bisect, cmd/bisect: add new library and tool

cmd/bisect automates culprit finding in a large set of
independent changes that together provoke a failure
(either when all enabled or when all disabled, but not both).
By repeating a target command with different subsets of
the changes enabled, bisect identifies the smallest number
of changes that need to be toggled away from the passing
state to cause the failing state.

Package bisect provides functionality to help target commands that
want to support running under cmd/bisect.

This is based on work done by khr and drchase in the Go compiler
and by drchase in github.com/dr2chase/gossahash,
but generalized to support other kinds of targets and
simplify the invocation.

This tool will be useful for finding call sites where a GODEBUG
setting changes a test outcome, as well as source code locations
where applying per-iteration loop semantics changes a test outcome.

Package bisect could use some direct tests of its own, but it is tested
quite a bit via the cmd/bisect test.

Change-Id: Id29efad9936bebee17c1475d92cb167019905aa4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/491875
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: David Chase <drchase@google.com>

Author: rsc

bisect/bisect.go

@@ -0,0 +1,35 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bisect
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// In order for package bisect to be copied into the standard library
+// and used by very low-level packages such as internal/godebug,
+// it needs to have no imports at all.
+func TestNoImports(t *testing.T) {
+	files, err := filepath.Glob("*.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, file := range files {
+		if strings.HasSuffix(file, "_test.go") {
+			continue
+		}
+		data, err := os.ReadFile(file)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if strings.Contains(string(data), "\nimport") {
+			t.Errorf("%s contains imports; package bisect must not import other packages", file)
+		}
+	}
+}

bisect/bisect_test.go

@@ -0,0 +1,13 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.20
+
+package main
+
+import "os/exec"
+
+func cmdInterrupt(cmd *exec.Cmd) {
+	// cmd.Cancel and cmd.WaitDelay not available before Go 1.20.
+}

cmd/bisect/go119.go

@@ -0,0 +1,26 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.20
+
+package main
+
+import (
+	"os"
+	"os/exec"
+	"time"
+)
+
+func cmdInterrupt(cmd *exec.Cmd) {
+	cmd.Cancel = func() error {
+		// On timeout, send interrupt,
+		// in hopes of shutting down process tree.
+		// Ignore errors sending signal; it's all best effort
+		// and not even implemented on Windows.
+		// TODO(rsc): Maybe use a new process group and kill the whole group?
+		cmd.Process.Signal(os.Interrupt)
+		return nil
+	}
+	cmd.WaitDelay = 2 * time.Second
+}