internal/pkg: implementation revisions

This enables changing the behaviour of an artifact implementation without changing its IR structure. Unfortunately this changes the IR header structure resulting in many rebuilds.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/ir_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"io"
 	"io/fs"
+	"math"
 	"reflect"
 	"testing"
 
@@ -135,3 +136,36 @@ func TestIRRoundtrip(t *testing.T) {
 	}
 	checkWithCache(t, testCasesCache)
 }
+
+func TestRevision(t *testing.T) {
+	checkWithCache(t, []cacheTestCase{{"revision", 0, nil,
+		func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
+			a := pkg.NewFile("", nil)
+			var buf bytes.Buffer
+			if err := c.EncodeAll(&buf, a); err != nil {
+				t.Fatal(err)
+			}
+
+			if _, err := c.NewDecoder(
+				bytes.NewReader(buf.Bytes()),
+			).Decode(); err != nil {
+				t.Fatalf("Decode: error = %v", err)
+			}
+
+			p := buf.Bytes()
+			p[8] = 0xfd
+			wantErr := pkg.RevisionError{math.MaxUint64 - 2, math.MaxUint64}
+			if _, err := c.NewDecoder(
+				bytes.NewReader(p),
+			).Decode(); !reflect.DeepEqual(err, wantErr) {
+				t.Fatalf("Decode: error = %v, want %v", err, wantErr)
+			}
+		}, expectsFS{
+			".":          {Mode: fs.ModeDir | 0700},
+			"checksum":   {Mode: fs.ModeDir | 0700},
+			"identifier": {Mode: fs.ModeDir | 0700},
+			"substitute": {Mode: fs.ModeDir | 0700},
+			"work":       {Mode: fs.ModeDir | 0700},
+		},
+	}})
+}