cmd/go/internal/clean: fix clean -testcache does not clean test cache

Truncate changes the size of the file. It does not change the I/O offset.

Fixes #29757

Change-Id: I1aa9223a86d6a8ce3c0efc3ac1d7d7647b77f589
Reviewed-on: https://go-review.googlesource.com/c/158117
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -152,7 +152,9 @@ func runClean(cmd *base.Command, args []string) {
 				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 _, err = f.Seek(0, 0); err == nil {
+							_, err = fmt.Fprintf(f, "%d\n", now)
+						}
 					}
 				}
 				if closeErr := f.Close(); err == nil {

src/cmd/go/testdata/script/clean_testcache.txt

@@ -0,0 +1,16 @@
+# go clean -testcache
+# should work (see golang.org/issue/29757).
+cd x
+go test x_test.go
+go clean -testcache
+go test x_test.go
+! stdout 'cached'
+
+
+-- x/x_test.go --
+package x_test
+import (
+    "testing"
+)
+func TestMain(t *testing.T) {
+}