proto: Update 'Compression' to int to support different compression algorithms

With nexedi/neoppod@fd80cc30 NEO/py added support to encode the compression
algorithm with the 'Compression' parameter. Before this, compression could
only be true (= with compression) or false (= without compression). Now
the absence of compression is encoded with 0. Any other number than 0
encodes a compression algorithm. The mapping is currently:

	1 = zlib

In the future, 2 could mean zstd [1].

[1] https://github.com/facebook/zstd/issues/1134

go/neo/client.go

@@ -381,7 +381,10 @@ func (c *Client) Load(ctx context.Context, xid zodb.Xid) (buf *mem.Buf, serial z
 		}
 	}
 
-	if resp.Compression {
+	if resp.Compression != 0 {
+		if resp.Compression != 1 {
+			return nil, 0, fmt.Errorf("unsupported compression algorithm: %v", resp.Compression)
+		}
 		buf2 := &mem.Buf{Data: nil}
 		udata, err := xzlib.Decompress(buf.Data)
 		buf.Release()

go/neo/neo_test.go

@@ -257,7 +257,7 @@ func _TestMasterStorage(t0 *tEnv) {
 		Oid:		xid1.Oid,
 		Serial:		serial1,
 		NextSerial:	proto.INVALID_TID,
-		Compression:	false,
+		Compression:	0,
 		Data:		buf1,
 		DataSerial:	0,		// XXX
 		Checksum:	sha1.Sum(buf1.Data),
@@ -292,7 +292,7 @@ func _TestMasterStorage(t0 *tEnv) {
 		Oid:		xid1prev.Oid,
 		Serial:		serial1prev,
 		NextSerial:	serial1,
-		Compression:	false,
+		Compression:	0,
 		Data:		buf1prev,
 		DataSerial:	0,		// XXX
 		Checksum:	sha1.Sum(buf1prev.Data),

go/neo/proto/proto.go

@@ -336,6 +336,11 @@ func (a *Address) neoDecodeN(b []byte) (uint64, bool) {
 // Checksum is a SHA1 hash.
 type Checksum [20]byte
 
+// Compression is an integer that tells the compression algorithm:
+//	0 = no compression algorithm is used
+//	1 = zlib is used
+type Compression uint64
+
 // PTid is Partition Table identifier.
 //
 // Zero value means "invalid id" (<-> None in py.PPTID, nil in msgpack)
@@ -715,7 +720,7 @@ type AnswerRebaseObject struct {
 	Serial         zodb.Tid
 	ConflictSerial zodb.Tid
 	// FIXME POption('data')
-		Compression bool
+		Compression Compression
 		Checksum    Checksum
 		Data        *mem.Buf
 }
@@ -729,7 +734,7 @@ type AnswerRebaseObject struct {
 type StoreObject struct {
 	Oid         zodb.Oid
 	Serial      zodb.Tid
-	Compression bool
+	Compression Compression
 	Checksum    Checksum
 	Data        []byte // TODO -> msg.Buf, separately (for writev)
 	DataSerial  zodb.Tid
@@ -784,7 +789,7 @@ type AnswerObject struct {
 	Oid         zodb.Oid
 	Serial      zodb.Tid
 	NextSerial  zodb.Tid
-	Compression bool
+	Compression Compression
 	Checksum    Checksum
 	Data        *mem.Buf // TODO encode -> separately (for writev)
 	DataSerial  zodb.Tid
@@ -1219,7 +1224,7 @@ type AddTransaction struct {
 type AddObject struct {
 	Oid         zodb.Oid
 	Serial      zodb.Tid
-	Compression bool
+	Compression Compression
 	Checksum    Checksum
 	Data        *mem.Buf
 	DataSerial  zodb.Tid

go/neo/storage/fs1/fs1.go

@@ -87,7 +87,7 @@ func (f *Backend) Load(ctx context.Context, xid zodb.Xid) (*proto.AnswerObject,
 		Serial:     serial,
 		NextSerial: nextSerial,
 
-		Compression: false,
+		Compression: 0,
 		Data:        buf,
 		Checksum:    xsha1.NEOSum(buf.Data),	// XXX computing every time