How to simulate a deadlock in SQL Server in a single process?

(Apparently I don't have enough reputation to add a comment. So posting as an answer.)

A deadlock requires at least two processes. the only exception being is the intra-query parallel deadlocks which are kind of impossible to reproduce.

However you can simulate a deadlock on two processes running the exact same query (or sp). Some ideas here

As many have pointed out, the answer is no, a single process cannot reliably deadlock itself. I came up with the following solution to simulate a deadlock on a development or test system..

Run the script below in a SQL Server Management Studio window. (Tested on 2008 R2 only.) You can leave it running as long as necessary.

In the place you want to simulate a deadlock, insert a call to sp_simulatedeadlock. Run your process, and the deadlock should occur.

When done testing, stop the SSMS query and run the cleanup code at the bottom.

/*
This script helps simulate deadlocks.  Run the entire script in a SQL query window.  It will continue running until stopped.
In the target script, insert a call to sp_simulatedeadlock where you want the deadlock to occur.
This stored procedure, also created below, causes the deadlock.
When you are done, stop the execution of this window and run the code in the cleanup section at the bottom.
*/
set nocount on

if object_id('DeadlockTest') is not null
    drop table DeadlockTest

create table DeadlockTest
(
    Deadlock_Key int primary key clustered,
    Deadlock_Count int
)
go

if exists (select * from sysobjects where id = object_id(N'sp_simulatedeadlock')
           AND objectproperty(id, N'IsProcedure') = 1)
drop procedure sp_simulatedeadlock
GO

create procedure sp_simulatedeadlock
(
    @MaxDeadlocks int = -1 -- specify the number of deadlocks you want; -1 = constant deadlocking
)
as begin

    set nocount on

    if object_id('DeadlockTest') is null
        return

    -- Volunteer to be a deadlock victim.
    set deadlock_priority low

    declare @DeadlockCount int

    select @DeadlockCount = Deadlock_Count -- this starts at 0
    from DeadlockTest
    where Deadlock_Key = 2

    -- Trace the start of each deadlock event.
    -- To listen to the trace event, setup a SQL Server Profiler trace with event class "UserConfigurable:0".
    -- Note that the user running this proc must have ALTER TRACE permission.
    -- Also note that there are only 128 characters allowed in the trace text.
    declare @trace nvarchar(128)

    if @MaxDeadlocks > 0 AND @DeadlockCount > @MaxDeadlocks
    begin

        set @trace = N'Deadlock Test @MaxDeadlocks: ' + cast(@MaxDeadlocks as nvarchar) + N' @DeadlockCount: ' + cast(@DeadlockCount as nvarchar) + N' Resetting deadlock count.  Will not cause deadlock.'
        exec sp_trace_generateevent
            @eventid = 82,  -- 82 = UserConfigurable:0 through 91 = UserConfigurable:9
            @userinfo = @trace

        -- Reset the number of deadlocks.
        -- Hopefully if there is an outer transaction, it will complete and persist this change.
        update DeadlockTest
        set Deadlock_Count = 0
        where Deadlock_Key = 2
        return
    end

    set @trace = N'Deadlock Test @MaxDeadlocks: ' + cast(@MaxDeadlocks as nvarchar) + N' @DeadlockCount: ' + cast(@DeadlockCount as nvarchar) + N' Simulating deadlock.'
    exec sp_trace_generateevent
        @eventid = 82,  -- 82 = UserConfigurable:0 through 91 = UserConfigurable:9
        @userinfo = @trace

    declare @StartedTransaction bit
    set @StartedTransaction = 0
    if @@trancount = 0
    begin
        set @StartedTransaction = 1
        begin transaction
    end

    -- lock 2nd record
    update DeadlockTest
    set Deadlock_Count = Deadlock_Count
    from DeadlockTest
    where Deadlock_Key = 2

    -- lock 1st record to cause deadlock
    update DeadlockTest
    set Deadlock_Count = Deadlock_Count
    from DeadlockTest
    where Deadlock_Key = 1

    if @StartedTransaction = 1
        rollback    
end
go

insert into DeadlockTest(Deadlock_Key, Deadlock_Count)
select 1, 0
union select 2, 0

