cmd/fix, cmd/go, cmd/gofmt: refactor common code into new internal diff package

Change-Id: Idac8473d1752059bf2f617fd7a781000ee2c3af4
GitHub-Last-Rev: 02a3aa1a3241d3ed4422518f1c954cd54bbe858e
GitHub-Pull-Request: golang/go#35141
Reviewed-on: https://go-review.googlesource.com/c/go/+/203218
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 58 insertions, 0 deletions

diff --git a/src/cmd/internal/diff/diff.go b/src/cmd/internal/diff/diff.go
new file mode 100644
index 0000000000..e9d2c23780
--- /dev/null
+++ b/src/cmd/internal/diff/diff.go
@@ -0,0 +1,58 @@
+// Copyright 2019 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 diff implements a Diff function that compare two inputs
+// using the 'diff' tool.
+package diff
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"runtime"
+)
+
+// Returns diff of two arrays of bytes in diff tool format.
+func Diff(prefix string, b1, b2 []byte) ([]byte, error) {
+	f1, err := writeTempFile(prefix, b1)
+	if err != nil {
+		return nil, err
+	}
+	defer os.Remove(f1)
+
+	f2, err := writeTempFile(prefix, b2)
+	if err != nil {
+		return nil, err
+	}
+	defer os.Remove(f2)
+
+	cmd := "diff"
+	if runtime.GOOS == "plan9" {
+		cmd = "/bin/ape/diff"
+	}
+
+	data, err := exec.Command(cmd, "-u", f1, f2).CombinedOutput()
+	if len(data) > 0 {
+		// diff exits with a non-zero status when the files don't match.
+		// Ignore that failure as long as we get output.
+		err = nil
+	}
+	return data, err
+}
+
+func writeTempFile(prefix string, data []byte) (string, error) {
+	file, err := ioutil.TempFile("", prefix)
+	if err != nil {
+		return "", err
+	}
+	_, err = file.Write(data)
+	if err1 := file.Close(); err == nil {
+		err = err1
+	}
+	if err != nil {
+		os.Remove(file.Name())
+		return "", err
+	}
+	return file.Name(), nil
+}