Re: [Xen-devel] [HACKERS] [OSSTEST PATCH 0/1] PostgreSQL db: Retry on constraint violation

[Thomas Munro <thomas.munro@xxxxxxxxxxxxxxxx>]

On Tue, Dec 13, 2016 at 10:47 AM, Kevin Grittner <kgrittn@xxxxxxxxx> wrote:
> On Mon, Dec 12, 2016 at 1:06 PM, Kevin Grittner <kgrittn@xxxxxxxxx> wrote:
>> On Mon, Dec 12, 2016 at 12:32 PM, Kevin Grittner <kgrittn@xxxxxxxxx> wrote:
>>
>>> As you can see, this generated a serialization failure.
>>
>> That was on 9.6.  On earlier versions it does indeed allow the
>> transaction on connection 2 to commit, yielding a non-serializable
>> result.  This makes a pretty strong case for back-patching this
>> commit:
>>
>> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=fcff8a575198478023ada8a48e13b50f70054766
>
> I have confirmed that this patch applies cleanly to all supported
> branches (with some offsets), that the bug is manifest without the
> patch, and that it is fixed with this patch in all supported
> branches.  This patch has been in the development code base for
> about 8 months and in production with the 9.6 release, so it has
> been in active production for 3 months with no sign of trouble.  If
> you ignore the code comment, doc changes, and new regression tests
> it consists of adding one line of code to nbtinsert.c.  What it
> does is that when a "unique_violation" error is about to fire, it
> adds a check to see whether the conditions for a serialization
> failure also exist; if so, it fires that error instead.
>
> This was not initially back-patched, in spite of numerous requests
> to do so, because it was a behavior change and not clearly a bug --
> it has a least some minimal chance of changing behavior someone
> might be relying on; however, Ian has constructed a use-case where
> without this patch we clearly allow a serialization anomaly which
> is not allowed with the patch, so this patch should, IMO, be
> considered a bug fix on that basis.
>
> Barring objections I will back-patch to 9.2 through 9.5 tomorrow.
> (9.1 is out of support and the fix is already in 9.6 and forward.)

+1

Ian's test case uses an exception handler to convert a difference in
error code into a difference in committed effect, thereby converting a
matter of programmer convenience into a bug.

For the record, read-write-unique-4.spec's permutation r2 w1 w2 c1 c2
remains an open question for further work.

-- 
Thomas Munro
http://www.enterprisedb.com

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