TarSum: versioning

This introduces Versions for TarSum checksums.
Fixes: https://github.com/docker/docker/issues/7526

It preserves current functionality and abstracts the interface for
future flexibility of hashing algorithms. As a POC, the VersionDev
Tarsum does not include the mtime in the checksum calculation, and would
solve https://github.com/docker/docker/issues/7387 though this is not a
settled Version is subject to change until a version number is assigned.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

tarsum/versioning_test.go

@@ -0,0 +1,49 @@
+package tarsum
+
+import (
+	"testing"
+)
+
+func TestVersion(t *testing.T) {
+	expected := "tarsum"
+	var v Version
+	if v.String() != expected {
+		t.Errorf("expected %q, got %q", expected, v.String())
+	}
+
+	expected = "tarsum.dev"
+	v = 1
+	if v.String() != expected {
+		t.Errorf("expected %q, got %q", expected, v.String())
+	}
+}
+
+func TestGetVersion(t *testing.T) {
+	testSet := []struct {
+		Str      string
+		Expected Version
+	}{
+		{"tarsum+sha256:e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b", Version0},
+		{"tarsum+sha256", Version0},
+		{"tarsum", Version0},
+		{"tarsum.dev", VersionDev},
+		{"tarsum.dev+sha256:deadbeef", VersionDev},
+	}
+
+	for _, ts := range testSet {
+		v, err := GetVersionFromTarsum(ts.Str)
+		if err != nil {
+			t.Fatalf("%q : %s", err, ts.Str)
+		}
+		if v != ts.Expected {
+			t.Errorf("expected %d (%q), got %d (%q)", ts.Expected, ts.Expected, v, v)
+		}
+	}
+
+	// test one that does not exist, to ensure it errors
+	str := "weak+md5:abcdeabcde"
+	_, err := GetVersionFromTarsum(str)
+	if err != ErrNotVersion {
+		t.Fatalf("%q : %s", err, str)
+	}
+}