cmd/go: use .mod instead of .zip to determine if version has go.mod file

When checking for updates, the go command checks whether the highest
compatible version has a go.mod file in order to determine whether
+incompatible versions may be considered "latest". Previously, to
perform this check, the go command would download the content of the
module (the .zip file) to see whether a go.mod file was present at the
root. This is slower than necessary, and it caused 'go list -m -u' to
try to save the sum for the .zip file in go.sum in some cases.

With this change, the go command only downloads the .mod file and
checks whether it appears to be a fake file generated for a version
that didn't have a go.mod file. This is faster and requires less
verification. Fake files only have a "module" directive. It's possible
to commit a file that passes this test, but it would be difficult to
do accidentally: Go 1.12 and later at least add a "go" directive. A
false positive here would cause version queries to have slightly
different results but would not affect builds.

Fixes #47377

Change-Id: Ie5ffd0b45e39bd0921328a60af99a9f6e5ab6346
Reviewed-on: https://go-review.googlesource.com/c/go/+/337850
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

4 files changed, 114 insertions, 56 deletions

diff --git a/src/cmd/go/internal/modfetch/coderepo.go b/src/cmd/go/internal/modfetch/coderepo.go
index f817a04583..dfef9f73c2 100644
--- a/src/cmd/go/internal/modfetch/coderepo.go
+++ b/src/cmd/go/internal/modfetch/coderepo.go
@@ -864,22 +864,25 @@ func (r *codeRepo) GoMod(version string) (data []byte, err error) {
 	data, err = r.code.ReadFile(rev, path.Join(dir, "go.mod"), codehost.MaxGoMod)
 	if err != nil {
 		if os.IsNotExist(err) {
-			return r.legacyGoMod(rev, dir), nil
+			return LegacyGoMod(r.modPath), nil
 		}
 		return nil, err
 	}
 	return data, nil
 }
 
-func (r *codeRepo) legacyGoMod(rev, dir string) []byte {
-	// We used to try to build a go.mod reflecting pre-existing
-	// package management metadata files, but the conversion
-	// was inherently imperfect (because those files don't have
-	// exactly the same semantics as go.mod) and, when done
-	// for dependencies in the middle of a build, impossible to
-	// correct. So we stopped.
-	// Return a fake go.mod that simply declares the module path.
-	return []byte(fmt.Sprintf("module %s\n", modfile.AutoQuote(r.modPath)))
+// LegacyGoMod generates a fake go.mod file for a module that doesn't have one.
+// The go.mod file contains a module directive and nothing else: no go version,
+// no requirements.
+//
+// We used to try to build a go.mod reflecting pre-existing
+// package management metadata files, but the conversion
+// was inherently imperfect (because those files don't have
+// exactly the same semantics as go.mod) and, when done
+// for dependencies in the middle of a build, impossible to
+// correct. So we stopped.
+func LegacyGoMod(modPath string) []byte {
+	return []byte(fmt.Sprintf("module %s\n", modfile.AutoQuote(modPath)))
 }
 
 func (r *codeRepo) modPrefix(rev string) string {
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index d280945ea6..6145e8b2f0 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -595,47 +595,14 @@ func rawGoModSummary(m module.Version) (*modFileSummary, error) {
 	}
 	c := rawGoModSummaryCache.Do(m, func() interface{} {
 		summary := new(modFileSummary)
-		var f *modfile.File
-		if m.Version == "" {
-			// m is a replacement module with only a file path.
-			dir := m.Path
-			if !filepath.IsAbs(dir) {
-				dir = filepath.Join(ModRoot(), dir)
-			}
-			gomod := filepath.Join(dir, "go.mod")
-			var data []byte
-			var err error
-			if gomodActual, ok := fsys.OverlayPath(gomod); ok {
-				// Don't lock go.mod if it's part of the overlay.
-				// On Plan 9, locking requires chmod, and we don't want to modify any file
-				// in the overlay. See #44700.
-				data, err = os.ReadFile(gomodActual)
-			} else {
-				data, err = lockedfile.Read(gomodActual)
-			}
-			if err != nil {
-				return cached{nil, module.VersionError(m, fmt.Errorf("reading %s: %v", base.ShortPath(gomod), err))}
-			}
-			f, err = modfile.ParseLax(gomod, data, nil)
-			if err != nil {
-				return cached{nil, module.VersionError(m, fmt.Errorf("parsing %s: %v", base.ShortPath(gomod), err))}
-			}
-		} else {
-			if !semver.IsValid(m.Version) {
-				// Disallow the broader queries supported by fetch.Lookup.
-				base.Fatalf("go: internal error: %s@%s: unexpected invalid semantic version", m.Path, m.Version)
-			}
-
-			data, err := modfetch.GoMod(m.Path, m.Version)
-			if err != nil {
-				return cached{nil, err}
-			}
-			f, err = modfile.ParseLax("go.mod", data, nil)
-			if err != nil {
-				return cached{nil, module.VersionError(m, fmt.Errorf("parsing go.mod: %v", err))}
-			}
+		name, data, err := rawGoModData(m)
+		if err != nil {
+			return cached{nil, err}
+		}
+		f, err := modfile.ParseLax(name, data, nil)
+		if err != nil {
+			return cached{nil, module.VersionError(m, fmt.Errorf("parsing %s: %v", base.ShortPath(name), err))}
 		}
-
 		if f.Module != nil {
 			summary.module = f.Module.Mod
 			summary.deprecated = f.Module.Deprecated
@@ -671,6 +638,42 @@ func rawGoModSummary(m module.Version) (*modFileSummary, error) {
 
