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 <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 405b7935e02..9b6a81dd7c5 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/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}