Add gn arg archive_seed_corpus

archive_seed_corpus is false by default. If it is true then seed
corpora won't get built. This will be used to reduce build sizes on
most fuzzer build bots since they don't all need seed corpora.

Bug: 952412
Change-Id: I67b925756763e6221e88a3d47961f20c5a831054
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566418
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650500}

build/config/sanitizers/sanitizers.gni

@@ -113,6 +113,9 @@ declare_args() {
   # happens with no extra flags. This is useful when in component build
   # enabling sanitizers only in some of the components.
   use_sanitizer_configs_without_instrumentation = false
+
+  # When true, seed corpora archives are built.
+  archive_seed_corpus = true
 }
 
 # Disable sanitizers for non-default toolchains.

testing/libfuzzer/archive_corpus.py

@@ -25,8 +25,10 @@ def main():
                       nargs='+')
   parser.add_argument('--output', metavar='output_archive_name.zip',
                       required=True)
-  args = parser.parse_args()
+  parser.add_argument('--dry_run', default=False, action='store_true',
+                      help="Don't actually create the output archive.")
 
+  args = parser.parse_args()
   corpus_files = []
   seed_corpus_path = args.output
 
@@ -39,6 +41,9 @@ def main():
         full_filename = os.path.join(dirpath, filename)
         corpus_files.append(full_filename)
 
+  if args.dry_run:
+    return
+
   with zipfile.ZipFile(seed_corpus_path, 'w') as z:
     # Turn warnings into errors to interrupt the build: crbug.com/653920.
     with warnings.catch_warnings():

testing/libfuzzer/fuzzer_test.gni

@@ -74,6 +74,12 @@ template("fuzzer_test") {
           }
         }
 
+        # Tell the script not to actually zip the seed corpus if we have been
+        # instructed not to build it.
+        if (!archive_seed_corpus) {
+          args += [ "--dry_run" ]
+        }
+
         outputs = [
           out,
         ]