-- Force other processes to be the deadlock victim.
set deadlock_priority high

begin transaction

while 1 = 1
begin

    begin try

        begin transaction

        -- lock 1st record
        update DeadlockTest
        set Deadlock_Count = Deadlock_Count
        from DeadlockTest
        where Deadlock_Key = 1

        waitfor delay '00:00:10'

        -- lock 2nd record (which will be locked when the target proc calls sp_simulatedeadlock)
        update DeadlockTest
        set Deadlock_Count = Deadlock_Count
        from DeadlockTest
        where Deadlock_Key = 2

        rollback

    end try
    begin catch
        print 'Error ' + convert(varchar(20), ERROR_NUMBER()) + ': ' + ERROR_MESSAGE()
        goto cleanup
    end catch

end

cleanup:

if @@trancount > 0
    rollback

drop procedure sp_simulatedeadlock
drop table DeadlockTest

You can exploit a bug that Microsoft seems in no hurry to fix by running

begin tran
go

CREATE TYPE dbo.IntIntSet AS TABLE(
    Value0 Int NOT NULL,
    Value1 Int NOT NULL
)
go

declare @myPK dbo.IntIntSet;
go

rollback

This SQL will cause a deadlock with itself. Lots more details at Aaron Bertand's blog http://sqlperformance.com/2013/11/t-sql-queries/single-tx-deadlock

This works reliably from a single session. Use service broker activation to invoke the second thread which is required for a deadlock.

NOTE1: cleanup script not included
NOTE2: service broker has to be enabled: ALTER DATABASE dbname SET ENABLE_BROKER WITH ROLLBACK IMMEDIATE;

EXEC sp_executesql N'
CREATE OR ALTER PROCEDURE DeadlockReceive 
AS
DECLARE @MessageBody NVARCHAR(1000);
RECEIVE @MessageBody = CAST(message_body AS NVARCHAR(1000) )FROM DeadlockQueue
SELECT @MessageBody
EXEC sp_executesql @MessageBody;'

IF EXISTS (SELECT * FROM sys.services WHERE name = 'DeadlockService') DROP SERVICE DeadlockService
IF OBJECT_ID('DeadlockQueue') IS NOT NULL DROP QUEUE dbo.DeadlockQueue
IF EXISTS (SELECT * FROM sys.service_contracts WHERE name = 'DeadlockContract') DROP CONTRACT DeadlockContract
IF EXISTS (SELECT * FROM sys.service_message_types WHERE name = 'DeadlockMessage') DROP MESSAGE TYPE DeadlockMessage
DROP TABLE IF EXISTS DeadlockTable1 ;
DROP TABLE IF EXISTS DeadlockTable2 ;

CREATE MESSAGE TYPE DeadlockMessage VALIDATION = NONE;
CREATE QUEUE DeadlockQueue WITH STATUS = ON, ACTIVATION (PROCEDURE_NAME = DeadlockReceive, EXECUTE AS SELF, MAX_QUEUE_READERS = 1);
CREATE CONTRACT DeadlockContract AUTHORIZATION dbo (DeadlockMessage SENT BY ANY);
CREATE SERVICE DeadlockService ON QUEUE DeadlockQueue (DeadlockContract);

CREATE TABLE DeadlockTable1 (Value INT); INSERT dbo.DeadlockTable1 SELECT 1;
CREATE TABLE DeadlockTable2 (Value INT); INSERT dbo.DeadlockTable2 SELECT 1;

DECLARE @ch UNIQUEIDENTIFIER
BEGIN DIALOG @ch FROM SERVICE DeadlockService TO SERVICE 'DeadlockService' ON CONTRACT DeadlockContract WITH ENCRYPTION = OFF ;
SEND ON CONVERSATION @ch MESSAGE TYPE DeadlockMessage (N'
set deadlock_priority high;
begin tran; 
update DeadlockTable2 set value = 5;
waitfor delay ''00:00:01'';
update DeadlockTable1 set value = 5;
commit')

SET DEADLOCK_PRIORITY LOW
BEGIN TRAN
    UPDATE dbo.DeadlockTable1 SET Value = 2
    waitfor delay '00:00:01';
    UPDATE dbo.DeadlockTable2 SET Value = 2
COMMIT