cmd/go: print require chains in build list errors

mvs.BuildList and functions that invoke it directly (UpgradeAll) now
return an *mvs.BuildListError when there is an error retrieving the
requirements for a module. This new error prints the chain of
requirements from the main module to the module where the error
occurred.

These errors come up most commonly when a go.mod file has an
unexpected module path or can't be parsed for some other reason. It's
currently difficult to debug these errors because it's not clear where
the "bad" module is required from. Tools like "go list -m" and
"go mod why" don't work without the build graph.

Fixes #30661

Change-Id: I3c9d4683dcd9a5d7c259e5e4cc7e1ee209700b10
Reviewed-on: https://go-review.googlesource.com/c/go/+/166984
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/cmd/go/internal/mvs/mvs_test.go b/src/cmd/go/internal/mvs/mvs_test.go
index 2a27dfb288..cab4bb241b 100644
--- a/src/cmd/go/internal/mvs/mvs_test.go
+++ b/src/cmd/go/internal/mvs/mvs_test.go
@@ -5,6 +5,7 @@
 package mvs
 
 import (
+	"fmt"
 	"reflect"
 	"strings"
 	"testing"
@@ -446,7 +447,7 @@ func (r reqsMap) Upgrade(m module.Version) (module.Version, error) {
 		}
 	}
 	if u.Path == "" {
-		return module.Version{}, &MissingModuleError{module.Version{Path: m.Path, Version: ""}}
+		return module.Version{}, fmt.Errorf("missing module: %v", module.Version{Path: m.Path})
 	}
 	return u, nil
 }
@@ -467,7 +468,7 @@ func (r reqsMap) Previous(m module.Version) (module.Version, error) {
 func (r reqsMap) Required(m module.Version) ([]module.Version, error) {
 	rr, ok := r[m]
 	if !ok {
-		return nil, &MissingModuleError{m}
+		return nil, fmt.Errorf("missing module: %v", m)
 	}
 	return rr, nil
 }