cmd/go/internal/{clean,test}: lock testexpire.txt

Also check to make sure we don't overwrite a newer timestamp with an
older one.

testexpire.txt may be written concurrently, and a partially-written
timestamp may appear much older than the actual intended one. We don't
want to re-run tests that should still be cached.

Updates #26794

Change-Id: If56348e799f0e7be3c5bc91b4a336e23ad99f791
Reviewed-on: https://go-review.googlesource.com/c/146379
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/clean/clean.go

@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -17,6 +18,7 @@ import (
 	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
+	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/work"
@@ -146,7 +148,20 @@ func runClean(cmd *base.Command, args []string) {
 		// right now are to be ignored.
 		dir := cache.DefaultDir()
 		if dir != "off" {
-			err := ioutil.WriteFile(filepath.Join(dir, "testexpire.txt"), []byte(fmt.Sprintf("%d\n", time.Now().UnixNano())), 0666)
+			f, err := lockedfile.Edit(filepath.Join(dir, "testexpire.txt"))
+			if err == nil {
+				now := time.Now().UnixNano()
+				buf, _ := ioutil.ReadAll(f)
+				prev, _ := strconv.ParseInt(strings.TrimSpace(string(buf)), 10, 64)
+				if now > prev {
+					if err = f.Truncate(0); err == nil {
+						_, err = fmt.Fprintf(f, "%d\n", now)
+					}
+				}
+				if closeErr := f.Close(); err == nil {
+					err = closeErr
+				}
+			}
 			if err != nil {
 				base.Errorf("go clean -testcache: %v", err)
 			}

src/cmd/go/internal/test/test.go

@@ -27,6 +27,7 @@ import (
 	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
+	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/str"
 	"cmd/go/internal/work"
@@ -566,7 +567,7 @@ func runTest(cmd *base.Command, args []string) {
 	// (We implement go clean -testcache by writing an expiration date
 	// instead of searching out and deleting test result cache entries.)
 	if dir := cache.DefaultDir(); dir != "off" {
-		if data, _ := ioutil.ReadFile(filepath.Join(dir, "testexpire.txt")); len(data) > 0 && data[len(data)-1] == '\n' {
+		if data, _ := lockedfile.Read(filepath.Join(dir, "testexpire.txt")); len(data) > 0 && data[len(data)-1] == '\n' {
 			if t, err := strconv.ParseInt(string(data[:len(data)-1]), 10, 64); err == nil {
 				testCacheExpire = time.Unix(0, t)
 			}