Thread: xml_is_document and selective pg_re_throw

xml_is_document and selective pg_re_throw

From
Nikhil Sontakke
Date:
Hi, <br /><br />Consider:<br /><br />SELECT xml '<foo>bar</foo><bar>foo</bar>' IS DOCUMENT;<br
/><br/>And I was looking at xml_is_document() source code. It calls xml_parse which throws an error with code set to
ERRCODE_INVALID_XML_DOCUMENT.The catch block of xml_parse then rethrows. <br /><br />Now xml_is_document does a
selectiverethrow only if the error is not ERRCODE_INVALID_XML_DOCUMENT. I can understand that this function does this
toreturn true/false, but doesn't this behavior of not propagating the error up all the way dangerous?
InterruptHoldoffCountinconsistencies for instance? <br /><br />A better way would have been to modify xml_parse to take
anadditional boolean argument "to_rethrow" and not to rethrow if that is false? Thoughts? <br /><br />Regards,<br
/>Nikhils<br /> 

Re: xml_is_document and selective pg_re_throw

From
Tom Lane
Date:
Nikhil Sontakke <nikkhils@gmail.com> writes:
> Consider:

> SELECT xml '<foo>bar</foo><bar>foo</bar>' IS DOCUMENT;

> And I was looking at xml_is_document() source code. It calls xml_parse
> which throws an error with code set to ERRCODE_INVALID_XML_DOCUMENT. The
> catch block of xml_parse then rethrows.

> Now xml_is_document does a selective rethrow only if the error is not
> ERRCODE_INVALID_XML_DOCUMENT. I can understand that this function does this
> to return true/false, but doesn't this behavior of not propagating the
> error up all the way dangerous? InterruptHoldoffCount inconsistencies for
> instance?

No, I don't see any particular risk there.  The places that might throw
ERRCODE_INVALID_XML_DOCUMENT are sufficiently few (as in, exactly one,
in this usage) that we can have reasonable confidence we know what the
system state is when we catch that error.

> A better way would have been to modify xml_parse to take an additional
> boolean argument "to_rethrow" and not to rethrow if that is false?

We could do that, but it would greatly complicate xml_parse IMO, since
it still needs its own PG_TRY block to handle other error cases, and
only one of those error cases ought to optionally return failure instead
of re-throwing.
        regards, tom lane


Re: xml_is_document and selective pg_re_throw

From
Nikhil Sontakke
Date:
 
No, I don't see any particular risk there.  The places that might throw
ERRCODE_INVALID_XML_DOCUMENT are sufficiently few (as in, exactly one,
in this usage) that we can have reasonable confidence we know what the
system state is when we catch that error.


Hmmm, I was writing some code in which I happened to hold a LWLock when this function was called. The first catch/rethrow cleaned up the InterruptHoldoffCount value. A subsequent release of that LWLock tripped up the (Assert(InterruptHoldoffCount > 0);) inside RESUME_INTERRUPTS().

I know holding an lwlock like this might not be a good idea, but this behavior just got me thinking about other probable issues.

Regards,
Nikhils
 
> A better way would have been to modify xml_parse to take an additional
> boolean argument "to_rethrow" and not to rethrow if that is false?

We could do that, but it would greatly complicate xml_parse IMO, since
it still needs its own PG_TRY block to handle other error cases, and
only one of those error cases ought to optionally return failure instead
of re-throwing.

                       regards, tom lane