cmdutil: simplify SeedArgFor to take a string instead of a namer

Simplify the previous commit so that SeedArgFor() takes strings
for the image type and distribution instead of the `namer`
interface. This makes the code a little bit easier to follow but
now the `distro.Distro` and `distro.ImageType` cannot be directly
passed into the helper anymore so it's a tradeoff.

Ideally it would just take `distro.{Distro,ImageType}` but for
that we would need an easy way to construct fake objects of
these types in our tests which there (currently) is not (AFAICT).

So settle with this approach as a compromise for now.

cmd/build/main.go

@@ -57,7 +57,7 @@ func makeManifest(
 	if config.Blueprint != nil {
 		bp = blueprint.Blueprint(*config.Blueprint)
 	}
-	seedArg, err := cmdutil.SeedArgFor(config, imgType, distribution, archName)
+	seedArg, err := cmdutil.SeedArgFor(config, imgType.Name(), distribution.Name(), archName)
 	if err != nil {
 		return nil, err
 	}

cmd/gen-manifests/main.go

@@ -190,7 +190,7 @@ func makeManifestJob(
 	cacheDir := filepath.Join(cacheRoot, archName+distribution.Name())
 
 	// ensure that each file has a unique seed based on filename
-	seedArg, err := cmdutil.SeedArgFor(bc, imgType, distribution, archName)
+	seedArg, err := cmdutil.SeedArgFor(bc, imgType.Name(), distribution.Name(), archName)
 	if err != nil {
 		panic(err)
 	}

internal/cmdutil/rand.go

@@ -32,20 +32,16 @@ func newRNGSeed() (int64, error) {
 	return randSeed.Int64(), nil
 }
 
-type namer interface {
-	Name() string
-}
-
-func SeedArgFor(bc *buildconfig.BuildConfig, imgType namer, distribution namer, archName string) (int64, error) {
+func SeedArgFor(bc *buildconfig.BuildConfig, imgTypeName, distributionName, archName string) (int64, error) {
 	rngSeed, err := newRNGSeed()
 	if err != nil {
 		return 0, err
 	}
 
 	h := fnv.New64()
-	h.Write([]byte(distribution.Name()))
+	h.Write([]byte(distributionName))
 	h.Write([]byte(archName))
-	h.Write([]byte(imgType.Name()))
+	h.Write([]byte(imgTypeName))
 	h.Write([]byte(bc.Name))
 
 	return rngSeed + int64(h.Sum64()), nil

internal/cmdutil/rand_test.go

@@ -52,8 +52,8 @@ func TestSeedArgFor(t *testing.T) {
 	t.Setenv(cmdutil.RNG_SEED_ENV_KEY, "1234")
 
 	for _, tc := range []struct {
-		bcName, imgType, distro, archName string
-		expectedSeed                      int64
+		bcName, imgTypeName, distroName, archName string
+		expectedSeed                              int64
 	}{
 		{"bcName", "fakeImgType", "fakeDistro", "x86_64", 9170052743323116054},
 		{"bcName1", "fakeImgType", "fakeDistro", "x86_64", -7134826073208782961},
@@ -62,7 +62,7 @@ func TestSeedArgFor(t *testing.T) {
 		{"bcName", "fakeImgType", "fakeDistro1", "aarch64", 47752167762999679},
 	} {
 		bc := &buildconfig.BuildConfig{Name: tc.bcName}
-		seedArg, err := cmdutil.SeedArgFor(bc, fakeNamer{tc.imgType}, fakeNamer{tc.distro}, tc.archName)
+		seedArg, err := cmdutil.SeedArgFor(bc, tc.imgTypeName, tc.distroName, tc.archName)
 		assert.NoError(t, err)
 		assert.Equal(t, tc.expectedSeed, seedArg)
 	}