cmd/go: avoid indexing modules in GOROOT

Scanning GOROOT modules for changes appears to be causing Windows
builders to time out in x/tools tests. We may try a per-package index
instead, but for now just skip GOROOT modules (as we do for main
modules).

Fixes #53493.

Change-Id: Ic5bb90b4ce173a24fc6564e85fcde96e1f9b975f
Reviewed-on: https://go-review.googlesource.com/c/go/+/413634
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/go_test.go

@@ -18,7 +18,6 @@ import (
 	"io"
 	"io/fs"
 	"log"
-	"math/rand"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -129,19 +128,6 @@ func TestMain(m *testing.M) {
 	}
 	os.Setenv("CMDGO_TEST_RUN_MAIN", "true")
 
-	if strings.HasPrefix(runtime.Version(), "devel ") && godebug.Get("goindexsalt") == "" {
-		// We're going to execute a lot of cmd/go tests, so set a consistent salt
-		// via GODEBUG so that the modindex package can avoid walking an entire
-		// GOROOT module whenever it tries to use an index for that module.
-		indexSalt := rand.Int63()
-		v := os.Getenv("GODEBUG")
-		if v == "" {
-			os.Setenv("GODEBUG", fmt.Sprintf("goindexsalt=%d", indexSalt))
-		} else {
-			os.Setenv("GODEBUG", fmt.Sprintf("%s,goindexsalt=%d", v, indexSalt))
-		}
-	}
-
 	// $GO_GCFLAGS a compiler debug flag known to cmd/dist, make.bash, etc.
 	// It is not a standard go command flag; use os.Getenv, not cfg.Getenv.
 	if os.Getenv("GO_GCFLAGS") != "" {

src/cmd/go/internal/modindex/read.go

@@ -15,7 +15,6 @@ import (
 	"internal/godebug"
 	"internal/goroot"
 	"internal/unsafeheader"
-	"io/fs"
 	"math"
 	"path"
 	"path/filepath"
@@ -29,7 +28,6 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
-	"cmd/go/internal/fsys"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/par"
 	"cmd/go/internal/str"
@@ -55,63 +53,29 @@ var fcache par.Cache
 
 var salt = godebug.Get("goindexsalt")
 
+// moduleHash returns an ActionID corresponding to the state of the module
+// located at filesystem path modroot.
 func moduleHash(modroot string, ismodcache bool) (cache.ActionID, error) {
 	// We expect modules stored within the module cache to be checksummed and
 	// immutable, and we expect released modules within GOROOT to change only
 	// infrequently (when the Go version changes).
-	if !ismodcache && !str.HasFilePathPrefix(modroot, cfg.GOROOT) {
+	if !ismodcache {
 		// The contents of this module may change over time. We don't want to pay
 		// the cost to detect changes and re-index whenever they occur, so just
 		// don't index it at all.
+		//
+		// Note that this is true even for modules in GOROOT/src: non-release builds
+		// of the Go toolchain may have arbitrary development changes on top of the
+		// commit reported by runtime.Version, or could be completly artificial due
+		// to lacking a `git` binary (like "devel gomote.XXXXX", as synthesized by
+		// "gomote push" as of 2022-06-15). (Release builds shouldn't have
+		// modifications, but we don't want to use a behavior for releases that we
+		// haven't tested during development.)
 		return cache.ActionID{}, ErrNotIndexed
 	}
 
 	h := cache.NewHash("moduleIndex")
 	fmt.Fprintf(h, "module index %s %s %s %v\n", runtime.Version(), salt, indexVersion, modroot)
-
-	if str.HasFilePathPrefix(modroot, cfg.GOROOT) && strings.HasPrefix(runtime.Version(), "devel ") && salt == "" {
-		// This copy of the standard library is a development version, not a
-		// release. It could be based on a Git commit (like
-		// "devel go1.19-2a78e8afc0 Wed Jun 15 00:06:24 2022 +0000") with or
-		// without changes on top of that commit, or it could be completly
-		// artificial due to lacking a `git` binary (like "devel gomote.XXXXX", as
-		// synthesized by "gomote push" as of 2022-06-15).
-		//
-		// If the user provided a unique salt via GODEBUG, we can trust that it is
-		// unique and just go with it. Otherwise, we compute an inexpensive hash of
-		// its files using mtimes so that during development we can continue to
-		// exercise the logic for cached GOROOT indexes.
-		//
-		// mtimes may be granular, imprecise, and loosely updated (see
-		// https://apenwarr.ca/log/20181113), but we don't expect Go contributors to
-		// be mucking around with the import graphs in GOROOT often enough for mtime
-		// collisions to matter essentially ever.
-		//
-		// Note that fsys.Walk walks paths in deterministic order, so this hash
-		// should be completely deterministic if the files are unchanged.
-		err := fsys.Walk(modroot, func(path string, info fs.FileInfo, err error) error {
-			if err := moduleWalkErr(modroot, path, info, err); err != nil {
-				return err
-			}
-
-			if info.IsDir() {
-				return nil
-			}
-			fmt.Fprintf(h, "file %v %v\n", info.Name(), info.ModTime())
-			if info.Mode()&fs.ModeSymlink != 0 {
-				targ, err := fsys.Stat(path)
-				if err != nil {
-					return err
-				}
-				fmt.Fprintf(h, "target %v %v\n", targ.Name(), targ.ModTime())
-			}
-			return nil
-		})
-		if err != nil {
-			return cache.ActionID{}, err
-		}
-	}
-
 	return h.Sum(), nil
 }