Cancel problems of query to pg_stat_statements

Hi!

Recently we faced a problem in out production psql installation, which was that we had to cancel all requests to the db, including performance monitoring requests, that uses ps_stat_statements. But we could not cancel the request in usual way, and had to kill -9 the pg process of it.

We've noticed that the the query execution stuck on PGSS_TEXT_FILE file reading in function qtext_load_file, which doesn't have CHECK_FOR_INTERRUPTS in the read cycle. In addition to our case with large PGSS_TEXT_FILE (and maybe the problems with virtual disk i/o) that can explain uncancellable pg_stat_statements queries.

Also, the reading block size can be reduced from 1GB to 32MB in order to increase the frequency of CHECK_FOR_INTERRUPTS calls without qtext_load_file performance degradation. To check that I did some little testing with fio like:

fio --name=readtest --filename=./random-bytes-file --rw=read --bs=32m --size=10G --ioengine=libaio --direct=1 --numjobs=1

So, I made a simple patch that adds CHECK_FOR_INTERRUPTS call in read cycle of qtext_load_file and change max value of toread from 1GB to 32MB.

I would appreciate your feedback on these changes.