Allow plugins to override server selection when a client joins

Closes #129.

1 files changed, 66 insertions, 0 deletions

diff --git a/plugin_srvselect.go b/plugin_srvselect.go
new file mode 100644
index 0000000..327efd5
--- /dev/null
+++ b/plugin_srvselect.go
@@ -0,0 +1,66 @@
+package proxy
+
+import (
+	"errors"
+	"sync"
+)
+
+var (
+	ErrEmptySrvSelectorName = errors.New("server selector name is empty")
+)
+
+var (
+	srvSelectors    map[string]func(*ClientConn) (string, Server)
+	srvSelectorMu   sync.RWMutex
+	srvSelectorOnce sync.Once
+)
+
+// RegisterSrvSelector registers a server selection handler
+// which can be enabled using the `SrvSelector` config option.
+// Empty names are an error.
+// If the handler returns an empty server name,
+// the regular server selection procedure is used.
+// Only one server selector can be active at a time.
+func RegisterSrvSelector(name string, sel func(*ClientConn) (string, Server)) error {
+	initSrvSelectors()
+
+	if name == "" {
+		return ErrEmptySrvSelectorName
+	}
+
+	srvSelectorMu.Lock()
+	defer srvSelectorMu.Unlock()
+
+	srvSelectors[name] = sel
+	return nil
+}
+
+func selectSrv(cc *ClientConn) (string, Server) {
+	sel := Conf().SrvSelector
+
+	if sel == "" {
+		return "", Server{}
+	}
+
+	initSrvSelectors()
+
+	srvSelectorMu.RLock()
+	defer srvSelectorMu.RUnlock()
+
+	handler, ok := srvSelectors[sel]
+	if !ok {
+		cc.Log("<-", "server selector not registered")
+		return "", Server{}
+	}
+
+	return handler(cc)
+}
+
+func initSrvSelectors() {
+	srvSelectorOnce.Do(func() {
+		srvSelectorMu.Lock()
+		defer srvSelectorMu.Unlock()
+
+		srvSelectors = make(map[string]func(*ClientConn) (string, Server))
+	})
+}