Avoid serialization of isc_attach_database calls issued by EXECUTE STATEMENT 
implementation
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                 Key: CORE-5648
                 URL: http://tracker.firebirdsql.org/browse/CORE-5648
             Project: Firebird Core
          Issue Type: Improvement
          Components: Engine
    Affects Versions: 4.0 Alpha 1, 3.0.2, 2.5.7, 3.0.1, 2.5.6, 3.0.0, 4.0 
Initial, 2.5.5, 2.5.4, 2.1.7, 2.5.3, 2.5.2 Update 1, 2.5.2, 2.5.1, 2.5.0
         Environment: Super Server and Super Classic architectures. 
Classic is not affected.
            Reporter: Vlad Khorsun


When EXECUTE STATEMENT ... ON EXTERNAL DATA SOURCE was introiduced in v2.5.0 
there was informal agreement that calls of 
isc_attach_database should be serialized to avoid deadlocks in fbclient 
(reported by users from time to time, but never reproduced).
Such serialization is almost not visible by end users when all works as 
expected, in particular when remote hosts can be reached via network.
But when there is some network issues, or when remote address is not available 
there could be significant delays before isc_attach_database 
returns corresponding error. Due to serialization of call of 
isc_attach_database by the engine all other attachments running EXECUTE 
STATEMENT 
and connecting to the their remote databases should wait for the unlucky one.

Starting from v3 it seems we can remove this artificial limitation in the 
engine and allow all attachments to call isc_attach_database 
in parallel without serialization.

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