centipede: create workdir in prepare instead of the constructor

[paulsemel]

It turns out that the same class can be used for different fuzzing rounds, and the parent directory of the temp dir is being cleared in between each round. For that reason, we need to re-create a workdir in prepare.

[alhijazi]

For engine fuzzers (do_engine_fuzzing in fuzz_task.py [1]):
we run the fuzzing engine in multiple rounds (run_engine_fuzzer[2])
at the end of each round, run_engine_fuzzer cleans up the temp directory using fuzzer_utils.cleanup()[3]
This deletes centipede's workdir [4]

[1]

clusterfuzz/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py

Line 1509 in ca14f3a

for fuzzing_round in range(_get_max_testcases()):

[2]https://github.com/google/clusterfuzz/blob/master/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py#L1212
[3]https://github.com/google/clusterfuzz/blob/master/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py#L1262
[4]

clusterfuzz/src/clusterfuzz/_internal/bot/fuzzers/centipede/engine.py

Line 152 in ca14f3a

self.workdir = self._create_temp_dir('workdir')

[paulsemel]

For engine fuzzers (do_engine_fuzzing in fuzz_task.py [1]): we run the fuzzing engine in multiple rounds (run_engine_fuzzer[2]) at the end of each round, run_engine_fuzzer cleans up the temp directory using fuzzer_utils.cleanup()[3] This deletes centipede's workdir [4]

[1]

clusterfuzz/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py

Line 1509 in ca14f3a

for fuzzing_round in range(_get_max_testcases()):

[2]https://github.com/google/clusterfuzz/blob/master/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py#L1212
[3]https://github.com/google/clusterfuzz/blob/master/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py#L1262
[4]

clusterfuzz/src/clusterfuzz/_internal/bot/fuzzers/centipede/engine.py

Line 152 in ca14f3a

self.workdir = self._create_temp_dir('workdir')

Indeed, that's what's happening and why this change is necessary, thanks for making this crystal clear!

[jonathanmetzman]

lgtm