all: zero mtimes in testdata gz files

Fixes #14937.

Change-Id: Iea11a32230d44d5a43f8aec812d25f13bce85895
Reviewed-on: https://go-review.googlesource.com/21038Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/compress/gzip/issue14937_test.go

+package gzip
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+// Per golang.org/issue/14939, check that every .gz file
+// in the tree has a zero mtime.
+func TestGZIPFilesHaveZeroMTimes(t *testing.T) {
+	var files []string
+	err := filepath.Walk(runtime.GOROOT(), func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if !info.IsDir() && strings.HasSuffix(path, ".gz") {
+			files = append(files, path)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal("error collecting list of .gz files in GOROOT: ", err)
+	}
+	if len(files) == 0 {
+		t.Fatal("expected to find some .gz files under GOROOT")
+	}
+	for _, path := range files {
+		checkZeroMTime(t, path)
+	}
+}
+
+func checkZeroMTime(t *testing.T, path string) {
+	f, err := os.Open(path)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer f.Close()
+	gz, err := NewReader(f)
+	if err != nil {
+		t.Errorf("cannot read gzip file %s: %s", path, err)
+		return
+	}
+	defer gz.Close()
+	if !gz.ModTime.Equal(time.Unix(0, 0)) {
+		t.Errorf("gzip file %s has non-zero mtime (%s)", path, gz.ModTime)
+	}
+}