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, 20 insertions, 3 deletions

diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index b822e74eb5..ddacf49ead 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -1236,7 +1236,7 @@ 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); err == nil {
+				if _, _, err = importFromModules(ctx, pkg.path, rs, nil); err == nil {
 					changed = true
 					break
 				}
@@ -1429,7 +1429,7 @@ 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)
+			m, _, err := importFromModules(ctx, path, ld.requirements, nil)
 			if err != nil {
 				var missing *ImportMissingError
 				if errors.As(err, &missing) && ld.ResolveMissingImports {
@@ -1516,7 +1516,24 @@ func (ld *loader) load(ctx context.Context, pkg *loadPkg) {
 		return
 	}
 
-	pkg.mod, pkg.dir, pkg.err = importFromModules(ctx, pkg.path, ld.requirements)
+	var mg *ModuleGraph
+	if ld.requirements.depth == eager {
+		var err error
+		mg, err = ld.requirements.Graph(ctx)
+		if err != nil {
+			// We already checked the error from Graph in loadFromRoots and/or
+			// updateRequirements, so we ignored the error on purpose and we should
+			// keep trying to push past it.
+			//
+			// However, because mg may be incomplete (and thus may select inaccurate
+			// versions), we shouldn't use it to load packages. Instead, we pass a nil
+			// *ModuleGraph, which will cause mg to first try loading from only the
+			// main module and root dependencies.
+			mg = nil
+		}
+	}
+
+	pkg.mod, pkg.dir, pkg.err = importFromModules(ctx, pkg.path, ld.requirements, mg)
 	if pkg.dir == "" {
 		return
 	}