 var rawGoModSummaryCache par.Cache // module.Version → rawGoModSummary result
 
+// rawGoModData returns the content of the go.mod file for module m, ignoring
+// all replacements that may apply to m.
+//
+// rawGoModData cannot be used on the Target module.
+//
+// Unlike rawGoModSummary, rawGoModData does not cache its results in memory.
+// Use rawGoModSummary instead unless you specifically need these bytes.
+func rawGoModData(m module.Version) (name string, data []byte, err error) {
+	if m.Version == "" {
+		// m is a replacement module with only a file path.
+		dir := m.Path
+		if !filepath.IsAbs(dir) {
+			dir = filepath.Join(ModRoot(), dir)
+		}
+		gomod := filepath.Join(dir, "go.mod")
+		if gomodActual, ok := fsys.OverlayPath(gomod); ok {
+			// Don't lock go.mod if it's part of the overlay.
+			// On Plan 9, locking requires chmod, and we don't want to modify any file
+			// in the overlay. See #44700.
+			data, err = os.ReadFile(gomodActual)
+		} else {
+			data, err = lockedfile.Read(gomodActual)
+		}
+		if err != nil {
+			return gomod, nil, module.VersionError(m, fmt.Errorf("reading %s: %v", base.ShortPath(gomod), err))
+		}
+	} else {
+		if !semver.IsValid(m.Version) {
+			// Disallow the broader queries supported by fetch.Lookup.
+			base.Fatalf("go: internal error: %s@%s: unexpected invalid semantic version", m.Path, m.Version)
+		}
+		data, err = modfetch.GoMod(m.Path, m.Version)
+	}
+	return "go.mod", data, err
+}
+
 // queryLatestVersionIgnoringRetractions looks up the latest version of the
 // module with the given path without considering retracted or excluded
 // versions.
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index dda9004a9f..e737ca90fc 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -5,13 +5,13 @@
 package modload
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
 	"io/fs"
 	"os"
 	pathpkg "path"
-	"path/filepath"
 	"sort"
 	"strings"
 	"sync"
@@ -931,14 +931,32 @@ func moduleHasRootPackage(ctx context.Context, m module.Version) (bool, error) {
 	return ok, err
 }
 
-func versionHasGoMod(ctx context.Context, m module.Version) (bool, error) {
-	needSum := false
-	root, _, err := fetch(ctx, m, needSum)
+// versionHasGoMod returns whether a version has a go.mod file.
+//
+// versionHasGoMod fetches the go.mod file (possibly a fake) and true if it
+// contains anything other than a module directive with the same path. When a
+// module does not have a real go.mod file, the go command acts as if it had one
+// that only contained a module directive. Normal go.mod files created after
+// 1.12 at least have a go directive.
+//
+// This function is a heuristic, since it's possible to commit a file that would
+// pass this test. However, we only need a heurstic for determining whether
+// +incompatible versions may be "latest", which is what this function is used
+// for.
+//
+// This heuristic is useful for two reasons: first, when using a proxy,
+// this lets us fetch from the .mod endpoint which is much faster than the .zip
+// endpoint. The .mod file is used anyway, even if the .zip file contains a
+// go.mod with different content. Second, if we don't fetch the .zip, then
+// we don't need to verify it in go.sum. This makes 'go list -m -u' faster
+// and simpler.
+func versionHasGoMod(_ context.Context, m module.Version) (bool, error) {
+	_, data, err := rawGoModData(m)
 	if err != nil {
 		return false, err
 	}
-	fi, err := os.Stat(filepath.Join(root, "go.mod"))
-	return err == nil && !fi.IsDir(), nil
+	isFake := bytes.Equal(data, modfetch.LegacyGoMod(m.Path))
+	return !isFake, nil
 }
 
 // A versionRepo is a subset of modfetch.Repo that can report information about
diff --git a/src/cmd/go/testdata/script/mod_update_sum_readonly.txt b/src/cmd/go/testdata/script/mod_update_sum_readonly.txt
new file mode 100644
index 0000000000..41f12e4084
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_update_sum_readonly.txt
@@ -0,0 +1,34 @@
+# When finding the latest version of a module, we should not download version
+# contents. Previously, we downloaded .zip files to determine whether a real
+# .mod file was present in order to decide whether +incompatible versions
+# could be "latest".
+#
+# Verifies #47377.
+
+# rsc.io/breaker has two versions, neither of which has a .mod file.
+go list -m -versions rsc.io/breaker
+stdout '^rsc.io/breaker v1.0.0 v2.0.0\+incompatible$'
+go mod download rsc.io/breaker@v1.0.0
+! grep '^go' $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v1.0.0.mod
+go mod download rsc.io/breaker@v2.0.0+incompatible
+! grep '^go' $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v2.0.0+incompatible.mod
+
+# Delete downloaded .zip files.
+go clean -modcache
+
+# Check for updates.
+go list -m -u rsc.io/breaker
+stdout '^rsc.io/breaker v1.0.0 \[v2.0.0\+incompatible\]$'
+
+# We should not have downloaded zips.
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v1.0.0.zip
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v2.0.0+incompatible.zip
+
+-- go.mod --
+module m
+
+go 1.16
+
+require rsc.io/breaker v1.0.0
+-- go.sum --
+rsc.io/breaker v1.0.0/go.mod h1:s5yxDXvD88U1/ESC23I2FK3Lkv4YIKaB1ij/Hbm805g=