A try bot is used to test proposed patches which are not yet committed. A subset of try bots form the commit queue, these try bots run on every patch before it is committed. Not every try bot needs to be part of the commit queue, some are optional. An optional try bot can be manually triggered by a user who wants some additional test coverage for their patch. Optional try bots will not be triggered by the commit queue, and failures on optional try bots will not prevent a patch from being committed.
Request try job access in order to trigger try jobs against your patch.
This section assumes you've read the docs in chromium/src for details about how the iOS try bots work and how to make common changes to things like gn_args or the tests being run. The following instructions are for the case where you actually need to add a brand new try bot.
First you will need to write a config for your try bot. The configs can be found in src/ios/build/bots/chromium.mac. All iOS try bots on tryserver.chromium.mac mirror iOS builders on chromium.mac, so you need to create a config for a chromium.mac bot even if you don't actually plan on creating a new builder on chromium.mac.
Once the config is created, you will need to add the builder to master.cfg and slaves.cfg. In master.cfg, find the b_ios_* block and copy/paste. Leave the factory property as is, but change the name. Remember, the name of the try bot here must match the name of the config from the previous step. Once you've created the builder definition, add it to c['builders']. In slaves.cfg, find the ios function and add an entry to the slave_map. The key should be the name of your new bot and the value should be a list of slaves that will back the bot.
In order to get slaves, Google employees should file a ticket at go/infrasys-bug requesting 1 or 2 Macs for tryserver.chromium.mac. You will need a lot more if you plan to make your try bot part of the commit queue.
After adding a non-CQ iOS try bot, ensure you have enough capacity. Ensure you have at least as many slaves as the existing iOS CQ try bots. You can find the list of current CQ try bots in src/infra/config/cq.cfg. Find the entry for the bucket named master.tryserver.chromium.mac.
Once you have enough capacity, enable a CQ experiment by adding a builders entry with an experiment_percentage field in addition to name. Start with a small experiment, like 10%. A CQ experiment will cause your try bot to be triggered on the specified percentage of CQ requests. Failures on an experimental try bot will not block the patch from being committed. Once your try bot is seeing a lot of successful executions, ramp up the experiment. You must enable a 100% experiment to properly load test your try bot before promoting it to the commit queue. To finally promote a 100% experiment to the CQ, remove the experiment_percentage.
At this point you should also have a matching builder on chromium.mac with the same name as your try bot. See the chromium.mac docs for details.