Clean artifacts in /out as part of build_fuzzers (#1384) (#1389)

* Clean artifacts in /out as part of build_fuzzers.

* Add prompt before build directory clean, also --no-clean option.

infra/helper.py

@@ -71,6 +71,8 @@ def main():
   build_fuzzers_parser.add_argument('project_name')
   build_fuzzers_parser.add_argument('source_path', help='path of local source',
                                     nargs='?')
+  build_fuzzers_parser.add_argument('--no-clean', action='store_true',
+                                    help='do not clean existing artifacts.')
 
   check_build_parser = subparsers.add_parser(
       'check_build', help='Checks that fuzzers execute without errors.')
@@ -298,11 +300,30 @@ def build_image(args):
 
 def build_fuzzers(args):
   """Build fuzzers."""
-  project_name = args.project_name
+  no_clean = args.no_clean
+  if not no_clean:
+    y_or_n = raw_input(
+        'WARNING: Remove existing build artifacts in /out '
+        '(recommended if build config changed) ? (y/N): ')
+    no_clean = y_or_n.lower() != 'y'
 
+  project_name = args.project_name
   if not _build_image(args.project_name):
     return 1
 
+  project_out_dir = os.path.join(BUILD_DIR, 'out', project_name)
+  if no_clean:
+    print('Keeping existing build artifacts as-is (if any).')
+  else:
+    print('Cleaning existing build artifacts.')
+
+    # Clean old and possibly conflicting artifacts in project's out directory.
+    docker_run([
+        '-v', '%s:/out' % project_out_dir,
+        '-t', 'gcr.io/oss-fuzz/%s' % project_name,
+        '/bin/bash', '-c', 'rm -rf /out/*'
+    ])
+
   env = [
       'FUZZING_ENGINE=' + args.engine,
       'SANITIZER=' + args.sanitizer
@@ -321,7 +342,7 @@ def build_fuzzers(args):
         '%s:/src/%s' % (_get_absolute_path(args.source_path), args.project_name)
     ]
   command += [
-      '-v', '%s:/out' % os.path.join(BUILD_DIR, 'out', project_name),
+      '-v', '%s:/out' % project_out_dir,
       '-v', '%s:/work' % os.path.join(BUILD_DIR, 'work', project_name),
       '-t', 'gcr.io/oss-fuzz/%s' % project_name
   ]