cmd/go/internal/modload: avoid loading the full module graph for imports satisfied by lazy roots

For #36460

Change-Id: Ibdbaa893ded772617e22f12db7a0463604db5195
Reviewed-on: https://go-review.googlesource.com/c/go/+/308516
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index cb206a3dea..f46c58f474 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -1125,6 +1125,22 @@ func keepSums(ctx context.Context, ld *loader, rs *Requirements, which whichSums
 				continue
 			}
 
+			if rs.depth == lazy && pkg.mod.Path != "" {
+				if v, ok := rs.rootSelected(pkg.mod.Path); ok && v == pkg.mod.Version {
+					// pkg was loaded from a root module, and because the main module is
+					// lazy we do not check non-root modules for conflicts for packages
+					// that can be found in roots. So we only need the checksums for the
+					// root modules that may contain pkg, not all possible modules.
+					for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {
+						if v, ok := rs.rootSelected(prefix); ok && v != "none" {
+							m := module.Version{Path: prefix, Version: v}
+							keep[resolveReplacement(m)] = true
+						}
+					}
+					continue
+				}
+			}
+
 			for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {
 				if v := mg.Selected(prefix); v != "none" {
 					m := module.Version{Path: prefix, Version: v}