cmd/go: reject relative paths in GOMODCACHE environment

Go already rejects relative paths in a couple environment variables,
It should reject relative paths in GOMODCACHE.

Fixes #43715

Change-Id: Id1ceff839c7ab21c00cf4ace45ce48324733a526
Reviewed-on: https://go-review.googlesource.com/c/go/+/284432
Run-TryBot: Baokun Lee <bk@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Baokun Lee <bk@golangcn.org>

src/cmd/go/internal/envcmd/env.go

@@ -428,7 +428,7 @@ func checkEnvWrite(key, val string) error {
 			return fmt.Errorf("GOPATH entry is relative; must be absolute path: %q", val)
 		}
 	// Make sure CC and CXX are absolute paths
-	case "CC", "CXX":
+	case "CC", "CXX", "GOMODCACHE":
 		if !filepath.IsAbs(val) && val != "" && val != filepath.Base(val) {
 			return fmt.Errorf("%s entry is relative; must be absolute path: %q", key, val)
 		}

src/cmd/go/internal/modfetch/cache.go

@@ -28,10 +28,8 @@ import (
 )
 
 func cacheDir(path string) (string, error) {
-	if cfg.GOMODCACHE == "" {
+	if err := checkCacheDir(); err != nil {
-		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
+		return "", err
-		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
-		return "", fmt.Errorf("internal error: cfg.GOMODCACHE not set")
 	}
 	enc, err := module.EscapePath(path)
 	if err != nil {
@@ -64,10 +62,8 @@ func CachePath(m module.Version, suffix string) (string, error) {
 // along with the directory if the directory does not exist or if the directory
 // is not completely populated.
 func DownloadDir(m module.Version) (string, error) {
-	if cfg.GOMODCACHE == "" {
+	if err := checkCacheDir(); err != nil {
-		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
+		return "", err
-		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
-		return "", fmt.Errorf("internal error: cfg.GOMODCACHE not set")
 	}
 	enc, err := module.EscapePath(m.Path)
 	if err != nil {
@@ -134,10 +130,8 @@ func lockVersion(mod module.Version) (unlock func(), err error) {
 // user's working directory.
 // If err is nil, the caller MUST eventually call the unlock function.
 func SideLock() (unlock func(), err error) {
-	if cfg.GOMODCACHE == "" {
+	if err := checkCacheDir(); err != nil {
-		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
+		base.Fatalf("go: %v", err)
-		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
-		base.Fatalf("go: internal error: cfg.GOMODCACHE not set")
 	}
 
 	path := filepath.Join(cfg.GOMODCACHE, "cache", "lock")
@@ -633,3 +627,16 @@ func rewriteVersionList(dir string) {
 		base.Fatalf("go: failed to write version list: %v", err)
 	}
 }
+
+func checkCacheDir() error {
+	if cfg.GOMODCACHE == "" {
+		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
+		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
+		return fmt.Errorf("internal error: cfg.GOMODCACHE not set")
+	}
+
+	if !filepath.IsAbs(cfg.GOMODCACHE) {
+		return fmt.Errorf("GOMODCACHE entry is relative; must be absolute path: %q.\n", cfg.GOMODCACHE)
+	}
+	return nil
+}

src/cmd/go/internal/modfetch/fetch.go

@@ -37,10 +37,8 @@ var downloadCache par.Cache
 // local download cache and returns the name of the directory
 // corresponding to the root of the module's file tree.
 func Download(ctx context.Context, mod module.Version) (dir string, err error) {
-	if cfg.GOMODCACHE == "" {
+	if err := checkCacheDir(); err != nil {
-		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
+		base.Fatalf("go: %v", err)
-		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
-		base.Fatalf("go: internal error: cfg.GOMODCACHE not set")
 	}
 
 	// The par.Cache here avoids duplicate work.

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

@@ -173,3 +173,9 @@ go env -w GOOS=linux GOARCH=mips
 env GOOS=windows
 ! go env -u GOOS
 stderr 'unsupported GOOS/GOARCH.*windows/mips$'
+
+# go env -w should reject relative paths in GOMODCACHE environment.
+! go env -w GOMODCACHE=~/test
+stderr 'go env -w: GOMODCACHE entry is relative; must be absolute path: "~/test"'
+! go env -w GOMODCACHE=./test
+stderr 'go env -w: GOMODCACHE entry is relative; must be absolute path: "./test"'

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

@@ -0,0 +1,11 @@
+env GO111MODULE=on
+
+# Go should reject relative paths in GOMODCACHE environment.
+
+env GOMODCACHE="~/test"
+! go get example.com/tools/cmd/hello
+stderr 'must be absolute path'
+
+env GOMODCACHE="./test"
+! go get example.com/tools/cmd/hello
+stderr 'must be absolute path'