syz-manager, syz-fuzzer: switch communication to flatrpc

[dvyukov]

syz-manager, syz-fuzzer: switch communication to flatrpc

Switch to flatrpc connection between manager and fuzzer.
With flatrpc we have a goroutine per connection instead of async RPC,
which makes things a bit simpler. Now don't reordered messages
(in particular start executing and finish executing for programs),
race on the program during printing is no longer possible
since we finish handlign start executing request before we even
receive finish executing.
We also don't need to lookup Runner for every RPC since it's
now local to the handling goroutine.
We also don't need to protect requests map since only single
goroutine accesses it.
We also send new programs to the fuzzer as soon as we receive
start executing message, which provides better buffering.
We also don't batch new requests and finish executing requests
in a single RPC, which makes things a bit simpler.

Update #1541

[dvyukov]

Note: this breaks syz-runtest. But since we don't use it, it looks pointless to fix. We better restore it as part of syz-manager. Now with dynamic request source, it should fit into syz-manager very nicely -- after checking we just switch to runtest source instead of fuzzer source.

[a-nogikh]

After checking we just switch to runtest source instead of fuzzer source.

We'd also need to deal with the web UI, corpus file operations and multiple ways in which the vm loop is interleaved with the fuzzer :( That's all doable, but unfortunately it's not "just switch" if we want to do it cleanly.

[dvyukov]

ranch o

After checking we just switch to runtest source instead of fuzzer source.

We'd also need to deal with the web UI, corpus file operations and multiple ways in which the vm loop is interleaved with the fuzzer :( That's all doable, but unfortunately it's not "just switch" if we want to do it cleanly.

I wouldn't bother about web UI. What do we need to do for the VM loop?

[a-nogikh]

Decouple it from syz-hub and bug reproduction (which we don't need for runtest).

[codecov]

Codecov Report

Attention: Patch coverage is 0.20619% with 484 lines in your changes are missing coverage. Please review.

Project coverage is 61.1%. Comparing base (c0f1611) to head (d98a661).
Report is 1 commits behind head on master.

Additional details and impacted files

Files	Coverage Δ
syz-manager/stats.go	0.0% <ø> (ø)
syz-manager/manager.go	0.0% <0.0%> (ø)
syz-fuzzer/fuzzer.go	4.7% <1.4%> (+0.7%)	⬆️
syz-fuzzer/proc.go	0.0% <0.0%> (ø)
syz-manager/rpc.go	0.0% <0.0%> (ø)

... and 1 file with indirect coverage changes