From 1c71899d35ca07586495ab5b33711a8b592faad7 Mon Sep 17 00:00:00 2001
From: Dmitri Shuralyov <dmitshur@golang.org>
Date: Wed, 13 Nov 2019 10:39:05 -0500
Subject: [PATCH] cmd/godoc: don't execute go mod download when GOMOD is
 /dev/null

When the GOMOD value is the operating system's null device, there
isn't a main module and thus there cannot be module requirements.
Don't try to fill module cache, since running 'go mod download'
would cause a "no modules specified (see 'go help mod download')"
error to be printed, which is confusing.

Fixes golang/go#35476

Change-Id: Ia1f5ee50797a4dc3e36c3b9773e19da781bd9d39
Reviewed-on: https://go-review.googlesource.com/c/tools/+/206886
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
---
 cmd/godoc/godoc_test.go | 34 +++++++++++++++++++++++++++++++++-
 cmd/godoc/main.go       |  9 +++++++--
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/cmd/godoc/godoc_test.go b/cmd/godoc/godoc_test.go
index 3654e37efd..3748388fd8 100644
--- a/cmd/godoc/godoc_test.go
+++ b/cmd/godoc/godoc_test.go
@@ -182,7 +182,7 @@ func TestURL(t *testing.T) {
 			cmd.Stderr = stderr
 			cmd.Args[0] = "godoc"
 
-			// Set GOPATH variable to non-existing absolute path
+			// Set GOPATH variable to a non-existing absolute path
 			// and GOPROXY=off to disable module fetches.
 			// We cannot just unset GOPATH variable because godoc would default it to ~/go.
 			// (We don't want the indexer looking at the local workspace during tests.)
@@ -444,6 +444,38 @@ package a; import _ "godoc.test/repo2/a"; const Name = "repo1a"`,
 	}
 }
 
+// Test for golang.org/issue/35476.
+func TestNoMainModule(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in -short mode")
+	}
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; for consistency with other tests that build godoc binary")
+	}
+	bin, cleanup := buildGodoc(t)
+	defer cleanup()
+	tempDir, err := ioutil.TempDir("", "godoc-test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	// Run godoc in an empty directory with module mode explicitly on,
+	// so that 'go env GOMOD' reports os.DevNull.
+	cmd := exec.Command(bin, "-url=/")
+	cmd.Dir = tempDir
+	cmd.Env = append(os.Environ(), "GO111MODULE=on")
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	err = cmd.Run()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(stderr.String(), "go mod download") {
+		t.Errorf("stderr contains 'go mod download', is that intentional?\nstderr=%q", stderr.String())
+	}
+}
+
 // Basic integration test for godoc -analysis=type (via HTTP interface).
 func TestTypeAnalysis(t *testing.T) {
 	bin, cleanup := buildGodoc(t)
diff --git a/cmd/godoc/main.go b/cmd/godoc/main.go
index 082fda85d7..1d402ba0bd 100644
--- a/cmd/godoc/main.go
+++ b/cmd/godoc/main.go
@@ -222,7 +222,7 @@ func main() {
 		// This may fail if module downloading is disallowed (GOPROXY=off) or due to
 		// limited connectivity, in which case we print errors to stderr and show
 		// documentation only for packages that are available.
-		fillModuleCache(os.Stderr)
+		fillModuleCache(os.Stderr, goModFile)
 
 		// Determine modules in the build list.
 		mods, err := buildList()
@@ -414,7 +414,12 @@ func goMod() (string, error) {
 // It should only be used when operating in module mode.
 //
 // See https://golang.org/cmd/go/#hdr-Download_modules_to_local_cache.
-func fillModuleCache(w io.Writer) {
+func fillModuleCache(w io.Writer, goMod string) {
+	if goMod == os.DevNull {
+		// No module requirements, nothing to do.
+		return
+	}
+
 	cmd := exec.Command("go", "mod", "download", "-json")
 	var out bytes.Buffer
 	cmd.Stdout = &out