[tor-bugs] #32823 [Core Tor/Tor]: support Android SharedPreferences xml as a torrc format

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#32823: support Android SharedPreferences xml as a torrc format
-------------------------------------------------+-------------------------
 Reporter:  eighthave                            |          Owner:  (none)
     Type:  enhancement                          |         Status:  new
 Priority:  Medium                               |      Component:  Core
                                                 |  Tor/Tor
  Version:                                       |       Severity:  Normal
 Keywords:  Android, xml, tbb-mobile, torrc,     |  Actual Points:
  sharedpreferences                              |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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 With Tor Browser, Orbot, and other apps on Android, some Tor settings are
 managed via the `SharedPreferences`/`PreferenceFragment` Android classes.
 When used in their standard setup, these handle the whole lifecycle of
 preferences.  Right now, we have to do a lot of tricks to get the settings
 from the Android classes, convert them to `torrc` format, then handle
 reloading Tor at the right time.

 Android's `SharedPreferences` writes out to a standard xml file, e.g
 `/data/data/org.torproject.android/shared_prefs/org.torproject.android_preferences.xml`.
 This XML file has a simple format that has been the same since the
 beginning.  If Tor can read this format as an optional `torrc` format, and
 also potentially automatically reload the config when that file changes,
 then the Android code will be drastically simpler.  This will greatly
 improve the interaction around any kind of user-facing setting, like
 bridges, exit countries, pluggable transports, etc.

 Here's an example of what the XML would look like:
 {{{
 <?xml version='1.0' encoding='utf-8' standalone='yes' ?>
 <map>
     <boolean name="RunAsDaemon" value="true" />
     <boolean name="AvoidDiskWrites" value="true" />
     <string name="TransPort">auto</string>
     <int name="DNSPort" value="5400" />
     <string name="TransPort" value="auto" />
     <string name="Bridges">obfs4 77.81.104.251:443
 115C90EBD0EB631C177560A872535772215478D9
 cert=UsuF7oN4KNKviZP54JOyTCoCphrdM5gwZK4vT8GnCAcmqLUJEJxyw1dpko9a/ii6He4iZg
 iat-mode=0,obfs4 5.249.146.133:80 FAF3A0073330D6AD92F3B4874B0D945562A633EF
 cert=TRe8bAODtjcGij7EPQaUayWEOqR99wDh2l3B4hFtCsn1JTJCph03pRZ9tx8wynpLYKWMQg
 iat-mode=0,meek_lite 0.0.2.0:2 97700DFE9F483596DDA6264C4D7DF7641E1E39CE
 url=https://meek.azureedge.net/ front=ajax.aspnetcdn.com</string>
 </map>
 }}}

 We might want an alternate format for bridge lines, where would be a
 dedicated XML file for only bridge lines with the `name` specifying the
 line number:
 {{{
 <?xml version='1.0' encoding='utf-8' standalone='yes' ?>
 <map>
     <string name="0">obfs4 77.81.104.251:443
 115C90EBD0EB631C177560A872535772215478D9
 cert=UsuF7oN4KNKviZP54JOyTCoCphrdM5gwZK4vT8GnCAcmqLUJEJxyw1dpko9a/ii6He4iZg
 iat-mode=0</string>
     <string name="1">obfs4 5.249.146.133:80
 FAF3A0073330D6AD92F3B4874B0D945562A633EF
 cert=TRe8bAODtjcGij7EPQaUayWEOqR99wDh2l3B4hFtCsn1JTJCph03pRZ9tx8wynpLYKWMQg
 iat-mode=0</string>
     <string name="2">meek_lite 0.0.2.0:2
 97700DFE9F483596DDA6264C4D7DF7641E1E39CE url=https://meek.azureedge.net/
 front=ajax.aspnetcdn.com</string>
 </map>
 }}}

 Yxml is a 6k C XML parser in a single file under an MIT license.  It has
 been maintained since 2013.  It would fit the bill quite nicely.
 https://dev.yorhel.nl/yxml

 Yes, XML is not an ideal format.  We don't need the most problematic parts
 of XML for this, e.g. namespaces, custom entities, etc. and Yxml does not
 support them anyway.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32823>
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