[Pidgin] #17331: Allow % sign in username for XMPP account creation

[Pidgin]

#17331: Allow % sign in username for XMPP account creation
-------------------------+-------------------------------------------------
 Reporter:  Byakushin    |      Owner:  EionRobb
     Type:  defect       |     Status:  new
Milestone:               |  Component:  unclassified
  Version:  3.0.0hg      |   Keywords:  username inputvalidation
                         |  specialcharacters
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 Hi,

 Pidgin 3.0.0dev introduces a regression by stricter input validation.

 The 'Add account' dialog, even when selecting protocol type XMPP, refuses
 to allow '%' characters in the username. In other words, if the username
 field has special characters such as % and _ which are fine for many XMPP
 servers, the 'Add' button remains greyed out.

 To reproduce:
 - Accounts -> Manage accounts -> Add...
 - Select protocol as XMPP, fill in username and domain fields with text,
 but then add a % character into the username
 - Observe how 'Add' button becomes greyed out even though it was active as
 soon as username was provided (without %)

 Wanted outcome:
 - Do not block account adding if account has e.g. % and _ characters in it
 - i.e. allow any characters in username that are allowed by the XMPP
 protocol.

 Impact:

 The '%' character is a common ('de facto standard' the way it feels to me,
 even) replacement used for converting email-based cross-organizational
 (enterprise) user accounts into XMPP server accounts, e.g. john at doe.com
 would get an account on collaborativexmppserver.com that would take the
 form of john%doe.com at collaborativexmppserver.com. I've run into this
 limitation multiple times this year already.

 Existing workaround is to add the user as johndoe.com, save, close Pidgin,
 edit .purple/accounts.xml manually to add the % character, and then
 restart Pidgin.

 The % sign in username works fine on current stable Pidgin as far as I've
 heard. This impacts only 3dev but I figure it will impact the future
 stable 3 branch unless it's fixed before that. :)

 Thank you, have a great day!

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Ticket URL: <https://developer.pidgin.im/ticket/17331>
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