基本的 DBus 偵錯技巧
經過幾年發展，DBus 已經取代早年 Linux 桌面環境所用的 GNOME Bonobo, KDE DCOP，用於許多應用程式，成為主流 IPC 系統。隨著軟體原件逐漸成熟，眾多程式語言都已經支援 DBus APIs，DBus daemon 的 footprint 也逐漸能夠被嵌入式所接受，而被行動裝置作業系統如 MeeGo, WebOS所採用。
Linux 開發者難免因爲介接需求，需要測試或使用 DBus 除錯。這裡分享幾個常用的小技巧。DBus 使用物件導向的 API 界面，所有 Services 的 Object 函式都是以 Method, Signals, Properties 的概念揭露給外界存取，配合 Introspectable API，很容易讓第三方介接。
最常用到的工具之一是 dbus-send，它可以用來從指令列測試接傳 DBus messages，像是列出系統上所有註冊在 Session Bus 的 Services
$ dbus-send --session --print-reply --reply-timeout=2000 \
--type=method_call --dest=org.freedesktop.DBus /org/freedesktop/DBus \
org.freedesktop.DBus.ListActivatableNames
有了 Services 名稱，接下來你就可以用 Service 為名以 Introspection 界面去查詢其所開放之 API，如
$ dbus-send --session --print-reply --reply-timeout=2000 --type=method_call \
--dest=org.gnome.ScreenSaver / org.freedesktop.DBus.Introspectable.Introspect
method return sender=:1.7256 -> dest=:1.7286 reply_serial=2
string "<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
<interface name="org.freedesktop.DBus.Introspectable">
<method name="Introspect">
<arg name="data" direction="out" type="s"/>
</method>
</interface>
<interface name="org.gnome.ScreenSaver">
<method name="Lock">
</method>
<method name="Cycle">
</method>
<method name="SimulateUserActivity">
</method>
<method name="Inhibit">
<arg name="application_name" direction="in" type="s"/>
<arg name="reason" direction="in" type="s"/>
<arg name="cookie" direction="out" type="u"/>
</method>
<method name="UnInhibit">
<arg name="cookie" direction="in" type="u"/>
</method>
<method name="GetInhibitors">
<arg name="list" direction="out" type="as"/>
</method>
<method name="Throttle">
<arg name="application_name" direction="in" type="s"/>
<arg name="reason" direction="in" type="s"/>
<arg name="cookie" direction="out" type="u"/>
</method>
<method name="UnThrottle">
<arg name="cookie" direction="in" type="u"/>
</method>
<method name="GetActive">
<arg name="value" direction="out" type="b"/>
</method>
<method name="GetActiveTime">
<arg name="seconds" direction="out" type="u"/>
</method>
<method name="SetActive">
<arg name="value" direction="in" type="b"/>
</method>
<method name="ShowMessage">
<arg name="summary" direction="in" type="s"/>
<arg name="body" direction="in" type="s"/>
<arg name="icon" direction="in" type="s"/>
</method>
<signal name="ActiveChanged">
<arg name="new_value" type="b"/>
</signal>
</interface>
</node>
"
$ dbus-send --session --print-reply --reply-timeout=2000 \
--type=method_call --dest=org.gnome.Tomboy \
/org/gnome/Tomboy/RemoteControl org.freedesktop.DBus.Introspectable.Introspect
string "<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<!-- NDesk.DBus 0.6.0 -->
<node>
<interface name="org.freedesktop.DBus.Introspectable">
<method name="Introspect">
<arg name="data" direction="out" type="s" />
</method>
</interface>
<interface name="org.gnome.Tomboy.RemoteControl">
<method name="Version">
<arg name="ret" direction="out" type="s" />
</method>
<method name="DisplayNote">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="HideNote">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="DisplayNoteWithSearch">
<arg name="uri" direction="in" type="s" />
<arg name="search" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="FindNote">
<arg name="linked_title" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="FindStartHereNote">
<arg name="ret" direction="out" type="s" />
</method>
<method name="CreateNote">
<arg name="ret" direction="out" type="s" />
</method>
<method name="CreateNamedNote">
<arg name="linked_title" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="CreateNamedNoteWithUri">
<arg name="linked_title" direction="in" type="s" />
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="DeleteNote">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="DisplaySearch" />
<method name="DisplaySearchWithText">
<arg name="search_text" direction="in" type="s" />
</method>
<method name="NoteExists">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="ListAllNotes">
