func (env *Env) BuildKernel(compilerBin, userspaceDir, cmdlineFile, sysctlFile string,
+func (env *env) BuildKernel(compilerBin, userspaceDir, cmdlineFile, sysctlFile string,
 	kernelConfig []byte) (string, error) {
 	cfg := env.cfg
 	imageDir := filepath.Join(cfg.Workdir, "image")
@@ -170,7 +176,7 @@ func (err *CrashError) Error() string {
 // Test boots numVMs VMs, tests basic kernel operation, and optionally tests the provided reproducer.
 // TestError is returned if there is a problem with kernel/image (crash, reboot loop, etc).
 // CrashError is returned if the reproducer crashes kernel.
-func (env *Env) Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error) {
+func (env *env) Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error) {
 	if err := mgrconfig.Complete(env.cfg); err != nil {
 		return nil, err
 	}