cmd/go: print package import chains for some build list errors

When we construct the build list by loading packages (e.g., in
"go build", "go list", or "go test"), we may load additional modules
not mentioned in the original build list. If we encounter an error
loading one of these modules, mvs.BuildList currently returns a
BuildListError with a chain of requirments. Unfortunately, this is not
helpful, since the graph is structured such that these missing modules
are direct requirements of the main module.

With this change, loader.load keeps track of the package that caused
each "missing" module to be added. If an error occurs in a missing
module, the chain of package imports is printed instead of the module
requirements.

Fixes #31475

Change-Id: Ie484814af42ceea3e85fedc38e705ba3a38cd495
Reviewed-on: https://go-review.googlesource.com/c/go/+/171859
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, 12 insertions, 3 deletions

diff --git a/src/cmd/go/internal/mvs/mvs.go b/src/cmd/go/internal/mvs/mvs.go
index 284a6fc339..d1c3d8c08a 100644
--- a/src/cmd/go/internal/mvs/mvs.go
+++ b/src/cmd/go/internal/mvs/mvs.go
@@ -65,7 +65,7 @@ type Reqs interface {
 // while constructing a build list. BuildListError prints the chain
 // of requirements to the module where the error occurred.
 type BuildListError struct {
-	err   error
+	Err   error
 	stack []buildListErrorElem
 }
 
@@ -77,9 +77,18 @@ type buildListErrorElem struct {
 	nextReason string
 }
 
+// Module returns the module where the error occurred. If the module stack
+// is empty, this returns a zero value.
+func (e *BuildListError) Module() module.Version {
+	if len(e.stack) == 0 {
+		return module.Version{}
+	}
+	return e.stack[0].m
+}
+
 func (e *BuildListError) Error() string {
 	b := &strings.Builder{}
-	errMsg := e.err.Error()
+	errMsg := e.Err.Error()
 	stack := e.stack
 
 	// Don't print modules at the beginning of the chain without a
@@ -177,7 +186,7 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 
 			if node.err != nil {
 				err := &BuildListError{
-					err:   node.err,
+					Err:   node.err,
 					stack: []buildListErrorElem{{m: node.m}},
 				}
 				for n, prev := neededBy[node], node; n != nil; n, prev = neededBy[n], n {