aci, builder: utilize the new tar for ACI files

Enables dgr to work on hosts that don't have any tar. (See also #217.)

Sorts the contents to have a reproducible order, but pulls the manifest
file to the front for fast access.

Sorted contents of ACIs allow for easier comparison, and usage of
tools such as zsync, and deduplication on the server. The price,
sorting by 'tar', is cheap.

zap the "chdir-acrobatique", use `tar -C`:
    dgr changes paths and performs needless renames, which result in mayhem
if the process quits prematurely or the timing were off. The solution is to
use tar's `-C` param and transform the filenames.

closes #210

aci-builder/bin-run/builder.go

@@ -136,39 +136,36 @@ func (b *Builder) tarAci() error {
 	}
 
 	upperPath := b.pod.Root + PATH_OVERLAY + "/" + upperId + PATH_UPPER
-	upperNamedRootfs := upperPath + "/" + manifestApp(b.pod).Name.String()
-	upperRootfs := upperPath + common.PathRootfs
-
-	if err := os.RemoveAll(upperNamedRootfs + PATH_TMP); err != nil {
-		logs.WithEF(err, b.fields.WithField("path", upperNamedRootfs+PATH_TMP)).Warn("Failed to clean tmp directory")
-	}
-
-	if err := os.Rename(upperNamedRootfs, upperRootfs); err != nil { // TODO this is dirty and can probably be renamed during tar
-		return errs.WithEF(err, b.fields.WithField("path", upperNamedRootfs), "Failed to rename rootfs")
-	}
-	defer os.Rename(upperRootfs, upperNamedRootfs)
-
-	dir, err := os.Getwd()
-	if err != nil {
-		return errs.WithEF(err, b.fields, "Failed to get current working directory")
-	}
-	defer func() {
-		if err := os.Chdir(dir); err != nil {
-			logs.WithEF(err, b.fields.WithField("path", dir)).Warn("Failed to chdir back")
+	rootfsAlias := manifestApp(b.pod).Name.String()
+	destination := b.aciTargetPath + common.PathImageAci // absolute dir, outside upperPath (think: /tmp/…)
+
+	params := []string{"--sort=name", "--numeric-owner", "--exclude", rootfsAlias + PATH_TMP + "/*"}
+	params = append(params, "-C", upperPath, "--transform", "s@^"+rootfsAlias+"@rootfs@")
+	params = append(params, "-cf", destination, common.PathManifest[1:], rootfsAlias)
+
+	logs.WithF(b.fields).Debug("Calling tar to collect all files")
+	if err := common.ExecCmd("tar", params...); err != nil {
+		// In case the host's tar is too old, try the builder's if it exists.
+		stage1Tar := rktcommon.Stage1RootfsPath(b.pod.Root)
+		var buildersTar string
+		for _, t := range []string{"/dgr/usr/bin/tar", "/bin/tar", "/usr/bin/tar"} {
+			buildersTar = filepath.Join(stage1Tar, t)
+			if _, err := os.Stat(buildersTar); err == nil {
+				break
+			}
 		}
-	}()
 
-	if err := os.Chdir(upperPath); err != nil {
-		return errs.WithEF(err, b.fields.WithField("path", upperPath), "Failed to chdir to upper base path")
-	}
-	if err := common.Tar(b.aciTargetPath+common.PathImageAci, common.PathManifest[1:], common.PathRootfs[1:]+"/"); err != nil {
-		return errs.WithEF(err, b.fields, "Failed to tar aci")
+		if err2 := common.ExecCmd(buildersTar, params...); err2 != nil {
+			// If that failed, output the original error nevertheless.
+			logs.WithFields(b.fields).WithField("params", params).Debug("Parameters to 'tar' within the builder")
+			return errs.WithEF(err, b.fields, "Failed to tar aci")
+		}
+		logs.WithF(b.fields).Debug("Had to resort to 'tar' from the builder to create the aci file")
 	}
 	// common.ExecCmd sometimes silently fails, hence the redundant check.
-	if _, err := os.Stat(b.aciTargetPath + common.PathImageAci); os.IsNotExist(err) {
+	if _, err := os.Stat(destination); os.IsNotExist(err) {
 		return errs.WithEF(err, b.fields, "Expected aci has not been created")
 	}
-	logs.WithField("path", dir).Debug("chdir")
 	return nil
 }