Introduce gopkginSource, plus its maybe

gopkginSource are basically thin wrappers around a gitSource. The main
difference is that the gopkginSource performs filtering on the versions
that come through according to the constraint stated in the import path
URL.

This violates both name uniqueness and name/version orthogonality, but
there's not really a ton we can do - we have to at least somewhat
respect the guidelines that gopkg.in set up, as they represent valid
user intent.

deduce.go

@@ -7,6 +7,7 @@ import (
 	"net/url"
 	"path"
 	"regexp"
+	"strconv"
 	"strings"
 )
 
@@ -256,7 +257,7 @@ func (m gopkginDeducer) deduceSource(p string, u *url.URL) (maybeSource, error)
 
 	// Putting a scheme on gopkg.in would be really weird, disallow it
 	if u.Scheme != "" {
-		return nil, fmt.Errorf("Specifying alternate schemes on gopkg.in imports is not permitted")
+		return nil, fmt.Errorf("specifying alternate schemes on gopkg.in imports is not permitted")
 	}
 
 	// gopkg.in is always backed by github
@@ -267,6 +268,11 @@ func (m gopkginDeducer) deduceSource(p string, u *url.URL) (maybeSource, error)
 	} else {
 		u.Path = path.Join(v[2], v[3])
 	}
+	major, err := strconv.ParseInt(v[4][1:], 10, 64)
+	if err != nil {
+		// this should only be reachable if there's an error in the regex
+		return nil, fmt.Errorf("could not parse %q as a gopkg.in major version", v[4][1:])
+	}
 
 	mb := make(maybeSources, len(gitSchemes))
 	for k, scheme := range gitSchemes {
@@ -275,7 +281,10 @@ func (m gopkginDeducer) deduceSource(p string, u *url.URL) (maybeSource, error)
 			u2.User = url.User("git")
 		}
 		u2.Scheme = scheme
-		mb[k] = maybeGitSource{url: &u2}
+		mb[k] = maybeGopkginSource{
+			url:   &u2,
+			major: major,
+		}
 	}
 
 	return mb, nil

maybe_source.go

@@ -9,6 +9,13 @@ import (
 	"github.com/Masterminds/vcs"
 )
 
+// A maybeSource represents a set of information that, given some
+// typically-expensive network effort, could be transformed into a proper source.
+//
+// Wrapping these up as their own type kills two birds with one stone:
+//
+// * Allows control over when deduction logic triggers network activity
+// * Makes it easy to attempt multiple URLs for a given import path
 type maybeSource interface {
 	try(cachedir string, an ProjectAnalyzer) (source, string, error)
 }
@@ -84,6 +91,43 @@ func (m maybeGitSource) try(cachedir string, an ProjectAnalyzer) (source, string
 	return src, ustr, nil
 }
 
+type maybeGopkginSource struct {
+	url   *url.URL
+	major int64
+}
+
+func (m maybeGopkginSource) try(cachedir string, an ProjectAnalyzer) (source, string, error) {
+	ustr := m.url.String()
+	path := filepath.Join(cachedir, "sources", sanitizer.Replace(ustr))
+	r, err := vcs.NewGitRepo(ustr, path)
+	if err != nil {
+		return nil, "", err
+	}
+
+	src := &gopkginSource{
+		gitSource: gitSource{
+			baseVCSSource: baseVCSSource{
+				an: an,
+				dc: newMetaCache(),
+				crepo: &repo{
+					r:     r,
+					rpath: path,
+				},
+			},
+		},
+		major: m.major,
+	}
+
+	src.baseVCSSource.lvfunc = src.listVersions
+
+	_, err = src.listVersions()
+	if err != nil {
+		return nil, "", err
+	}
+
+	return src, ustr, nil
+}
+
 type maybeBzrSource struct {
 	url *url.URL
 }

vcs_source.go

@@ -284,7 +284,7 @@ func (s *gopkginSource) listVersions() (vlist []Version, err error) {
 	}
 
 	// Apply gopkg.in's filtering rules
-	vlist := make([]Version, len(ovlist))
+	vlist = make([]Version, len(ovlist))
 	k := 0
 	var dbranch int // index of branch to be marked default
 	var bsv *semver.Version
@@ -300,7 +300,9 @@ func (s *gopkginSource) listVersions() (vlist []Version, err error) {
 		case branchVersion:
 			// The semver lib isn't exactly the same as gopkg.in's logic, but
 			// it's close enough that it's probably fine to use. We can be more
-			// exact if real problems crop up.
+			// exact if real problems crop up. The most obvious vector for
+			// problems is that we totally ignore the "unstable" designation
+			// right now.
 			sv, err := semver.NewVersion(tv.name)
 			if err != nil || sv.Major() != s.major {
 				// not a semver-shaped branch name at all, or not the same major
@@ -326,7 +328,11 @@ func (s *gopkginSource) listVersions() (vlist []Version, err error) {
 
 	vlist = vlist[:k]
 	if bsv != nil {
-		vlist[dbranch].(versionPair).v.(branchVersion).isDefault = true
+		dbv := vlist[dbranch].(versionPair)
+		vlist[dbranch] = branchVersion{
+			name:      dbv.v.(branchVersion).name,
+			isDefault: true,
+		}.Is(dbv.r)
 	}
 
 	// Process the filtered version data into the cache