Filter out pseudo versions from storage on list endpoint (#1114)

* list should not return pseudo versions

* rm empty line

* handle case when repo does not exist anymore

pkg/download/protocol.go

@@ -3,6 +3,8 @@ package download
 import (
 	"context"
 	"io"
+	"regexp"
+	"strings"
 	"sync"
 
 	"github.com/gomods/athens/pkg/errors"
@@ -101,7 +103,35 @@ func (p *protocol) List(ctx context.Context, mod string) ([]string, error) {
 		return nil, errors.E(op, errors.M(mod), errors.KindNotFound, goErr)
 	}
 
-	return union(goList, strList), nil
+	strListSemVers := removePseudoVersions(strList)
+	// if the repo does not exist but athens already saved some versions
+	// return those so that running go get github.com/my/mod gives us the newest saved version
+	// we should only do that if exclusively pseudo-versions have been saved
+	// otherwise @latest would not return the latest stable version but latest commit
+	if isRepoNotFoundErr && len(strListSemVers) == 0 {
+		return strList, nil
+	}
+	// if the repo exists we have to filter out pseudo versions to prevent following scenario:
+	// user does go get github.com/my/mod
+	// there is no sem-ver and so the /list endpoint returns nothing, then /latests gets hit
+	// Athens saves the pseudo version x1
+	// from now on every time user runs go get github.com/my/mod she/he will get pseudo version x1 even if a newer version x2 exists
+	// this is because /list returns non-empty list of versions (x1) and so /latest wont get hit
+	return union(goList, strListSemVers), nil
+}
+
+var pseudoVersionRE = regexp.MustCompile(`^v[0-9]+\.(0\.0-|\d+\.\d+-([^+]*\.)?0\.)\d{14}-[A-Za-z0-9]+(\+incompatible)?$`)
+
+func removePseudoVersions(allVersions []string) []string {
+	var vers []string
+	for _, v := range allVersions {
+		// copied from go cmd https://github.com/golang/go/blob/master/src/cmd/go/internal/modfetch/pseudo.go#L93
+		isPseudoVersion := strings.Count(v, "-") >= 2 && pseudoVersionRE.MatchString(v)
+		if !isPseudoVersion {
+			vers = append(vers, v)
+		}
+	}
+	return vers
 }
 
 func (p *protocol) Latest(ctx context.Context, mod string) (*storage.RevInfo, error) {