go/zodb/zeo: Load: loadBefore RPC can return None to indicate non-existing object

Before this patch and with updated DrvTestLoad (see previous patch) it was failing as:

--- FAIL: TestLoad (0.52s)
    --- FAIL: TestLoad/py/msgpack=false (0.25s)
        xtesting.go:306: load 0285cbac258bf265:0000000000000000: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac258bf265:0000000000000000: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac258bf265:0000000000000000: 0000000000000000: no such object
        xtesting.go:306: load 0285cbac3d0369e5:0000000000000001: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac3d0369e5:0000000000000001: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac3d0369e5:0000000000000001: 0000000000000001: no such object
        xtesting.go:306: load 0285cbac41b4e832:0000000000000002: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac41b4e832:0000000000000002: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac41b4e832:0000000000000002: 0000000000000002: no such object
        xtesting.go:306: load 0285cbac4666667f:0000000000000003: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac4666667f:0000000000000003: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac4666667f:0000000000000003: 0000000000000003: no such object
        xtesting.go:306: load 0285cbac4fc96318:0000000000000004: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac4fc96318:0000000000000004: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac4fc96318:0000000000000004: 0000000000000004: no such object
        xtesting.go:306: load 0285cbac547ae165:0000000000000005: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac547ae165:0000000000000005: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac547ae165:0000000000000005: 0000000000000005: no such object
        xtesting.go:306: load 0285cbac628f5c4b:0000000000000006: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac628f5c4b:0000000000000006: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbac628f5c4b:0000000000000006: 0000000000000006: no such object
        xtesting.go:306: load 0285cbaca444447f:0000000000000007: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbaca444447f:0000000000000007: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbaca444447f:0000000000000007: 0000000000000007: no such object
        xtesting.go:306: load 0285cbacbbbbbbff:0000000000000008: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbacbbbbbbff:0000000000000008: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbacbbbbbbff:0000000000000008: 0000000000000008: no such object
        xtesting.go:306: load 0285cbad80da7498:0000000000000009: returned err unexpected:
            have: /tmp/zeo535364855/1.fs.zeosock: load 0285cbad80da7498:0000000000000009: /tmp/zeo535364855/1.fs.zeosock: call loadBefore: unexpected reply: got ogórek.None{}; expect 3-tuple
            want: /tmp/zeo535364855/1.fs.zeosock: load 0285cbad80da7498:0000000000000009: 0000000000000009: no such object
    --- FAIL: TestLoad/py/msgpack=true (0.26s)
        xtesting.go:306: load 0285cbac258bf265:0000000000000000: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac258bf265:0000000000000000: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac258bf265:0000000000000000: 0000000000000000: no such object
        xtesting.go:306: load 0285cbac3d0369e5:0000000000000001: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac3d0369e5:0000000000000001: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac3d0369e5:0000000000000001: 0000000000000001: no such object
        xtesting.go:306: load 0285cbac41b4e832:0000000000000002: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac41b4e832:0000000000000002: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac41b4e832:0000000000000002: 0000000000000002: no such object
        xtesting.go:306: load 0285cbac4666667f:0000000000000003: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac4666667f:0000000000000003: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac4666667f:0000000000000003: 0000000000000003: no such object
        xtesting.go:306: load 0285cbac4fc96318:0000000000000004: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac4fc96318:0000000000000004: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac4fc96318:0000000000000004: 0000000000000004: no such object
        xtesting.go:306: load 0285cbac547ae165:0000000000000005: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac547ae165:0000000000000005: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac547ae165:0000000000000005: 0000000000000005: no such object
        xtesting.go:306: load 0285cbac628f5c4b:0000000000000006: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac628f5c4b:0000000000000006: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbac628f5c4b:0000000000000006: 0000000000000006: no such object
        xtesting.go:306: load 0285cbaca444447f:0000000000000007: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbaca444447f:0000000000000007: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbaca444447f:0000000000000007: 0000000000000007: no such object
        xtesting.go:306: load 0285cbacbbbbbbff:0000000000000008: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbacbbbbbbff:0000000000000008: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbacbbbbbbff:0000000000000008: 0000000000000008: no such object
        xtesting.go:306: load 0285cbad80da7498:0000000000000009: returned err unexpected:
            have: /tmp/zeo794664426/1.fs.zeosock: load 0285cbad80da7498:0000000000000009: /tmp/zeo794664426/1.fs.zeosock: call loadBefore: zlink is closed
            want: /tmp/zeo794664426/1.fs.zeosock: load 0285cbad80da7498:0000000000000009: 0000000000000009: no such object

go/zodb/storage/zeo/proto.go

@@ -421,3 +421,37 @@ func (e encoding) asString(xs interface{}) (string, bool) {
 		}
 	}
 }
+
+
+// None returns object that represents None in encoding e.
+func (e encoding) None() interface{} {
+	switch e {
+	default:
+		panic("bug")
+
+		// pickle: None
+	case 'Z':
+		return pickle.None{}
+
+		// msgpack: nil
+	case 'M':
+		return nil
+	}
+}
+
+// isNone verifies whether object corresponds to None encoded via encoding e.
+func (e encoding) isNone(xv interface{}) bool {
+	switch e {
+	default:
+		panic("bug")
+
+	case 'Z':
+		// pickle: None
+		_, ok := xv.(pickle.None)
+		return ok
+
+		// msgpack: nil
+	case 'M':
+		return (xv == nil)
+	}
+}

go/zodb/storage/zeo/zeo.go

@@ -98,9 +98,12 @@ func (z *zeo) Load(ctx context.Context, xid zodb.Xid) (buf *mem.Buf, serial zodb
 	}
 
 	// (data, serial, next_serial | None)
+	if enc.isNone(xres) {
+		return nil, 0, &zodb.NoObjectError{Oid: xid.Oid}
+	}
 	res, ok := enc.asTuple(xres)
 	if !ok || len(res) != 3 {
-		return nil, 0, rpc.ereplyf("got %#v; expect 3-tuple", xres)
+		return nil, 0, rpc.ereplyf("got %#v; expect 3-tuple | None", xres)
 	}
 
 	data, ok1 := enc.asBytes(res[0])

go/zodb/storage/zeo/zrpc.go

@@ -132,7 +132,7 @@ func (zl *zLink) shutdown(err error) {
 		zl.callMu.Unlock()
 
 		for _, rxc := range callTab {
-			rxc <- msg{arg: nil} // notify link was closed	XXX ok? or err explicitly?
+			close(rxc) // notify link was closed
 		}
 	})
 }
@@ -276,14 +276,14 @@ func (zl *zLink) Call(ctx context.Context, method string, argv ...interface{}) (
 	case <-ctx.Done():
 		return msg{}, ctx.Err()
 
-	case reply = <-rxc:
-		if reply.arg == nil {
+	case reply, ok := <-rxc:
+		if !ok {
 			// we were woken up because of shutdown
 			return msg{}, errLinkClosed
 		}
-	}
 
-	return reply, nil
+		return reply, nil
+	}
 }
 
 // reply sends reply to a call received with msgid.