Ah, it's chromium_perf_fyi, not chromium_fyi. Continue looking...

It seems that other ToT bots actually get clobbered. Not sure what's special about 'CFI Linux ToT'. At the same time, we probably need to apply 'clobber' config to 'clang_tot' anyway:
https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/chromium/config.py?q=%5B%27%5Dclang.tot%5B%27%5D&sq=package:chromium&dr=C&l=521

This might fix 'CFI Linux ToT'.

> At the same time, we probably need to apply 'clobber' config to 'clang_tot'

The clobbering of Clang ToT bots (and Clang builds in general) rely on the build config to add -DCR_CLANG_REVISION=123456 to the build flags. When the revision changes, that causes the files to be recompiled. This is also how developers builds get clobbered when we roll Clang. I don't know why this doesn't work for the CFI bot.

Never mind, I found it:

https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn?q=_CLANG_REVISION&sq=package:chromium&l=234&dr=C


  if (is_clang && !is_nacl) {
    # This is here so that all files get recompiled after a clang roll and
    # when turning clang on or off. (defines are passed via the command line,
    # and build system rebuild things when their commandline changes). Nothing
    # should ever read this define.
    defines += [ "CR_CLANG_REVISION=$clang_revision" ]
  }

Yup, that's the one.

From 'generate build files' step on ClangToTLinux:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTLinux/builds/5499/steps/generate_build_files/logs/stdio

Writing """\
clang_use_chrome_plugins = false
is_clang = true
is_component_build = true
is_debug = false
llvm_force_head_revision = true
symbol_level = 2
""" to /b/build/slave/ClangToTLinux/build/src/out/Release/args.gn.

And the same step from 'CFI Linux ToT':

Writing """\
is_cfi = true
is_component_build = false
is_debug = false
""" to /b/build/slave/CFI_Linux_ToT/build/src/out/Release/args.gn.

Hmm, but is_clang should be true anyway when targeting Linux, supposedly because of https://cs.chromium.org/chromium/src/build/config/BUILDCONFIG.gn?rcl=0&l=125

Okay, I know what to do; just a sec.

The issue is in mb_config.pyl.

#8: possibly, the lack of llvm_force_head_revision sets the stable LLVM revision inside GN.

My best guess for the fix: https://codereview.chromium.org/2039103003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0475505c4370a6186fd73ffc9c8ae07af55bf0ae

commit 0475505c4370a6186fd73ffc9c8ae07af55bf0ae
Author: krasin <krasin@google.com>
Date: Tue Jun 07 00:42:32 2016

Add clang_tot config to 'CFI Linux ToT' bot.

Otherwise, GN does not get that it's a ToT build and the build dir
is not clobbered.

BUG= 617771 

Review-Url: https://codereview.chromium.org/2039103003
Cr-Commit-Position: refs/heads/master@{#398177}

[modify] https://crrev.com/0475505c4370a6186fd73ffc9c8ae07af55bf0ae/tools/mb/mb_config.pyl

It seems to be clobbered now.