[release-branch.go1.12] cmd/go/internal/cache: disable builds if GOCACHE is not an absolute path

If GOCACHE is set but is not an absolute path, we cannot build.
And GOCACHE=off also returns the error message "build cache is
disabled by GOCACHE=off".

Fixes #30493

Change-Id: I24f64bc886599ca0acd757acada4714aebe4d3ae
Reviewed-on: https://go-review.googlesource.com/c/164200
Run-TryBot: Baokun Lee <nototon@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 13d24b68)
Reviewed-on: https://go-review.googlesource.com/c/164717
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/cache/default.go

@@ -37,7 +37,7 @@ See golang.org to learn more about Go.
 // the first time Default is called.
 func initDefaultCache() {
 	dir := DefaultDir()
-	if dir == "off" || dir == "" {
+	if dir == "off" {
 		if defaultDirErr != nil {
 			base.Fatalf("build cache is required, but could not be located: %v", defaultDirErr)
 		}
@@ -74,7 +74,12 @@ func DefaultDir() string {
 
 	defaultDirOnce.Do(func() {
 		defaultDir = os.Getenv("GOCACHE")
+		if filepath.IsAbs(defaultDir) || defaultDir == "off" {
+			return
+		}
 		if defaultDir != "" {
+			defaultDir = "off"
+			defaultDirErr = fmt.Errorf("GOCACHE is not an absolute path")
 			return
 		}
 

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

@@ -10,6 +10,11 @@ env HOME=
 ! go build -o triv triv.go
 stderr 'build cache is required, but could not be located: GOCACHE is not defined and .*'
 
+# If GOCACHE is set but is not an absolute path, and we cannot build.
+env GOCACHE=test
+! go build -o triv triv.go
+stderr 'build cache is required, but could not be located: GOCACHE is not an absolute path'
+
 # An explicit GOCACHE=off also disables builds.
 env GOCACHE=off
 ! go build -o triv triv.go