internal/testenv: check that external 'diff' tool is the GNU version

TestVerifyUnified in internal/lsp/diff/difftest requires specific
behaviour of the 'diff' command which is known to be satisfied by
GNU diff. The plan9 'diff' command has no '-u' option, and the
illumos 'diff -u' produces output in a different format. Checking
specifically for the GNU version in the HasTool function ensures
the expected behaviour, and otherwise causes the test to be skipped.

Updates golang/go#38772

Change-Id: I5493fa8cfc48a112dc0b7356618c62d3ccb0366f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/232479
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/lsp/diff/difftest/difftest_test.go

@@ -2,10 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// As of writing illumos uses a version of diff for which `diff -u` reports
-// locations differently from GNU diff.
-// +build !illumos
-
 // Package difftest supplies a set of tests that will operate on any
 // implementation of a diff algorithm as exposed by
 // "golang.org/x/tools/internal/lsp/diff"

internal/testenv/testenv.go

@@ -7,6 +7,7 @@
 package testenv
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -77,6 +78,17 @@ func hasTool(tool string) error {
 		if checkGoGoroot.err != nil {
 			return checkGoGoroot.err
 		}
+
+	case "diff":
+		// Check that diff is the GNU version, needed for the -u argument and
+		// to report missing newlines at the end of files.
+		out, err := exec.Command(tool, "-version").Output()
+		if err != nil {
+			return err
+		}
+		if !bytes.Contains(out, []byte("GNU diffutils")) {
+			return fmt.Errorf("diff is not the GNU version")
+		}
 	}
 
 	return nil