Make the json_view comparison fair to fresh documents and robust

- compare.py: make --data, --corpus, and --build-dir absolute, since the
  benchmarks run in the build directory; download into a .part file and
  remove an archive whose SHA-256 does not match, so that an interrupted
  download is not kept
- bench_view/bench_corpus/bench_edit: report files that cannot be opened instead of
  aborting; run each engine once untimed before its timed call, so that
  no engine pays for the allocator cleaning up after the previous one
  (with glibc, json_view after json::parse looked 1.7x slower on
  citm_catalog traverse); add "simdjson DOM (fresh)" and time
  "json_view (reused)" for traverse and select too
- README: explain fresh vs. reused documents and page faults on Linux

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/benchmarks/json_view/compare.py

@@ -154,11 +154,14 @@ def download_library(name, work):
     if not os.path.isfile(archive):
         print(f'downloading {pin["url"]}', flush=True)
         # the URLs are the https constants in PINNED, and the SHA-256 is checked below
-        urllib.request.urlretrieve(pin['url'], archive)  # nosec B310
+        # (into a .part file first, so that an interrupted download is not kept)
+        urllib.request.urlretrieve(pin['url'], archive + '.part')  # nosec B310
+        os.replace(archive + '.part', archive)
     with open(archive, 'rb') as f:
         digest = hashlib.sha256(f.read()).hexdigest()
     if digest != pin['sha256']:
-        sys.exit(f'error: SHA-256 of {archive} is {digest}, expected {pin["sha256"]}')
+        os.remove(archive)  # downloaded again by the next run
+        sys.exit(f'error: SHA-256 of {archive} is {digest}, expected {pin["sha256"]} (removed)')
     src = os.path.join(work, 'download', pin['dir'])
     if not os.path.isdir(src):
         with tarfile.open(archive) as t:
@@ -214,6 +217,10 @@ def main():
     ap.add_argument('--corpus', nargs='*', default=[], help='more files for bench_corpus')
     ap.add_argument('--build-dir', default=os.path.join(HERE, 'build'), help='where to build (default: build/ next to this script)')
     args = ap.parse_args()
+    # the benchmarks run in the build directory: make the paths absolute
+    args.data = os.path.abspath(args.data)
+    args.corpus = [os.path.abspath(f) for f in args.corpus]
+    args.build_dir = os.path.abspath(args.build_dir)
 
     cxx = os.environ.get('CXX', 'c++')
     cc = os.environ.get('CC', 'cc')