cmd/go: proceed with GOPATH unset if the command doesn't use it

For #43938

Change-Id: I0937b9bb6de3d29d7242ee61f053d4803277dc0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/351329
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>

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

@@ -720,7 +720,7 @@ 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")
+		return fmt.Errorf("module cache not found: neither GOMODCACHE nor GOPATH is set")
 	}
 	if !filepath.IsAbs(cfg.GOMODCACHE) {
 		return fmt.Errorf("GOMODCACHE entry is relative; must be absolute path: %q.\n", cfg.GOMODCACHE)

src/cmd/go/internal/modload/init.go

@@ -246,6 +246,12 @@ func ModFile() *modfile.File {
 
 func BinDir() string {
 	Init()
+	if cfg.GOBIN != "" {
+		return cfg.GOBIN
+	}
+	if gopath == "" {
+		return ""
+	}
 	return filepath.Join(gopath, "bin")
 }
 
@@ -381,12 +387,11 @@ func Init() {
 		"verify, graph, and why. Implement support for go mod download and add test cases" +
 		"to ensure verify, graph, and why work properly.")
 	list := filepath.SplitList(cfg.BuildContext.GOPATH)
-	if len(list) == 0 || list[0] == "" {
-		base.Fatalf("missing $GOPATH")
-	}
-	gopath = list[0]
-	if _, err := fsys.Stat(filepath.Join(gopath, "go.mod")); err == nil {
-		base.Fatalf("$GOPATH/go.mod exists but should not")
+	if len(list) > 0 && list[0] != "" {
+		gopath = list[0]
+		if _, err := fsys.Stat(filepath.Join(gopath, "go.mod")); err == nil {
+			base.Fatalf("$GOPATH/go.mod exists but should not")
+		}
 	}
 
 	if inWorkspaceMode() {

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

@@ -31,11 +31,18 @@ env GOPATH=
 go env GOMODCACHE
 stdout $HOME[/\\]go[/\\]pkg[/\\]mod
 
-# If GOMODCACHE isn't set and GOPATH starts with the path list separator, it's an error.
+# If GOMODCACHE isn't set and GOPATH starts with the path list separator,
+# GOMODCACHE is empty and any command that needs it errors out.
 env GOMODCACHE=
 env GOPATH=${:}$WORK/this/is/ignored
-! go env GOMODCACHE
-stderr 'missing \$GOPATH'
+
+go env GOMODCACHE
+stdout '^$'
+! stdout .
+! stderr .
+
+! go mod download rsc.io/[email protected]
+stderr '^go: module cache not found: neither GOMODCACHE nor GOPATH is set$'
 
 # If GOMODCACHE isn't set and GOPATH has multiple elements only the first is used.
 env GOMODCACHE=

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

@@ -0,0 +1,15 @@
+# https://golang.org/issue/43938: 'go build' should succeed
+# if GOPATH and the variables needed for its default value
+# are all unset but not relevant to the specific command.
+
+env HOME=''
+env home=''
+env GOPATH=''
+
+go list -deps main.go
+stdout '^io$'
+
+-- main.go --
+package main
+
+import _ "io"