config/module: git support

config/module/get.go

@@ -1,8 +1,11 @@
 package module
 
 import (
+	"bytes"
 	"fmt"
 	"net/url"
+	"os/exec"
+	"syscall"
 )
 
 // Getter defines the interface that schemes must implement to download
@@ -24,6 +27,7 @@ var Getters map[string]Getter
 func init() {
 	Getters = map[string]Getter{
 		"file": new(FileGetter),
+		"git":  new(GitGetter),
 	}
 }
 
@@ -51,3 +55,27 @@ func Get(dst, src string) error {
 
 	return err
 }
+
+// getRunCommand is a helper that will run a command and capture the output
+// in the case an error happens.
+func getRunCommand(cmd *exec.Cmd) error {
+	var buf bytes.Buffer
+	cmd.Stdout = &buf
+	cmd.Stderr = &buf
+	err := cmd.Run()
+	if err == nil {
+		return nil
+	}
+	if exiterr, ok := err.(*exec.ExitError); ok {
+		// The program has exited with an exit code != 0
+		if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
+			return fmt.Errorf(
+				"%s exited with %d: %s",
+				cmd.Path,
+				status.ExitStatus(),
+				buf.String())
+		}
+	}
+
+	return fmt.Errorf("error running %s: %s", cmd.Path, buf.String())
+}

config/module/get_git.go

@@ -0,0 +1,39 @@
+package module
+
+import (
+	"fmt"
+	"net/url"
+	"os"
+	"os/exec"
+)
+
+// GitGetter is a Getter implementation that will download a module from
+// a git repository.
+type GitGetter struct{}
+
+func (g *GitGetter) Get(dst string, u *url.URL) error {
+	if _, err := exec.LookPath("git"); err != nil {
+		return fmt.Errorf("git must be available and on the PATH")
+	}
+
+	_, err := os.Stat(dst)
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+	if err == nil {
+		return g.update(dst, u)
+	}
+
+	return g.clone(dst, u)
+}
+
+func (g *GitGetter) clone(dst string, u *url.URL) error {
+	cmd := exec.Command("git", "clone", u.String(), dst)
+	return getRunCommand(cmd)
+}
+
+func (g *GitGetter) update(dst string, u *url.URL) error {
+	cmd := exec.Command("git", "pull", "--ff-only")
+	cmd.Dir = dst
+	return getRunCommand(cmd)
+}