cmd/go: describe dependencies in build list error messages

mvs.BuildList reports errors with a chain of modules to make it
clear why the module where the error occurred was part of the
build. This is a little confusing with "go get -u" since there are
edges in the module graph for requirements and for updates.

With this change, we now print "requires" or "updates to" between
each module version in the chain.

Updates #30661

Change-Id: Ie689500ea86857e715b250b9e0cae0bc6686dc32
Reviewed-on: https://go-review.googlesource.com/c/go/+/171150
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, 30 insertions, 15 deletions

diff --git a/src/cmd/go/internal/mvs/mvs.go b/src/cmd/go/internal/mvs/mvs.go
index 160e6089db..284a6fc339 100644
--- a/src/cmd/go/internal/mvs/mvs.go
+++ b/src/cmd/go/internal/mvs/mvs.go
@@ -65,33 +65,41 @@ 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
-	Stack []module.Version
+	err   error
+	stack []buildListErrorElem
+}
+
+type buildListErrorElem struct {
+	m module.Version
+
+	// nextReason is the reason this module depends on the next module in the
+	// stack. Typically either "requires", or "upgraded to".
+	nextReason string
 }
 
 func (e *BuildListError) Error() string {
 	b := &strings.Builder{}
-	errMsg := e.Err.Error()
-	stack := e.Stack
+	errMsg := e.err.Error()
+	stack := e.stack
 
 	// Don't print modules at the beginning of the chain without a
 	// version. These always seem to be the main module or a
 	// synthetic module ("target@").
-	for len(stack) > 0 && stack[len(stack)-1].Version == "" {
+	for len(stack) > 0 && stack[len(stack)-1].m.Version == "" {
 		stack = stack[:len(stack)-1]
 	}
 
 	// Don't print the last module if the error message already
 	// starts with module path and version.
-	if len(stack) > 0 && strings.HasPrefix(errMsg, fmt.Sprintf("%s@%s: ", stack[0].Path, stack[0].Version)) {
-		// error already mentions module
-		stack = stack[1:]
+	errMentionsLast := len(stack) > 0 && strings.HasPrefix(errMsg, fmt.Sprintf("%s@%s: ", stack[0].m.Path, stack[0].m.Version))
+	for i := len(stack) - 1; i >= 1; i-- {
+		fmt.Fprintf(b, "%s@%s %s\n\t", stack[i].m.Path, stack[i].m.Version, stack[i].nextReason)
 	}
-
-	for i := len(stack) - 1; i >= 0; i-- {
-		fmt.Fprintf(b, "%s@%s ->\n\t", stack[i].Path, stack[i].Version)
+	if errMentionsLast || len(stack) == 0 {
+		b.WriteString(errMsg)
+	} else {
+		fmt.Fprintf(b, "%s@%s: %s", stack[0].m.Path, stack[0].m.Version, errMsg)
 	}
-	b.WriteString(errMsg)
 	return b.String()
 }
 
@@ -168,9 +176,16 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 			q = q[1:]
 
 			if node.err != nil {
-				err := &BuildListError{Err: node.err}
-				for n := node; n != nil; n = neededBy[n] {
-					err.Stack = append(err.Stack, n.m)
+				err := &BuildListError{
+					err:   node.err,
+					stack: []buildListErrorElem{{m: node.m}},
+				}
+				for n, prev := neededBy[node], node; n != nil; n, prev = neededBy[n], n {
+					reason := "requires"
+					if n.upgrade == prev.m {
+						reason = "updating to"
+					}
+					err.stack = append(err.stack, buildListErrorElem{m: n.m, nextReason: reason})
 				}
 				return nil, err
 			}