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Chromium Clobber Landmines

What are they?

Clobber landmines are an easy way to send a targeted clobber to a class of builder machines (i.e. bot machines on build.chromium.org). This can be useful in the following circumstances:
  • A build breakage is discovered and requires all the bots for a given operating system to clobber
  • You know you're making a change which will introduce binary incompatibility
Landmines have the property that if a bot 'rolls over' the revision containing them (either forwards or backwards), it will trigger a clobber. This is quite useful for trybots which can be assigned random base revisions, or may even just switch between HEAD and LKGR multiple times per day. Simply clobbering the bot manually once may not be enough to make sure that it's 'fixed' for good.

How do I use them?

Simply add logic to build/get_landmines.py in the print_landmines() function. There are a variety of parameters which you can use to fine-tune the effect of a landmine:
 Landmine parameter/function  Example values  Description
 platform()  'win', 'mac', 'linux', 'ios', or 'android'  target platform being built.
 gyp_defines()  {'component': 'static_library'} ...  dict()-ification of the GYP_DEFINES env var.
 distributor()      'goma', 'ib', or None  the distributed compile technology in use for this build.
 builder()  'make', 'ninja', 'xcode', or 'msvs'  the builder technology for this build. 
 target  'Release', 'Debug'  the target that print_landmines() is generating landmines for.

To add a landmine for the given configuration, just add() a message in print_landmines(), conditioned on the configuration. Try to make the reason for the landmine as descriptive as possible, citing relevant revisions, CLs or bugs.

Example:

def print_landmines(target):
  # ... Other landmines here
  if platform() == 'win' and builder() == 'ninja':
    add('Compile on cc_unittests fails due to symbols removed in r185063.')

Frequently Asked Questions:

Q: What happens if I delete/clean up/replace landmines in get_landmines()?
A: This is OK (as in, it won't break anything), but note that it will cause bots which match the configuration of the removed landmines to clobber again. 

Sometimes this is done intentionally: if there's a frequently-clobbered configuration it may be expedient to only have one landmine which is replaced when a clobber is needed.

Q: Can I include a landmine in a CL which does other things?
A: If you know that your change will definitely require some bots to clobber, then yes, you SHOULD include it with the rest of the changes.

Q: How do I include a landmine for a change in a different repository (e.g., Blink, Skia)
A: The change to build/get_landmines.py should be included in the roll (i.e., the CL that changes DEPS to update the revision). You will need to contact the current gardener of your component (listed at the top left of the BuildBot waterfall) and have them apply the change in their roll. See for example 19836002 for an example of adding a landmine during a Blink roll. If a roll lands without a landmine (say thanks to autoroll bot), it's ok to commit a separate CL with just the landmine, ideally as close as possible after the roll, since this leaves little gap between them (you do not need to roll back and have a manual roll+landmine CL, as this is rarely necessary).

Q: I have a question that's not here, whom do I bug?
A: Please bug iannucci@chromium.org.

Details, please...

For a given build:
  1. Check out some version of the code
  2. landmines.py runs with the current GYP environment
    1. This calls get_landmines() for each POSSIBLE target (not just the one that's going to be built).
    2. If <build_dir>/<target>/.landmines doesn't exist, it's written with the result of get_landmines(<target>)
    3. Else
      1. If the result of get_landmines(<target>) differs from the content of the .landmines file, the diff is written out to <build_dir>/<target>/.landmines_triggered .
      2. Else <build_dir>/<target>/.landmines_triggered is deleted
  3. compile.py runs with a --target passed to it
    1. if <build_dir>/<target>/.landmines_triggered exists, compile.py prints the contents of the file and behaves as if --clobber was specified on the command line.
      1. clobbering includes removing both .landmines and .landmines_triggered

Use cases

Moving generated files

If you move generated files, then you need to clobber the build, otherwise stale files may be used (if they are found earlier during header search). Subtly, this shows up only on later CLs that change the generated file in the new location, but don't overwrite the stale one. See for example Issue 381111 (comment). Further, if this change happens in a separate repository (e.g., Blink), then CLs that change the generated files don't work until the repo has rolled and a landmine has been added. I.e., the steps are:
  1. Submit Blink CL moving generated files.
  2. Roll Blink to Chromium, including landmine (or include in separate followup CL).
  3. Submit Blink CL changing generated files.
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