<arg name="ret" direction="out" type="as" />
</method>
<method name="GetNoteContents">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="GetNoteTitle">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="GetNoteCreateDate">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="x" />
</method>
<method name="GetNoteChangeDate">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="x" />
</method>
<method name="GetNoteContentsXml">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="GetNoteCompleteXml">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="SetNoteContents">
<arg name="uri" direction="in" type="s" />
<arg name="text_contents" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="SetNoteContentsXml">
<arg name="uri" direction="in" type="s" />
<arg name="xml_contents" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="SetNoteCompleteXml">
<arg name="uri" direction="in" type="s" />
<arg name="xml_contents" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="GetTagsForNote">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="as" />
</method>
<method name="AddTagToNote">
<arg name="uri" direction="in" type="s" />
<arg name="tag_name" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="RemoveTagFromNote">
<arg name="uri" direction="in" type="s" />
<arg name="tag_name" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="GetAllNotesWithTag">
<arg name="tag_name" direction="in" type="s" />
<arg name="ret" direction="out" type="as" />
</method>
<method name="GetNotebookForNote">
<arg name="uri" direction="in" type="s" />
<arg name="ret" direction="out" type="s" />
</method>
<method name="AddNoteToNotebook">
<arg name="uri" direction="in" type="s" />
<arg name="notebook_name" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="GetAllNotesInNotebook">
<arg name="notebook_name" direction="in" type="s" />
<arg name="ret" direction="out" type="as" />
</method>
<method name="AddNotebook">
<arg name="notebook_name" direction="in" type="s" />
<arg name="ret" direction="out" type="b" />
</method>
<method name="SearchNotes">
<arg name="query" direction="in" type="s" />
<arg name="case_sensitive" direction="in" type="b" />
<arg name="ret" direction="out" type="as" />
</method>
<signal name="NoteDeleted">
<arg name="uri" direction="out" type="s" />
<arg name="title" direction="out" type="s" />
</signal>
<signal name="NoteAdded">
<arg name="uri" direction="out" type="s" />
</signal>
<signal name="NoteSaved">
<arg name="uri" direction="out" type="s" />
</signal>
</interface>
</node>"
注意其中 object name 因各 Service 定義不同，首次查詢可從 / 開始查，Dbus 會答覆其子路徑。
有了 API 界面之後，就可以直接用 dbus-send 送些指令給這些 Services 啦。如下啟動螢幕保護程式。
dbus-send --session --dest=org.gnome.ScreenSaver \
--type=method_call --print-reply --reply-timeout=20000 \
/ org.gnome.ScreenSaver.SetActive boolean:true
參數的格式可參考 DBus 規格中的 Type Signatures。
如果你是在 Desktop 環境想查詢測試 DBus APIs, 比較容易的工具是使用 d-feet，這是 Python/GTK 所寫的工具，只消滑鼠點點就可以查詢各種 API 與送出指令訊息。(以下圖片出自 d-feet)
知道了基本的測試工具後，測試期間通常需要觀測訊息的傳送與反應是否正確，此時可以利用 dbus-monitor。它可以用來監測系統中所有的 Dbus messages，方便查詢軟體是否運作正常。
由於安全性的考量，dbus-monitor 預設只能監錄 Session Bus 中的訊息，意即在每個 Login session 中使用者所開啟的軟體。至於 System Bus，像是 Network Manager 等預設是無法存取的，這是避免惡意軟體竊取隱私，如密碼等資訊。
必須手動開啟權限，設定方法是更改以下設定檔，開啟 eavesdrop policy，並重啟 Dbus daemon. 這樣才能竊聽相關通訊。
cat > /etc/dbus-1/system-local.conf
<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
<policy context="default">
<!-- All messages may be received by default -->
<allow receive_requested_reply="false" receive_type="method_call" eavesdrop="true"/>
<allow receive_requested_reply="false" receive_type="method_return" eavesdrop="true"/>
<allow receive_requested_reply="false" receive_type="error" eavesdrop="true"/>
<allow receive_requested_reply="false" receive_type="signal" eavesdrop="true"/>
<allow eavesdrop="true"/>
</policy>
<policy user="root">
<allow send_destination="*" eavesdrop="true"/>
<allow receive_sender="*" eavesdrop="true"/>
</policy>
</busconfig>
詳盡資訊請參考
- D-Bus Specification
- MeeGo D-Bus/Overview
- Scripting D-Bus