prosopopee: allow selecting number of threads used for thumbnail generation

Generating thumbnails is done in parallel threads via
multiprocessing.Pool. By default, Pool schedules tasks on as many
threads as there are cpu threads on the host machine.

Let's allow users to select the number of threads Pool can use.

Signed-off-by: Quentin Schulz <foss@0leil.net>

Author: QSchulz

prosopopee/prosopopee.py

@@ -47,7 +47,15 @@ def loglevel(string):
     help="Configure the logging level",
 )
 subparser = parser.add_subparsers(dest="cmd")
-subparser.add_parser("build", help="Generate static site")
+parser_build = subparser.add_parser("build", help="Generate static site")
+parser_build.add_argument(
+    "-j",
+    "--jobs",
+    default=None,
+    type=int,
+    help="Specifies number of jobs (thumbnail generations) to run simultaneously. Default: number "
+    "of threads available on the system",
+)
 subparser.add_parser("test", help="Verify all your yaml data")
 subparser.add_parser("preview", help="Start preview webserver on port 9000")
 subparser.add_parser("deploy", help="Deploy your website")
@@ -923,7 +931,12 @@ def main():
         logger.info("Success: HTML file building without error")
         sys.exit(0)
 
-    with Pool() as pool:
+    # If prosopopee is started without any argument, 'build' is assumed but the jobs parameter
+    # is not part of the namespace, so set its default to None (or 'number of available CPU
+    # treads')
+    jobs = args.jobs if args.cmd else None
+
+    with Pool(jobs) as pool:
         # Pool splits the iterable into pre-defined chunks which are then assigned to processes.
         # There is no other scheduling in play after that. This is an issue when chunks are
         # outrageously unbalanced in terms of CPU time which happens when most galleries are