SQL> recreate table t(id int); commit;
SQL> insert into t values(1); commit;
SQL> update t set id=id where id=1;

commit; set transaction snapshot lock timeout 2;
set list on;
set term ^;
execute block returns( txt varchar(40), gds int ) as
declare x int;
begin
    gds = null;
    begin
        txt='try select locked row for update:';
        select id from t where id=1 for update with lock into x;
    when any do 
        begin
           gds = gdscode;
           suspend;
        end  
    end

    begin
        txt='try to direct update locked row:';
        update t set id=id where id=1;
    when any do 
        begin
           gds = gdscode;
           suspend;
        end  
    end
end
^set term ;^
set list off;

TXT                             try select locked row for update:
GDS                             335544878 -- это "concurrent_transaction"

TXT                             try to direct update locked row:
GDS                             335544336 -- а это "deadlock"

SQL> select id from t where id=1 for update with lock;

          ID
============
Statement failed, SQLSTATE = HY000
concurrent transaction number is 4772130


SQL> update t set id=id where id=1;
Statement failed, SQLSTATE = 40001
 deadlock 
 -update conflicts with concurrent update 
-concurrent transaction number is 4772130

commit; set transaction snapshot lock timeout 2;
set list on;
set term ^;
execute block returns( txt varchar(40), gds int ) as
declare x int;
begin
    gds = null;
    begin
        txt='try select locked row for update:';
        select id from t where id=1 for update with lock into x;
    when  gdscode concurrent_transaction, gdscode deadlock do
        begin
           gds = gdscode;
           suspend;
        end
    when any do
        begin
            txt='something else happens...';
            gds = gdscode;
            suspend;
        end
    end
end
^set term ;^
set list off;

Statement failed, SQLSTATE = HY000
request synchronization error
After line 3 in file trylock.sql

recreate exception ex_nowait_or_timeout_required 'transaction must start in NO WAIT mode or with LOCK_TIMEOUT.';
recreate table test_log(dts timestamp default 'now', msg varchar(50), fb_gdscode int);
recreate table t(id int);
insert into t values(1);
commit;
set term ^;
create or alter procedure try_lock_test(
  a_table varchar(31),
  a_id dm_ids
) as
    declare v_dbkey dm_dbkey;
    declare v_retries int;
    declare v_timeout int;
    declare v_stt varchar(255);
    declare i int;
begin

    -- Check that current Tx run in NO wait or with lock_timeout.
    -- Otherwise raise error: performance degrades almost to zero.
    --execute procedure sp_check_nowait_or_timeout;
    if ( rdb$get_context('SYSTEM', 'LOCK_TIMEOUT') < 0 ) then
        exception ex_nowait_or_timeout_required;
    
    -- Tries to LOCK record if current Tx was started with LOCK TIMEOUT > 0
    -- or NO wait; log errors if they occur.
    -- Raises: concurrent_transaction OR deadlock (TWO kind of exceptions!)

    v_timeout = cast( rdb$get_context('SYSTEM', 'LOCK_TIMEOUT') as int );
    if ( v_timeout >= 0) then -- ">=" - do not forget NO wait mode!
    begin

        v_retries = iif( v_timeout > 0, 5, 1);
        i=1;
        while (i <= v_retries) do
        begin
            begin
                v_stt = 'select rdb$db_key from '||a_table||' where id = :x for update with lock';
                -- this can raise exc. "concurrent_transaction (335544878)":
                 execute statement (v_stt) ( x := :a_id ) into v_dbkey; 

                if ( v_dbkey is NOT null) then
                begin
                    v_stt = 'update '||a_table||' set id=id where rdb$db_key = :x';
                    -- this can raise TWO exceptions at the same time:
                    -- 1) deadlock  (335544336)
                    -- 2) -update conflicts with concurrent update (335544451)
                    -- But in when-block we'll get only first of them ("deadlock").
                    -- Test: sql.ru/forum/actualutils.aspx?action=gotomsg&tid=1088890&msg=15879042
                    execute statement (v_stt) ( x := v_dbkey );
                    --if ( i > 1 ) then
                       --execute procedure sp_add_to_abend_log( v_prefix||' - achieve lock after '||(i-1)||' fault attempt(s).' );
                end
                i = v_retries;
                LEAVE;

            when gdscode concurrent_transaction, gdscode deadlock do
                begin
                    if ( i = v_retries ) then
                    begin
                        in autonomous transaction
                        do insert into test_log(msg, fb_gdscode) values('can`t lock row after '||:v_retries||' attempts', gdscode);
                         exception;  -- all number of retries exceeded: raise concurrent_transaction OR deadlock
                    end
                end
            when any do
                begin
                    in autonomous transaction
                    do insert into test_log(msg, fb_gdscode) values('smth else happens on iter = '||:i, gdscode);
                     exception; 
                end
            end
            i = i +1;
        end
    end
end
^ set term ;^
commit;

 session #1 
SQL> update t set id=id where id=1;

 session #2 
SQL> delete from test_log;  commit; 
SQL> set transaction read committed lock timeout 1;  
SQL> execute procedure try_lock_test('t', 1);

Statement failed, SQLSTATE = 40001
deadlock
-concurrent transaction number is 4772739
-At procedure 'TRY_LOCK_TEST' line: 33, col: 17
-At procedure  'TRY_LOCK_TEST' line: 56, col: 25 

                      DTS MSG                                                  FB_GDSCODE
========================= ================================================== ============
2014-04-14 14:53:48.0010  smth else happens on iter = 1                                 0
2014-04-14 14:53:50.0000  smth else happens on iter = 2                                 0
2014-04-14 14:53:50.0010  smth else happens on iter = 3                                 0
2014-04-14 14:53:52.0000  smth else happens on iter = 4                                 0
2014-04-14 14:53:52.0010  can`t lock row after 5 attempts                       335544336