hst/dbus: validate interface strings

This is relocated to hst to validate early.

Signed-off-by: Ophestra <cat@gensokyo.uk>

hst/dbus.go

@@ -1,5 +1,27 @@
 package hst
 
+import (
+	"strconv"
+	"strings"
+)
+
+// BadInterfaceError is returned when Interface fails an undocumented check in xdg-dbus-proxy,
+// which would have cause a silent failure.
+type BadInterfaceError struct {
+	// Interface is the offending interface string.
+	Interface string
+	// Segment is passed through from the [BusConfig.CheckInterfaces] argument.
+	Segment string
+}
+
+func (e *BadInterfaceError) Message() string { return e.Error() }
+func (e *BadInterfaceError) Error() string {
+	if e == nil {
+		return "<nil>"
+	}
+	return "bad interface string " + strconv.Quote(e.Interface) + " in " + e.Segment + " bus configuration"
+}
+
 // BusConfig configures the xdg-dbus-proxy process.
 type BusConfig struct {
 	// See set 'see' policy for NAME (--see=NAME)
@@ -14,6 +36,74 @@ type BusConfig struct {
 	// Broadcast set RULE for broadcasts from NAME (--broadcast=NAME=RULE)
 	Broadcast map[string]string `json:"broadcast"`
 
-	Log    bool `json:"log,omitempty"`
+	// Log turn on logging (--log)
+	Log bool `json:"log,omitempty"`
+	// Filter enable filtering (--filter)
 	Filter bool `json:"filter"`
 }
+
+// Interfaces iterates over all interface strings specified in [BusConfig].
+func (c *BusConfig) Interfaces(yield func(string) bool) {
+	if c == nil {
+		return
+	}
+
+	for _, iface := range c.See {
+		if !yield(iface) {
+			return
+		}
+	}
+	for _, iface := range c.Talk {
+		if !yield(iface) {
+			return
+		}
+	}
+	for _, iface := range c.Own {
+		if !yield(iface) {
+			return
+		}
+	}
+
+	for iface := range c.Call {
+		if !yield(iface) {
+			return
+		}
+	}
+	for iface := range c.Broadcast {
+		if !yield(iface) {
+			return
+		}
+	}
+}
+
+// CheckInterfaces checks for invalid interface strings based on an undocumented check in xdg-dbus-error,
+// returning [BadInterfaceError] if one is encountered.
+func (c *BusConfig) CheckInterfaces(segment string) error {
+	if c == nil {
+		return nil
+	}
+
+	for iface := range c.Interfaces {
+		/*
+			xdg-dbus-proxy fails without output when this condition is not met:
+				char *dot = strrchr (filter->interface, '.');
+				if (dot != NULL)
+				  {
+				    *dot = 0;
+				    if (strcmp (dot + 1, "*") != 0)
+				      filter->member = g_strdup (dot + 1);
+				  }
+
+			trim ".*" since they are removed before searching for '.':
+				if (g_str_has_suffix (name, ".*"))
+				  {
+				    name[strlen (name) - 2] = 0;
+				    wildcard = TRUE;
+				  }
+		*/
+		if strings.IndexByte(strings.TrimSuffix(iface, ".*"), '.') == -1 {
+			return &BadInterfaceError{iface, segment}
+		}
+	}
+	return nil
+}