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.go

@@ -278,10 +278,11 @@ func (ic *irCache) encode(
 		return *a == *b
 	})
 
-	// kind uint64 | deps_sz uint64
-	var buf [wordSize * 2]byte
+	// kind uint64 | rev uint64 | deps_sz uint64
+	var buf [wordSize * 3]byte
 	binary.LittleEndian.PutUint64(buf[:], uint64(a.Kind()))
-	binary.LittleEndian.PutUint64(buf[wordSize:], uint64(len(idents)))
+	binary.LittleEndian.PutUint64(buf[wordSize:], GetRevision(a))
+	binary.LittleEndian.PutUint64(buf[wordSize*2:], uint64(len(idents)))
 	if _, err = w.Write(buf[:]); err != nil {
 		return
 	}
@@ -740,6 +741,15 @@ func (ir *IRReader) ReadString() string {
 	return unsafe.String(unsafe.SliceData(p), len(p))
 }
 
+// A RevisionError describes the first [Artifact] in an IR stream claiming a
+// revision not supported by the registered implementation.
+type RevisionError [2]uint64
+
+func (e RevisionError) Error() string {
+	return "claimed revision " + strconv.FormatUint(e[0], 10) +
+		" differs from implementation value " + strconv.FormatUint(e[1], 10)
+}
+
 // decode decodes the next [Artifact] in the IR stream and returns any buffer
 // originating from [Cache] before returning. decode returns [io.EOF] if and
 // only if the underlying [io.Reader] is already read to EOF.
@@ -772,6 +782,9 @@ func (d *IRDecoder) decode() (a Artifact, err error) {
 		return
 	}
 
+	ir.mustRead(ir.buf[:])
+	rev := binary.LittleEndian.Uint64(ir.buf[:])
+
 	defer ir.putAll()
 	ir.mustRead(ir.buf[:])
 	sz := binary.LittleEndian.Uint64(ir.buf[:])
@@ -792,6 +805,10 @@ func (d *IRDecoder) decode() (a Artifact, err error) {
 		err = syscall.ENOTRECOVERABLE
 		return
 	}
+	if _rev := GetRevision(a); _rev != rev {
+		err = RevisionError{rev, _rev}
+		return
+	}
 
 	if ir.count != -1 {
 		err = ErrRemainingIR