cmd/go: with -mod=vendor, don't panic if there are duplicate requirements

In loadModFile with -mod=vendor, load the vendor list and use it to
initialize the module graph before calling updateRoots.

In updateLazyRoots with any mode other than "mod", return the original
*Requirements if no roots needed to be upgraded, even if there are
inconsistencies. This means 'go list -m -mod=readonly' and -mod=vendor
may succeed if there are duplicate requirements or requirements on
versions of the main module.

Fixes #47565

Change-Id: I4640dffc4a7359367cc910da0d29e3538bfd1ca4
Reviewed-on: https://go-review.googlesource.com/c/go/+/340252
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
index 604a57b437..bf69567316 100644
--- a/src/cmd/go/internal/modload/buildlist.go
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -191,6 +191,19 @@ func (rs *Requirements) rootSelected(path string) (version string, ok bool) {
 	return "", false
 }
 
+// hasRedundantRoot returns true if the root list contains multiple requirements
+// of the same module or a requirement on any version of the main module.
+// Redundant requirements should be pruned, but they may influence version
+// selection.
+func (rs *Requirements) hasRedundantRoot() bool {
+	for i, m := range rs.rootModules {
+		if m.Path == Target.Path || (i > 0 && m.Path == rs.rootModules[i-1].Path) {
+			return true
+		}
+	}
+	return false
+}
+
 // Graph returns the graph of module requirements loaded from the current
 // root modules (as reported by RootModules).
 //
@@ -882,6 +895,12 @@ func updateLazyRoots(ctx context.Context, direct map[string]bool, rs *Requiremen
 		// and (trivially) version.
 
 		if !rootsUpgraded {
+			if cfg.BuildMod != "mod" {
+				// The only changes to the root set (if any) were to remove duplicates.
+				// The requirements are consistent (if perhaps redundant), so keep the
+				// original rs to preserve its ModuleGraph.
+				return rs, nil
+			}
 			// The root set has converged: every root going into this iteration was
 			// already at its selected version, although we have have removed other
 			// (redundant) roots for the same path.