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.Introspectmethod 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

来源：http://people.debian.org.tw/~chihchun/2011/03/08/test-and-debug-dbus/