cmd/go/internal/modload: skip reading go.mod files for imports in 'go mod tidy' of modules before 'go 1.21'

This eliminate a network access in 'go mod tidy' of an already-tidy
module, which would otherwise be needed to fetch go.mod checksums for
the test dependencies whose go.mod checksums were omitted in Go
releases between Go 1.17 and 1.20 due to bug #56222.

For modules between 'go 1.17' and 'go 1.20' we intentionally preserve
the old 'go mod tidy' output (omitting go.sum entries for the go.mod
files of test dependencies of external packages). We should also avoid
performing extra sumdb lookups for checksums that would be discarded
anyway.

Updates #56222.

Change-Id: I7f0f1c8e902db0e3414c819621c4b99052f503f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/492741
Auto-Submit: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/modload/import.go

@@ -256,7 +256,13 @@ func (e *invalidImportError) Unwrap() error {
 // If the package is present in exactly one module, importFromModules will
 // return the module, its root directory, and a list of other modules that
 // lexically could have provided the package but did not.
-func importFromModules(ctx context.Context, path string, rs *Requirements, mg *ModuleGraph) (m module.Version, modroot, dir string, altMods []module.Version, err error) {
+//
+// If skipModFile is true, the go.mod file for the package is not loaded. This
+// allows 'go mod tidy' to preserve a minor checksum-preservation bug
+// (https://go.dev/issue/56222) for modules with 'go' versions between 1.17 and
+// 1.20, preventing unnecessary go.sum churn and network access in those
+// modules.
+func importFromModules(ctx context.Context, path string, rs *Requirements, mg *ModuleGraph, skipModFile bool) (m module.Version, modroot, dir string, altMods []module.Version, err error) {
 	invalidf := func(format string, args ...interface{}) (module.Version, string, string, []module.Version, error) {
 		return module.Version{}, "", "", nil, &invalidImportError{
 			importPath: path,
@@ -442,7 +448,7 @@ func importFromModules(ctx context.Context, path string, rs *Requirements, mg *M
 			// If the module graph is pruned and this is a test-only dependency
 			// of a package in "all", we didn't necessarily load that file
 			// when we read the module graph, so do it now to be sure.
-			if cfg.BuildMod != "vendor" && mods[0].Path != "" && !MainModules.Contains(mods[0].Path) {
+			if !skipModFile && cfg.BuildMod != "vendor" && mods[0].Path != "" && !MainModules.Contains(mods[0].Path) {
 				if _, err := goModSummary(mods[0]); err != nil {
 					return module.Version{}, "", "", nil, err
 				}

src/cmd/go/internal/modload/load.go

@@ -823,6 +823,10 @@ type loader struct {
 	// transitively *imported by* the packages and tests in the main module.)
 	allClosesOverTests bool
 
+	// skipImportModFiles indicates whether we may skip loading go.mod files
+	// for imported packages (as in 'go mod tidy' in Go 1.17–1.20).
+	skipImportModFiles bool
+
 	work *par.Queue
 
 	// reset on each iteration
@@ -1003,6 +1007,10 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 			// version higher than the go.mod version adds nothing.
 			ld.TidyCompatibleVersion = ld.GoVersion
 		}
+
+		if semver.Compare("v"+ld.GoVersion, tidyGoModSumVersionV) < 0 {
+			ld.skipImportModFiles = true
+		}
 	}
 
 	if semver.Compare("v"+ld.GoVersion, narrowAllVersionV) < 0 && !ld.UseVendorAll {
@@ -1398,7 +1406,7 @@ func (ld *loader) updateRequirements(ctx context.Context) (changed bool, err err
 				//
 				// In some sense, we can think of this as ‘upgraded the module providing
 				// pkg.path from "none" to a version higher than "none"’.
-				if _, _, _, _, err = importFromModules(ctx, pkg.path, rs, nil); err == nil {
+				if _, _, _, _, err = importFromModules(ctx, pkg.path, rs, nil, ld.skipImportModFiles); err == nil {
 					changed = true
 					break
 				}
@@ -1609,7 +1617,7 @@ func (ld *loader) preloadRootModules(ctx context.Context, rootPkgs []string) (ch
 			// If the main module is tidy and the package is in "all" — or if we're
 			// lucky — we can identify all of its imports without actually loading the
 			// full module graph.
-			m, _, _, _, err := importFromModules(ctx, path, ld.requirements, nil)
+			m, _, _, _, err := importFromModules(ctx, path, ld.requirements, nil, ld.skipImportModFiles)
 			if err != nil {
 				var missing *ImportMissingError
 				if errors.As(err, &missing) && ld.ResolveMissingImports {
@@ -1697,7 +1705,7 @@ func (ld *loader) load(ctx context.Context, pkg *loadPkg) {
 	}
 
 	var modroot string
-	pkg.mod, modroot, pkg.dir, pkg.altMods, pkg.err = importFromModules(ctx, pkg.path, ld.requirements, mg)
+	pkg.mod, modroot, pkg.dir, pkg.altMods, pkg.err = importFromModules(ctx, pkg.path, ld.requirements, mg, ld.skipImportModFiles)
 	if pkg.dir == "" {
 		return
 	}
@@ -1956,7 +1964,7 @@ func (ld *loader) checkTidyCompatibility(ctx context.Context, rs *Requirements)
 
 		pkg := pkg
 		ld.work.Add(func() {
-			mod, _, _, _, err := importFromModules(ctx, pkg.path, rs, mg)
+			mod, _, _, _, err := importFromModules(ctx, pkg.path, rs, mg, ld.skipImportModFiles)
 			if mod != pkg.mod {
 				mismatches := <-mismatchMu
 				mismatches[pkg] = mismatch{mod: mod, err: err}

src/cmd/go/testdata/script/mod_sum_issue56222.txt

@@ -36,11 +36,24 @@ go list -f '{{if eq .ImportPath "example.com/generics"}}{{.Module.GoVersion}}{{e
 stdout 1.18
 
 
+# Even at go 1.20 or earlier, 'go mod tidy' shouldn't need go.mod files or
+# checksums that it won't record.
+
+go mod tidy -go=1.20
+go clean -modcache  # Remove checksums from the module cache, so that only go.sum is used.
+
+env OLDSUMDB=$GOSUMDB
+env GOSUMDB=bad
+go mod tidy
+
+env GOSUMDB=$OLDSUMDB
+
+
 # Regardless of the go version in go.mod, 'go get -t' should fetch
 # enough checksums to run 'go test' on the named package.
 
 rm p
-go mod tidy
+go mod tidy -go=1.20
 go list -m all
 ! stdout example.com/generics
 go get -t example.com/m2/[email protected]