Implement (inventory) formspec plugin API

Node-based formspecs are not supported because there is no node API yet.
Proxy-level formspec functionality is invisible to upstream servers. The
API can be used to display a formspec that is submitted to the upstream
server. It is also able to intercept the submission of formspecs
displayed by the upstream server.

Closes #131.

3 files changed, 88 insertions, 0 deletions

diff --git a/client_conn.go b/client_conn.go
index 1a16d94..2f5ba04 100644
--- a/client_conn.go
+++ b/client_conn.go
@@ -66,6 +66,8 @@ type ClientConn struct {
 	modChsMu sync.RWMutex
 
 	cltInfo *mt.ToSrvCltInfo
+
+	FormspecPrepend string
 }
 
 // Name returns the player name of the ClientConn.
diff --git a/plugin_formspec.go b/plugin_formspec.go
new file mode 100644
index 0000000..7803108
--- /dev/null
+++ b/plugin_formspec.go
@@ -0,0 +1,82 @@
+package proxy
+
+import (
+	"strings"
+	"sync"
+
+	"github.com/HimbeerserverDE/mt"
+)
+
+var (
+	onPlayerReceiveFields     map[string][]func(*ClientConn, []mt.Field)
+	onPlayerReceiveFieldsMu   sync.Mutex
+	onPlayerReceiveFieldsOnce sync.Once
+)
+
+// ShowFormspec opens a formspec on the client.
+// The form name should follow the standard minetest naming convention,
+// i.e. "pluginname:formname".
+// If formname is empty, it reshows the inventory formspec
+// without updating it for future opens.
+// If formspec is empty, the formspec is closed.
+// If formname is empty at the same time, any formspec is closed
+// regardless of its name.
+func (cc *ClientConn) ShowFormspec(formname string, formspec string) {
+	cc.SendCmd(&mt.ToCltShowFormspec{
+		Formspec: cc.FormspecPrepend + formspec,
+		Formname: formname,
+	})
+}
+
+// FormspecEscape escapes characters that cannot be used in formspecs.
+func FormspecEscape(formspec string) string {
+	formspec = strings.ReplaceAll(formspec, "\\", "\\\\")
+	formspec = strings.ReplaceAll(formspec, "[", "\\[")
+	formspec = strings.ReplaceAll(formspec, "]", "\\]")
+	formspec = strings.ReplaceAll(formspec, ";", "\\;")
+	formspec = strings.ReplaceAll(formspec, ",", "\\,")
+	formspec = strings.ReplaceAll(formspec, "$", "\\$")
+	return formspec
+}
+
+// RegisterOnPlayerReceiveFields registers a callback that is called
+// when a client submits a specific formspec.
+// Events triggering this callback include a button being pressed,
+// Enter being pressed while a text field is focused,
+// a checkbox being toggled, an item being selected in a dropdown list,
+// selecting a new tab, changing the selection in a textlist or table,
+// selecting an entry in a textlist or table, a scrollbar being moved
+// or the form actively being closed by the player.
+func RegisterOnPlayerReceiveFields(formname string, handler func(*ClientConn, []mt.Field)) {
+	initOnPlayerReceiveFields()
+
+	onPlayerReceiveFieldsMu.Lock()
+	defer onPlayerReceiveFieldsMu.Unlock()
+
+	onPlayerReceiveFields[formname] = append(onPlayerReceiveFields[formname], handler)
+}
+
+func handleOnPlayerReceiveFields(cc *ClientConn, cmd *mt.ToSrvInvFields) bool {
+	onPlayerReceiveFieldsMu.Lock()
+	defer onPlayerReceiveFieldsMu.Unlock()
+
+	handlers, ok := onPlayerReceiveFields[cmd.Formname]
+	if !ok || len(handlers) == 0 {
+		return false
+	}
+
+	for _, handler := range handlers {
+		handler(cc, cmd.Fields)
+	}
+
+	return true
+}
+
+func initOnPlayerReceiveFields() {
+	onPlayerReceiveFieldsOnce.Do(func() {
+		onPlayerReceiveFieldsMu.Lock()
+		defer onPlayerReceiveFieldsMu.Unlock()
+
+		onPlayerReceiveFields = make(map[string][]func(*ClientConn, []mt.Field))
+	})
+}
diff --git a/process.go b/process.go
index 862f29d..a68238c 100644
--- a/process.go
+++ b/process.go
@@ -480,6 +480,10 @@ func (cc *ClientConn) process(pkt mt.Pkt) {
 	case *mt.ToSrvCltInfo:
 		// Store for any future hops (need to send it to the new server).
 		cc.cltInfo = cmd
+	case *mt.ToSrvInvFields:
+		if handleOnPlayerReceiveFields(cc, cmd) {
+			return
+		}
 	}
 
 	forward(pkt)