internal/modindex: suppress missing modcacheindex message

There are workflows where there will be no module cache, and no
place to put the index. Gopls should cope with this case silently.

(If there is no index, then there will be no unimported completions
found, but in these workflows, that seems to be ok.)

Fixes: golang/go#75505
Change-Id: Ie71951cb31b79542ff1480965292ac9d72b93f45
Reviewed-on: https://go-review.googlesource.com/c/tools/+/733520
Reviewed-by: Madeline Kalil <mkalil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: pjweinbgo

gopls/internal/cache/imports.go

@@ -191,8 +191,8 @@ func (s *modcacheState) refreshIndex() {
 	if err != nil {
 		if s.indexErr != nil {
 			s.indexErr = err // prefer most recent error
-		} else {
-			// Keep using stale s.index (if any).
+		} else if s.index != nil {
+			// Keep using stale s.index.
 			log.Printf("modcacheState.refreshIndex: %v", err)
 		}
 	} else {

gopls/internal/cache/snapshot.go

@@ -978,7 +978,7 @@ func (s *Snapshot) WorkspaceMetadata(ctx context.Context) ([]*metadata.Package,
 	defer s.mu.Unlock()
 
 	meta := make([]*metadata.Package, 0, s.workspacePackages.Len())
-	for id, _ := range s.workspacePackages.All() {
+	for id := range s.workspacePackages.All() {
 		meta = append(meta, s.meta.Packages[id])
 	}
 	return meta, nil

gopls/internal/test/integration/completion/completion_test.go

@@ -23,6 +23,7 @@ import (
 	. "golang.org/x/tools/gopls/internal/test/integration"
 	"golang.org/x/tools/gopls/internal/test/integration/fake"
 	"golang.org/x/tools/gopls/internal/util/bug"
+	"golang.org/x/tools/internal/modindex"
 	"golang.org/x/tools/internal/testenv"
 )
 
@@ -1624,3 +1625,69 @@ go 1.24.2
 		}
 	})
 }
+
+func TestModIndexDirEmpty(t *testing.T) {
+	// Test for issue #75505. Do not run this test in parallel.
+	// Test that if modindex.IndexDir is empty, we still get stdlib completions
+	// but not unimported completions from modules.
+
+	// Save/restore IndexDir, and set it to empty string.
+	// This simulates having no module cache.
+	old := modindex.IndexDir
+	modindex.IndexDir = ""
+	defer func() { modindex.IndexDir = old }()
+	// for the paranoid, with modindex.IndexDir = old here, the test fails
+
+	const mod = `
+-- go.mod --
+module mod.com
+go 1.22
+require example.com v1.2.3
+-- main.go --
+package main
+func main() {
+	_ = fmt.P
+	_ = blah.N
+}
+-- main2.go --
+package main
+import "example.com/blah"
+func _() {
+	_ = blah.Hello
+}
+`
+	WithOptions(
+		WriteGoSum("."),
+		ProxyFiles(proxy), // providing example.com through GOPROXY to the go command
+		Settings{"importsSource": settings.ImportsSourceGopls},
+	).Run(t, mod, func(t *testing.T, env *Env) {
+		// We need example.com in the module cache, but not
+		// otherwise available to the completion logic.
+		env.RemoveWorkspaceFile("main2.go")
+		env.RunGoCommand("mod", "tidy")
+		env.Await(env.DoneWithChangeWatchedFiles())
+
+		env.OpenFile("main.go")
+		env.Await(env.DoneWithOpen())
+
+		// 1. Check stdlib completion (should work)
+		loc := env.RegexpSearch("main.go", "fmt.P()")
+		completions := env.Completion(loc)
+		if len(completions.Items) == 0 {
+			t.Errorf("stdlib completion failed to find fmt.Print...")
+		}
+
+		// 2. Check unimported completion (should fail)
+		// We look for "blah.H" which is from example.com
+		loc = env.RegexpSearch("main.go", "blah.N()")
+		completions = env.Completion(loc)
+
+		// We expect NO unimported completions for blah.Name because modindex is disabled.
+		for _, item := range completions.Items {
+			// We shouldn't get candidate from example.com/blah.
+			if strings.Contains(item.Detail, "example.com/blah") {
+				t.Errorf("unexpected unimported completion for blah with empty IndexDir: %v", item)
+			}
+		}
+	})
+}

internal/modindex/index.go

@@ -10,7 +10,6 @@ import (
 	"encoding/csv"
 	"fmt"
 	"io"
-	"log"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -114,7 +113,7 @@ var IndexDir string = func() string {
 	}
 	dir = filepath.Join(dir, "goimports")
 	if err := os.MkdirAll(dir, 0777); err != nil {
-		log.Printf("failed to create modcache index dir: %v", err)
+		dir = "" // #75505, people complain about the error message
 	}
 	return dir
 }()
@@ -127,6 +126,9 @@ func Read(gomodcache string) (*Index, error) {
 	if err != nil {
 		return nil, err
 	}
+	if IndexDir == "" {
+		return nil, os.ErrNotExist
+	}
 
 	// Read the "link" file for the specified gomodcache directory.
 	// It names the payload file.
@@ -227,6 +229,9 @@ func readIndexFrom(gomodcache string, r io.Reader) (*Index, error) {
 
 // write writes the index file and updates the index directory to refer to it.
 func write(gomodcache string, ix *Index) error {
+	if IndexDir == "" {
+		return os.ErrNotExist
+	}
 	// Write the index into a payload file with a fresh name.
 	f, err := os.CreateTemp(IndexDir, fmt.Sprintf("index-%d-*", CurrentVersion))
 	if err != nil {