dbus: handle bizarre dbus proxy behaviour

There is a strange behaviour in xdg-dbus-proxy where if any interface string when stripped of a single ".*" suffix does not contain a '.' byte anywhere, the program will exit with code 1 without any output. This checks for such conditions to make the failure less confusing.

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

dbus/config.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"strings"
 )
 
 // ProxyPair is an upstream dbus address and a downstream socket path.
@@ -27,6 +28,61 @@ type Config struct {
 	Filter bool `json:"filter"`
 }
 
+func (c *Config) interfaces(yield func(string) bool) {
+	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
+		}
+	}
+}
+
+func (c *Config) checkInterfaces(segment string) error {
+	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
+}
+
 func (c *Config) Args(bus ProxyPair) (args []string) {
 	argc := 2 + len(c.See) + len(c.Talk) + len(c.Own) + len(c.Call) + len(c.Broadcast)
 	if c.Log {
@@ -63,9 +119,7 @@ func (c *Config) Args(bus ProxyPair) (args []string) {
 	return
 }
 
-func (c *Config) Load(r io.Reader) error {
+func (c *Config) Load(r io.Reader) error { return json.NewDecoder(r).Decode(&c) }
-	return json.NewDecoder(r).Decode(&c)
-}
 
 // NewConfigFromFile opens the target config file at path and parses its contents into *Config.
 func NewConfigFromFile(path string) (*Config, error) {

dbus/proxy.go

@@ -2,6 +2,7 @@ package dbus
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"os/exec"
 	"sync"
@@ -14,6 +15,15 @@ import (
 // Overriding ProxyName will only affect Proxy instance created after the change.
 var ProxyName = "xdg-dbus-proxy"
 
+type BadInterfaceError struct {
+	Interface string
+	Segment   string
+}
+
+func (e *BadInterfaceError) Error() string {
+	return fmt.Sprintf("bad interface string %q in %s bus configuration", e.Interface, e.Segment)
+}
+
 // Proxy holds the state of a xdg-dbus-proxy process, and should never be copied.
 type Proxy struct {
 	helper helper.Helper
@@ -66,9 +76,15 @@ func Finalise(sessionBus, systemBus ProxyPair, session, system *Config) (final *
 
 	var args []string
 	if session != nil {
+		if err = session.checkInterfaces("session"); err != nil {
+			return
+		}
 		args = append(args, session.Args(sessionBus)...)
 	}
 	if system != nil {
+		if err = system.checkInterfaces("system"); err != nil {
+			return
+		}
 		args = append(args, system.Args(systemBus)...)
 	}