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xml2: Improve error handling of libxml2 calls
The contrib module xml2/ has always been fuzzy with the cleanup of the memory allocated by the calls internal to libxml2, even if there are APIs in place giving a lot of control over the error behavior, all located in the backend's xml.c. The code paths fixed in the commit address multiple defects, while sanitizing the code: - In xpath.c, several allocations are done by libxml2 for xpath_workspace, whose memory cleanup could go out of sight as it relied on a single TRY/CATCH block done in pgxml_xpath(). workspace->res is allocated by libxml2, and may finish by not being freed at all upon a failure outside of a TRY area. This code is refactored so as the TRY/CATCH block of pgxml_xpath() is moved one level higher to its callers, which are responsible for cleaning up the contents of a workspace on failure. cleanup_workspace() now requires a volatile workspace, forcing as a rule that a TRY/CATCH block should be used. - Several calls, like xmlStrdup(), xmlXPathNewContext(), xmlXPathCtxtCompile(), etc. can return NULL on failures (for most of them allocation failures. These forgot to check for failures, or missed that pg_xml_error_occurred() should be called, to check if an error is already on the stack. - Some memory allocated by libxml2 calls was freed in an incorrect way, "resstr" in xslt_process() being one example. The class of errors fixed here are for problems that are unlikely going to happen in practice, so no backpatch is done. The changes have finished by being rather invasive, so it is perhaps not a bad thing to be conservative and to keep these changes only on HEAD anyway. Author: Michael Paquier <michael@paquier.xyz> Reported-by: Karavaev Alexey <maralist86@mail.ru> Reviewed-by: Jim Jones <jim.jones@uni-muenster.de> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/18943-2f2a04ab03904598@postgresql.org
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