I've got a fairly sizeable application that runs as a CGI app under Apache which I am attempting to convert to FastCGI.
"Once through and done" apps tend not to care much if deallocation is less than perfect, since exit(0) (or otherwise) tends to heal all wounds. Not so much when something's running for hours, days or weeks. There memory leaks are ruinous.
I've wrapped all of my internal functionality plus all calls to PQexecParams and the escape/unescape functions, all of which must be deallocated after being used. All counters after a pass through the code are zero (increment on allocate, decrement on PQfremem or PQclear.)
But -- I still have a lot of memory out on the heap according to jemalloc stats that is not being deallocated, and what's worse is that if I rig the code to call PQfinish and then PQconnect once again I get even more imbalanced allocate/free counts (and the memory use in said buckets to go with them.)
The application itself is nothing particularly fancy although it typically makes dozens of Postgres calls; single-threaded, no prepared statements or async requests.
This is under 14.1; I haven't rolled the code forward, but I see nothing in the notes implying there is a problem in libpq that has been corrected, or that there was one in the past in this regard. Its also possible that the FastCGI wrapper has a problem internally. The app, when run under valgrind to do cron processing, comes back clean -- it does show allocations on exit however, although "still accessible" and those which do come up are related from OpenSSL's error string initialization in /lib/libcrypto.so (I'm on FreeBSD and openssl was specified as "--with-openssl" when Postgres was built.)
The obvious question, given the warnings in the FastCGI library: Does libpq modify the process environment? Reading from it (provided you don't modify anything from the pointers you access; if you want to then you must copy them somewhere and make the modification outside of the environment itself) is perfectly fine but writing it, directly or indirectly, is NOT. A quick grep implies that indeed it may in backend/libpq/auth.c at least, but I do not have ENABLE_GSS defined in my configuration so that code shouldn't be there.