cmd/go/internal/modload: handle NotExist errors in (*mvsReqs).Previous

Previous is used during downgrading. If the module proxy does not
advertise any versions (for example, because it contains only
pseudo-versions), then Previous should return "none" instead of a
non-nil error.

For #37438

Change-Id: I4edfec19cfeb3ffe50df4979f99a01321c442509
Reviewed-on: https://go-review.googlesource.com/c/go/+/266370
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: Bryan C. Mills

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

@@ -7,6 +7,7 @@ package modload
 import (
 	"context"
 	"errors"
+	"os"
 	"sort"
 
 	"cmd/go/internal/modfetch"
@@ -102,6 +103,9 @@ func (*mvsReqs) Previous(m module.Version) (module.Version, error) {
 	// TODO(golang.org/issue/38714): thread tracing context through MVS.
 	list, err := versions(context.TODO(), m.Path, CheckAllowed)
 	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return module.Version{Path: m.Path, Version: "none"}, nil
+		}
 		return module.Version{}, err
 	}
 	i := sort.Search(len(list), func(i int) bool { return semver.Compare(list[i], m.Version) >= 0 })