go/build: import packages in module mode when GO111MODULE is "on"

go/build.Import locates package dirctories using 'go list' when in
module mode (finding, downloading, and extracting modules is
complicated, so go/build does not handle it).

Previously, Import used 'go list' if GO111MODULE was not explicitly
off and a go.mod file was present (plus some other conditions). With
this change, if GO111MODULE is "on", a go.mod file does not need to be
present.

Fixes #34669

Change-Id: I9e56871054d4b07c3fc04b6f14a5c8c8e9f3c333
Reviewed-on: https://go-review.googlesource.com/c/go/+/199818
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/go/build/build.go

@@ -1008,8 +1008,12 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode,
 		return errNoModules
 	}
 
-	// If modules are not enabled, then the in-process code works fine and we should keep using it.
-	switch os.Getenv("GO111MODULE") {
+	// Predict whether module aware mode is enabled by checking the value of
+	// GO111MODULE and looking for a go.mod file in the source directory or
+	// one of its parents. Running 'go env GOMOD' in the source directory would
+	// give a canonical answer, but we'd prefer not to execute another command.
+	go111Module := os.Getenv("GO111MODULE")
+	switch go111Module {
 	case "off":
 		return errNoModules
 	default: // "", "on", "auto", anything else
@@ -1031,8 +1035,9 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode,
 		}
 	}
 
-	// Look to see if there is a go.mod.
+	// Unless GO111MODULE=on, look to see if there is a go.mod.
 	// Since go1.13, it doesn't matter if we're inside GOPATH.
+	if go111Module != "on" {
 		parent := absSrcDir
 		for {
 			info, err := os.Stat(filepath.Join(parent, "go.mod"))
@@ -1045,6 +1050,7 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode,
 			}
 			parent = d
 		}
+	}
 
 	cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
 

src/go/build/build_test.go

@@ -420,6 +420,46 @@ func TestImportVendorParentFailure(t *testing.T) {
 	}
 }
 
+// Check that a package is loaded in module mode if GO111MODULE=on, even when
+// no go.mod file is present. It should fail to resolve packages outside std.
+// Verifies golang.org/issue/34669.
+func TestImportPackageOutsideModule(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// Disable module fetching for this test so that 'go list' fails quickly
+	// without trying to find the latest version of a module.
+	defer os.Setenv("GOPROXY", os.Getenv("GOPROXY"))
+	os.Setenv("GOPROXY", "off")
+
+	// Create a GOPATH in a temporary directory. We don't use testdata
+	// because it's in GOROOT, which interferes with the module heuristic.
+	gopath, err := ioutil.TempDir("", "gobuild-notmodule")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(gopath)
+	if err := os.MkdirAll(filepath.Join(gopath, "src/example.com/p"), 0777); err != nil {
+		t.Fatal(err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(gopath, "src/example.com/p/p.go"), []byte("package p"), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "on")
+	defer os.Setenv("GOPATH", os.Getenv("GOPATH"))
+	os.Setenv("GOPATH", gopath)
+	ctxt := Default
+	ctxt.GOPATH = gopath
+
+	want := "cannot find module providing package"
+	if _, err := ctxt.Import("example.com/p", gopath, FindOnly); err == nil {
+		t.Fatal("importing package when no go.mod is present succeeded unexpectedly")
+	} else if errStr := err.Error(); !strings.Contains(errStr, want) {
+		t.Fatalf("error when importing package when no go.mod is present: got %q; want %q", errStr, want)
+	}
+}
+
 func TestImportDirTarget(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
 	ctxt := Default