Delete modules in case of breaking changes or invalid checksums, compatibility with go mod download (#10)

* add module invalidation via DELETE request
* add go mod download vcs provider

pkg/vcs/gomod.go

@@ -0,0 +1,87 @@
+package vcs
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"io"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type goVCS struct {
+	dir    string
+	log    logger
+	module string
+}
+
+func NewGoMod(l logger, module string) VCS {
+	return &goVCS{log: l, module: module, dir: "/tmp/_go"}
+}
+
+func (g *goVCS) List(ctx context.Context) ([]Version, error) {
+	if err := g.download(ctx, "latest"); err != nil {
+		return nil, err
+	}
+	b, err := g.file("list")
+	if err != nil {
+		return nil, err
+	}
+	versions := []Version{}
+	for _, line := range strings.Split(string(b), "\n") {
+		versions = append(versions, Version(line))
+	}
+	return versions, nil
+}
+
+func (g *goVCS) Timestamp(ctx context.Context, version Version) (time.Time, error) {
+	if err := g.download(ctx, version.String()); err != nil {
+		return time.Time{}, err
+	}
+	b, err := g.file(version.String() + ".info")
+	if err != nil {
+		return time.Time{}, err
+	}
+	info := struct {
+		Version string
+		Time    time.Time
+	}{}
+	if json.Unmarshal(b, &info) == nil {
+		return info.Time, nil
+	}
+	if t, err := time.Parse(time.RFC3339, string(b)); err == nil {
+		return t, nil
+	}
+	if sec, err := strconv.ParseInt(string(b), 10, 64); err == nil {
+		return time.Unix(sec, 0), nil
+	}
+	return time.Time{}, nil
+}
+
+func (g *goVCS) Zip(ctx context.Context, version Version) (io.ReadCloser, error) {
+	if err := g.download(ctx, version.String()); err != nil {
+		return nil, err
+	}
+	b, err := g.file(version.String() + ".zip")
+	if err != nil {
+		return nil, err
+	}
+	return ioutil.NopCloser(bytes.NewReader(b)), nil
+}
+
+func (g *goVCS) download(ctx context.Context, version string) error {
+	cmd := exec.Command("go", "mod", "download", g.module+"@"+version)
+	cmd.Env = append(os.Environ(), "GOPATH="+g.dir)
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}
+
+func (g *goVCS) file(name string) ([]byte, error) {
+	path := filepath.Join(g.dir, "pkg", "mod", "cache", "download", g.module, "@v", name)
+	return ioutil.ReadFile(path)
+}