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.

2 files changed, 153 insertions, 7 deletions

diff --git a/pkg/bisect/bisect.go b/pkg/bisect/bisect.go
index 78cb58bd4..904be4b53 100644
--- a/pkg/bisect/bisect.go
+++ b/pkg/bisect/bisect.go
@@ -55,7 +55,7 @@ type env struct {
 	repo      vcs.Repo
 	bisecter  vcs.Bisecter
 	head      *vcs.Commit
-	inst      *instance.Env
+	inst      instance.BuilderTester
 	numTests  int
 	buildTime time.Duration
 	testTime  time.Duration
@@ -85,11 +85,30 @@ func Run(cfg *Config) ([]*vcs.Commit, *report.Report, error) {
 	if !ok {
 		return nil, nil, fmt.Errorf("bisection is not implemented for %v", cfg.Manager.TargetOS)
 	}
+	inst, err := instance.NewEnv(&cfg.Manager)
+	if err != nil {
+		return nil, nil, err
+	}
+	if _, err = repo.CheckoutBranch(cfg.Kernel.Repo, cfg.Kernel.Branch); err != nil {
+		return nil, nil, err
+	}
+
+	return runImpl(cfg, repo, bisecter, inst)
+}
+
+func runImpl(cfg *Config, repo vcs.Repo, bisecter vcs.Bisecter, inst instance.BuilderTester) (
+	[]*vcs.Commit, *report.Report, error) {
 	env := &env{
 		cfg:      cfg,
 		repo:     repo,
 		bisecter: bisecter,
+		inst:     inst,
 	}
+	head, err := repo.HeadCommit()
+	if err != nil {
+		return nil, nil, err
+	}
+	env.head = head
 	if cfg.Fix {
 		env.log("bisecting fixing commit since %v", cfg.Kernel.Commit)
 	} else {
@@ -135,12 +154,6 @@ func Run(cfg *Config) ([]*vcs.Commit, *report.Report, error) {
 func (env *env) bisect() ([]*vcs.Commit, *report.Report, error) {
 	cfg := env.cfg
 	var err error
-	if env.inst, err = instance.NewEnv(&cfg.Manager); err != nil {
-		return nil, nil, err
-	}
-	if env.head, err = env.repo.CheckoutBranch(cfg.Kernel.Repo, cfg.Kernel.Branch); err != nil {
-		return nil, nil, err
-	}
 	if err := build.Clean(cfg.Manager.TargetOS, cfg.Manager.TargetVMArch,
 		cfg.Manager.Type, cfg.Manager.KernelSrc); err != nil {
 		return nil, nil, fmt.Errorf("kernel clean failed: %v", err)
diff --git a/pkg/bisect/bisect_test.go b/pkg/bisect/bisect_test.go
new file mode 100644
index 000000000..472fde289
--- /dev/null
+++ b/pkg/bisect/bisect_test.go
@@ -0,0 +1,133 @@
+// Copyright 2019 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package bisect
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strconv"
+	"testing"
+
+	"github.com/google/syzkaller/pkg/build"
+	"github.com/google/syzkaller/pkg/instance"
+	"github.com/google/syzkaller/pkg/mgrconfig"
+	"github.com/google/syzkaller/pkg/report"
+	"github.com/google/syzkaller/pkg/vcs"
+)
+
+// testEnv will implement instance.BuilderTester. This allows us to
+// set bisect.env.inst to a testEnv object.
+type testEnv struct {
+	repo *vcs.TestRepo
+	r    vcs.Repo
+	t    *testing.T
+	// TODO: add a "fix bool" here so that Test() can return results according to
+	// whether fix/cause bisection is happening.
+}
+
+func (env *testEnv) BuildSyzkaller(repo, commit string) error {
+	return nil
+}
+
+func (env *testEnv) BuildKernel(compilerBin, userspaceDir, cmdlineFile, sysctlFile string,
+	kernelConfig []byte) (string, error) {
+	return "", nil
+}
+
+func (env *testEnv) Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error) {
+	hc, err := env.r.HeadCommit()
+	if err != nil {
+		env.t.Fatal(err)
+	}
+	// For cause bisection, if newer than or equal to 602, it crashes.
+	// -- 602 is the cause commit.
+	// TODO: for fix bisection(check env.fix), if older than 602, it crashes.
+	// -- 602 is the fix commit.
+	val, err := strconv.Atoi(hc.Title)
+	if err != nil {
+		env.t.Fatalf("invalid commit title: %v", val)
+	}
+	if val >= 602 {
+		var errors []error
+		for i := 0; i < numVMs; i++ {
+			errors = append(errors, &instance.CrashError{
+				Report: &report.Report{
+					Title: fmt.Sprintf("crashes at %v", hc.Title),
+				},
+			})
+		}
+		return errors, nil
+	}
+	var errors []error
+	for i := 0; i < numVMs; i++ {
+		errors = append(errors, nil)
+	}
+	return errors, nil
+}
+
+// TestBisectCause tests that bisection returns the correct cause
+// commit.
+func TestBisectCause(t *testing.T) {
+	t.Parallel()
+	build.CompilerIdentity = func(_ string) (string, error) {
+		return "unused-compiler-identity", nil
+	}
+	baseDir, err := ioutil.TempDir("", "syz-git-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(baseDir)
+	originRepo := vcs.CreateTestRepo(t, baseDir, "originRepo")
+	for rv := 4; rv < 10; rv++ {
+		for i := 0; i < 6; i++ {
+			originRepo.CommitChange(fmt.Sprintf("%v", rv*100+i))
+			if i == 0 {
+				originRepo.SetTag(fmt.Sprintf("v%v.0", rv))
+			}
+		}
+	}
+	repo := vcs.CloneTestRepo(t, baseDir, "repo", originRepo)
+	r, err := vcs.NewRepo("test", "64", repo.Dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	head, err := r.HeadCommit()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cfg := &Config{
+		Fix:   false,
+		Trace: new(bytes.Buffer),
+		Manager: mgrconfig.Config{
+			TargetOS:     "test",
+			TargetVMArch: "64",
+			Type:         "qemu",
+			KernelSrc:    repo.Dir,
+		},
+		Kernel: KernelConfig{
+			Commit: head.Hash,
+			Repo:   originRepo.Dir,
+		},
+	}
+	inst := &testEnv{
+		repo: repo,
+		r:    r,
+		t:    t,
+	}
+	commits, rep, err := runImpl(cfg, r, r.(vcs.Bisecter), inst)
+	if err != nil {
+		t.Fatalf("returned error: '%v'", err)
+	}
+	if len(commits) != 1 {
+		t.Fatalf("Got %d commits: %v", len(commits), commits)
+	}
+	if commits[0].Title != "602" {
+		t.Fatalf("Expected commit '602' got '%v'", commits[0].Title)
+	}
+	if rep == nil {
+		t.Fatal("returned rep==nil, report should not be empty")
+	}
+}