cmd/go/internal/mvs: reverse the order of BuildListError.stack

When we print the stack from a BuildListError, we print the main
module first and the error last. That was the opposite of the order in
which in was stored in memory, leading to (arguably) more complex code
and (definitely) my own inability to reason about the contents of the
slice.

For now, it's still more convenient to construct the stack reversed,
so we do that and then reverse it before packing it into the error.

For #36460

Change-Id: I6312fb67b2ad9bf9b64071fe829854833208bad7
Reviewed-on: https://go-review.googlesource.com/c/go/+/244759
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/mvs/mvs.go

@@ -83,7 +83,7 @@ func (e *BuildListError) Module() module.Version {
 	if len(e.stack) == 0 {
 		return module.Version{}
 	}
-	return e.stack[0].m
+	return e.stack[len(e.stack)-1].m
 }
 
 func (e *BuildListError) Error() string {
@@ -93,22 +93,22 @@ func (e *BuildListError) Error() string {
 	// 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].m.Version == "" {
-		stack = stack[:len(stack)-1]
+	for len(stack) > 0 && stack[0].m.Version == "" {
+		stack = stack[1:]
 	}
 
-	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)
-	}
 	if len(stack) == 0 {
 		b.WriteString(e.Err.Error())
 	} else {
+		for _, elem := range stack[:len(stack)-1] {
+			fmt.Fprintf(b, "%s@%s %s\n\t", elem.m.Path, elem.m.Version, elem.nextReason)
+		}
 		// Ensure that the final module path and version are included as part of the
 		// error message.
 		if _, ok := e.Err.(*module.ModuleError); ok {
 			fmt.Fprintf(b, "%v", e.Err)
 		} else {
-			fmt.Fprintf(b, "%v", module.VersionError(stack[0].m, e.Err))
+			fmt.Fprintf(b, "%v", module.VersionError(stack[len(stack)-1].m, e.Err))
 		}
 	}
 	return b.String()
@@ -202,16 +202,27 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) (m
 			q = q[1:]
 
 			if node.err != nil {
-				err := &BuildListError{
-					Err:   node.err,
-					stack: []buildListErrorElem{{m: node.m}},
-				}
+				// Construct the stack reversed (from the error to the main module),
+				// then reverse it to obtain the usual order (from the main module to
+				// the error).
+				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})
+					stack = append(stack, buildListErrorElem{m: n.m, nextReason: reason})
+				}
+				i, j := 0, len(stack)-1
+				for i < j {
+					stack[i], stack[j] = stack[j], stack[i]
+					i++
+					j--
+				}
+
+				err := &BuildListError{
+					Err:   node.err,
+					stack: stack,
 				}
 				return nil, err
 			}