xml_is_document and selective pg_re_throw

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

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