Implement blackbox in Golang (#250)

* Initial release

pkg/vcs/vcs.go

@@ -0,0 +1,82 @@
+package vcs
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"github.com/StackExchange/blackbox/v2/models"
+)
+
+// Vcs is the handle
+type Vcs interface {
+	models.Vcs
+}
+
+// NewFnSig function signature needed by reg.
+type NewFnSig func() (Vcs, error)
+
+// Item stores one item
+type Item struct {
+	Name     string
+	New      NewFnSig
+	Priority int
+}
+
+// Catalog is the list of registered vcs's.
+var Catalog []*Item
+
+// Discover polls the VCS plug-ins to determine the VCS of directory.
+// The first to succeed is returned.
+// It never returns nil, since "NONE" is always valid.
+func Discover() (Vcs, string) {
+	for _, v := range Catalog {
+		h, err := v.New()
+		if err != nil {
+			return nil, "" // No idea how that would happen.
+		}
+		if b, repodir := h.Discover(); b {
+
+			// Try to find the rel path from CWD to RepoBase
+			wd, err := os.Getwd()
+			if err != nil {
+				fmt.Printf("ERROR: Can not determine cwd! Failing!\n")
+				os.Exit(1)
+			}
+			//fmt.Printf("DISCCOVER: WD=%q REPO=%q\n", wd, repodir)
+			if repodir != wd && strings.HasSuffix(repodir, wd) {
+				// This is a terrible hack.  We're basically guessing
+				// at the filesystem layout.  That said, it works on macOS.
+				// TODO(tlim): Abstract this out into a separate function
+				// so we can do integration tests on it (to know if it fails on
+				// a particular operating system.)
+				repodir = wd
+			}
+			r, err := filepath.Rel(wd, repodir)
+			if err != nil {
+				// Wait, we're not relative to each other? Give up and
+				// just return the abs repodir.
+				return h, repodir
+			}
+			return h, r
+		}
+	}
+	// This can't happen. If it does, we'll panic and that's ok.
+	return nil, ""
+}
+
+// Register a new VCS.
+func Register(name string, priority int, newfn NewFnSig) {
+	//fmt.Printf("VCS registered: %v\n", name)
+	item := &Item{
+		Name:     name,
+		New:      newfn,
+		Priority: priority,
+	}
+	Catalog = append(Catalog, item)
+
+	// Keep the list sorted.
+	sort.Slice(Catalog, func(i, j int) bool { return Catalog[j].Priority < Catalog[i].Priority })
+}