SQL Server stored procedure return code oddity

Question

The client that calls this code is restricted and can only deal with return codes from stored procs. So, we modified our usual contract to RETURN -1 on error and default to RETURN 0 if no error

If the code hits the inner catch block, then the RETURN code default is -4 rather then 0

Does anyone know where this comes from please? With reference

Cheers gbn

IF OBJECT_ID('dbo.foo') IS NOT NULL DROP TABLE dbo.foo
GO
CREATE TABLE dbo.foo (
    KeyCol  char(12) NOT NULL,
    ValueCol xml NOT NULL,
    Comment varchar(1000) NULL,
    CONSTRAINT PK_foo PRIMARY KEY CLUSTERED (KeyCol)
)
GO

IF OBJECT_ID('dbo.bar') IS NOT NULL DROP PROCEDURE dbo.bar
GO
CREATE PROCEDURE dbo.bar
    @Key char(12),
    @Value xml,
    @Comment varchar(1000)
AS
SET NOCOUNT ON
DECLARE @StartTranCount tinyint;
BEGIN TRY
    SELECT @StartTranCount = @@TRANCOUNT;

    IF @StartTranCount = 0 BEGIN TRAN;

    BEGIN TRY
        --SELECT @StartTranCount = 'fish' --generates an error and goes to outer CATCH
        INSERT dbo.foo (KeyCol, ValueCol, Comment) VALUES (@Key, @Value, @Comment);
    END TRY
    BEGIN CATCH
        IF ERROR_NUMBER() = 2627    --PK violation
            UPDATE
                dbo.foo
            SET
                ValueCol = @Value, Comment = @Comment
            WHERE
                KeyCol = @Key;
        ELSE
            RAISERROR ('Tits up', 16, 1);
    END CATCH

    IF @StartTranCount = 0 COMMIT TRAN;
END TRY
BEGIN CATCH
    IF @StartTranCount = 0 AND XACT_STATE() <> 0 ROLLBACK TRAN;
    RETURN -1
END CATCH
--Without this, we'll send -4 if we hit the UPDATE CATCH block above
--RETURN 0
GO

--please run these **separately**

--Run with RETURN 0 and fish line commented out
DECLARE @rtn int
EXEC @rtn = dbo.bar 'abcdefghijkl', '<foobar />', 'testing'
SELECT @rtn; SELECT * FROM dbo.foo
GO

DECLARE @rtn int
EXEC @rtn = dbo.bar 'abcdefghijkl', '<foobar2 />', 'testing2'
--updated OK but we get @rtn = -4
SELECT @rtn; SELECT * FROM dbo.foo
GO

--uncomment fish line
DECLARE @rtn int
EXEC @rtn = dbo.bar 'abcdefghijkl', '<foobar />', 'testing'
--Hit outer CATCH, @rtn = -1 as expected
SELECT @rtn; SELECT * FROM dbo.foo

Answer 1

In playing around with the procedure, I can get a a return -6, if I insert a null into foo.KeyCol and remove the RAISERROR in the inner catch. This is something SQL Server is doing, and is documented here: Return Values from Stored Procedures.