begin: when provided file doesn't look like an ACI, it's a rootfs tar…

…ball

This commit modifies begin such that after a local file has been given
to acbuild and it's been extracted, acbuild will check for the existence
of a manifest and a rootfs. If either is missing, it's assumed that the
tarball acbuild just extracted is the rootfs for the container, and it
will move the files into the rootfs directory and create an empty
manifest.

Documentation/subcommands/begin.md

@@ -53,13 +53,16 @@ must start with `.`, `~`, or `/`. As an example, if the ACI is in the current
 directory, then instead of passing in `alpine-latest-linux-amd64.aci`, what
 would be passed in is `./alpine-latest-linux-amd64.aci`.
 
-## Starting with a pre-existing rootfs
+## Starting with a local, pre-existing rootfs
 
-If the user has a rootfs they wish to use in the ACI, perhaps produced by a
-tool like [buildroot](http://buildroot.org/), a directory can be passed to
-begin. The contents of the directory will be copied into the ACI, and the ACI
-will have a manifest identical to when the begin command is not passed a
-directory.
+A build can be started with a rootfs of a container. This rootfs could perhaps
+be produced by a tool like [buildroot](http://buildroot.org/), or downloaded
+from somewhere like the [Ubuntu Core
+releases](http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/).
+
+The rootfs can either be in a local directory, or a local tar file. In either
+case, the rootfs is copied into the container and an empty manifest is created
+that is identical to beginning with an empty ACI.
 
 When specifying something on the local filesystem to the begin command, the
 path to it _must_ start with `.`, `~`, or `/`. As an example, if the directory
@@ -74,4 +77,5 @@ acbuild begin ./my-app.aci
 acbuild begin quay.io/coreos/alpine-sh
 acbuild --work-path /tmp/mybuild begin
 acbuild begin ~/projects/buildroot/output/target
+acbuild begin ./ubuntu-core-14.04-core-amd64.tar.gz
 ```

lib/begin.go

@@ -108,7 +108,45 @@ func (a *ACBuild) beginFromLocalImage(start string) error {
 	if finfo.IsDir() {
 		return fmt.Errorf("provided starting ACI is a directory: %s", start)
 	}
-	return util.ExtractImage(start, a.CurrentACIPath, nil)
+	err = util.ExtractImage(start, a.CurrentACIPath, nil)
+	if err != nil {
+		return err
+	}
+
+	for _, file := range []struct {
+		FileName string
+		FilePath string
+	}{
+		{"manifest file", path.Join(a.CurrentACIPath, aci.ManifestFile)},
+		{"rootfs directory", path.Join(a.CurrentACIPath, aci.RootfsDir)},
+	} {
+		_, err = os.Stat(file.FilePath)
+		switch {
+		case os.IsNotExist(err):
+			fmt.Fprintf(os.Stderr, "%s is missing, assuming build is beginning with a tar of a rootfs\n", file.FileName)
+			return a.startedFromTar()
+		case err != nil:
+			return err
+		}
+	}
+	return nil
+}
+
+func (a *ACBuild) startedFromTar() error {
+	tmpPath := path.Join(a.ContextPath, "rootfs")
+	err := os.Rename(a.CurrentACIPath, tmpPath)
+	if err != nil {
+		return err
+	}
+	err = a.beginWithEmptyACI()
+	if err != nil {
+		return err
+	}
+	err = os.Remove(path.Join(a.CurrentACIPath, aci.RootfsDir))
+	if err != nil {
+		return err
+	}
+	return os.Rename(tmpPath, path.Join(a.CurrentACIPath, aci.RootfsDir))
 }
 
 func (a *ACBuild) beginFromLocalDirectory(start string) error {