compare: move FreeBSD loose keyword comparisons to gomtree command

FreeBSD has quite unfortunate behaviour when dealing with keywords that
are missing in one of the manifests being compared -- namely, they
ignore these instances.

Commit 21723a3974 ("*: fix comparison of missing keywords") re-added
this behaviour after the introduction of the Compare API, but
unfortunately it was implemented in the Compare API itself -- meaning
that library users (which didn't want this behaviour) were silently
opted into it.

This patch moves the behaviour to the command-line, where it belongs
(a future patch in this series will allow users to opt-out of this
unfortunate behaviour, as well as some other unfortunate FreeBSD
compatibility behaviours).

Fixes: 21723a3974 ("*: fix comparison of missing keywords")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

compare_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"slices"
 	"testing"
 	"time"
 
@@ -230,6 +231,77 @@ func TestCompareKeySubset(t *testing.T) {
 	}
 }
 
+func TestCompareKeyDelta(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create a bunch of objects.
+	tmpfile := filepath.Join(dir, "tmpfile")
+	require.NoError(t, os.WriteFile(tmpfile, []byte("some content here"), 0666))
+
+	tmpdir := filepath.Join(dir, "testdir")
+	require.NoError(t, os.Mkdir(tmpdir, 0755))
+
+	tmpsubfile := filepath.Join(tmpdir, "anotherfile")
+	require.NoError(t, os.WriteFile(tmpsubfile, []byte("aaa"), 0666))
+
+	// Walk the current state.
+	manifestKeywords := append(DefaultKeywords[:], "sha1digest")
+	old, err := Walk(dir, nil, manifestKeywords, nil)
+	require.NoErrorf(t, err, "walk %s", dir)
+
+	t.Run("Extra-Key", func(t *testing.T) {
+		extraKeyword := Keyword("sha256digest")
+		newManifestKeywords := append(manifestKeywords[:], extraKeyword)
+
+		new, err := Walk(dir, nil, newManifestKeywords, nil)
+		require.NoErrorf(t, err, "walk %s", dir)
+
+		diffs, err := Compare(old, new, nil)
+		require.NoError(t, err, "compare")
+
+		assert.NotEmpty(t, diffs, "extra keys in manifest should result in deltas")
+		for _, diff := range diffs {
+			if assert.Equal(t, Modified, diff.Type(), "extra keyword diff element should be 'modified'") {
+				kds := diff.Diff()
+				if assert.Len(t, kds, 1, "should only get a single key delta") {
+					kd := kds[0]
+					assert.Equalf(t, Extra, kd.Type(), "key %q", kd.Name())
+					assert.Equal(t, extraKeyword, kd.Name())
+					assert.Nil(t, kd.Old(), "Old for extra keyword delta")
+					assert.NotNil(t, kd.New(), "New for extra keyword delta")
+				}
+			}
+		}
+	})
+
+	t.Run("Missing-Key", func(t *testing.T) {
+		missingKeyword := Keyword("sha1digest")
+		newManifestKeywords := slices.DeleteFunc(manifestKeywords[:], func(kw Keyword) bool {
+			return kw == missingKeyword
+		})
+
+		new, err := Walk(dir, nil, newManifestKeywords, nil)
+		require.NoErrorf(t, err, "walk %s", dir)
+
+		diffs, err := Compare(old, new, nil)
+		require.NoError(t, err, "compare")
+
+		assert.NotEmpty(t, diffs, "missing keys in manifest should result in deltas")
+		for _, diff := range diffs {
+			if assert.Equal(t, Modified, diff.Type(), "missing keyword diff element should be 'modified'") {
+				kds := diff.Diff()
+				if assert.Len(t, kds, 1, "should only get a single key delta") {
+					kd := kds[0]
+					assert.Equalf(t, Missing, kd.Type(), "key %q", kd.Name())
+					assert.Equal(t, missingKeyword, kd.Name())
+					assert.NotNil(t, kd.Old(), "Old for missing keyword delta")
+					assert.Nil(t, kd.New(), "New for missing keyword delta")
+				}
+			}
+		}
+	})
+}
+
 //gocyclo:ignore
 func TestTarCompare(t *testing.T) {
 	dir := t.TempDir()