Fuzz testing COPY FROM parsing

Hi,

I've been mucking around with COPY FROM lately, and to test it, I wrote 
some tools to generate input files and load them with COPY FROM:

https://github.com/hlinnaka/pgcopyfuzz

I used a fuzz testing tool called honggfuzz [1] to generate test inputs 
for COPY FROM. At first I tried to use afl and libfuzzer, but honggfuzz 
was much easier to use with PostgreSQL. It has a "persistent fuzzing 
mode", which allows starting the server normally (well, in single-user 
mode), and calling a function to get the next input. With the other 
fuzzers I tried, you have to provide a callback function that the fuzzer 
calls for each test iteration, and that was hard to integrate into the 
PostgreSQL main processing loop.

I ran it for about 2 h on my laptop with the patch I was working on [2]. 
It didn't find any crashes, but it generated about 1300 input files that 
it considered "interesting" based on code coverage analysis. When I took 
those generated inputs, and ran them against unpatched and patched 
server, some inputs produced different results. So that revealed a 
couple of bugs in the patch. (I'll post a fixed patched version on that 
thread soon.)

I hope others find this useful, too.

[1] https://github.com/google/honggfuzz
[2] 
https://www.postgresql.org/message-id/11d39e63-b80a-5f8d-8043-fff04201fadc@iki.fi

- Heikki