A watchlist is a mechanism that allows a developer (a "watcher") to watch over portions of code that the watcher is interested in. A watcher will be cc-ed on changes that modify that portion of code, thereby giving that watcher an opportunity to make comments on codereview.chromium.org even before the change is committed.
Watchlists are defined using a WATCHLISTS file, which resides at the root of a repository. A typical WATCHLISTS file looks like:
'valgrind': ['firstname.lastname@example.org', 'email@example.com'],
In this case, watchlists named valgrind and mac are defined in WATCHLIST_DEFINITIONS and their corresponding watchers declared in WATCHLISTS.
In the example above, whenever a new changeset is created that refers to any file in tools/valgrind/, the 'valgrind' watchlist will be triggered and firstname.lastname@example.org & email@example.com will be cc-ed to the changeset for review. A regular expression can be used as the matching pattern. Matches are determined using python's re.search() function call, so matching A_WORD is the same as matching .*A_WORD.*
Each name in WATCHLISTS must be defined first in WATCHLIST_DEFINITIONS.
Watchlist processing takes place during 'gcl upload' or 'git-cl upload' and are non-binding; that is, an approval from that watcher is not needed for commit. It merely gives the watcher an opportunity to make comments, if any.
You create new watchlists or add yourself to existing watchlists by editing the WATCHLISTS file at the base of the repository.
It's advisable to run watchlists.py to verify that your new rules work.
Example (from src):
python ../depot_tools/watchlists.py PATH/TO/FILE1 PATH/TO/FILE2 ....
To override watchlist processing, use gcl upload with --no_watchlists flag, or git cl upload with --bypass-hooks.