centipede: create workdir in prepare instead of the constructor

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`.

Author: paulsemel

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

@@ -149,7 +149,6 @@ class Engine(engine.Engine):
 
   def __init__(self):
     super().__init__()
-    self.workdir = self._create_temp_dir('workdir')
 
   @property
   def name(self):
@@ -202,7 +201,8 @@ def prepare(self, corpus_dir, target_path, build_dir):
     # 1. Centipede-readable corpus file;
     # 2. Centipede-readable feature file;
     # 3. Crash reproducing inputs.
-    arguments[constants.WORKDIR_FLAGNAME] = str(self.workdir)
+    workdir = self._create_temp_dir('workdir')
+    arguments[constants.WORKDIR_FLAGNAME] = str(workdir)
 
     # Directory corpus_dir saves the corpus files required by ClusterFuzz.
     arguments[constants.CORPUS_DIR_FLAGNAME] = corpus_dir
@@ -301,7 +301,12 @@ def fuzz(self, target_path, options, reproducers_dir, max_time):  # pylint: disa
               int(fuzz_result.time_executed)))
 
     stats_filename = f'fuzzing-stats-{os.path.basename(target_path)}.000000.csv'
-    stats_file = os.path.join(self.workdir, stats_filename)
+    args = fuzzer_options.FuzzerArguments.from_list(options.arguments)
+    assert args is not None
+    assert constants.WORKDIR_FLAGNAME in args
+
+    workdir = args[constants.WORKDIR_FLAGNAME]
+    stats_file = os.path.join(workdir, stats_filename)
     stats = _parse_centipede_stats(stats_file)
     if not stats:
       stats = {}
@@ -505,9 +510,10 @@ def minimize_testcase(self, target_path, arguments, input_path, output_path,
       TimeoutError: If the testcase minimization exceeds max_time.
     """
     runner = _get_runner(target_path)
+    workdir = self._create_temp_dir('workdir')
     args = [
         f'--binary={target_path}',
-        f'--workdir={self.workdir}',
+        f'--workdir={workdir}',
         f'--minimize_crash={input_path}',
         f'--num_runs={constants.NUM_RUNS_PER_MINIMIZATION}',
         '--seed=1',
@@ -517,7 +523,7 @@ def minimize_testcase(self, target_path, arguments, input_path, output_path,
       logs.warning(
           'Testcase minimization timed out.', fuzzer_output=result.output)
       raise TimeoutError('Minimization timed out.')
-    minimum_testcase = self._get_smallest_crasher(self.workdir)
+    minimum_testcase = self._get_smallest_crasher(workdir)
     if minimum_testcase:
       shutil.copyfile(minimum_testcase, output_path)
     else: