Thanks for the report. This failure is unrelated to the coverage script or coverage instrumentation, you just need to run the tests in a way like:

$ python testing/xvfb.py out/Coverage/unit_tests --gtest_filter=EncryptionMigrationScreenHandlerTest.*

However, it's possible that the script may not work properly if you pass this command to the script via -c arg. Yuke, do we require all commands start with a binary executable?

If I put python into the -c arg I get an assertion error:

AssertionError: Target executable "python" in command: "python testing/xvfb.py out/Coverage/unit_tests --gtest_filter=CryptohomeUtilTest.*" is outside of the given build directory: "out/Coverage".

Also I independently discovered xvfb-run which I tried to use with the python call, but I think this won't work. I can't confirm at any rate because ToT doesn't compile atm.

Thanks for testing! I'm not surprised by the assert you've got. Assigning to Yuke now.

Just for fun I disabled that assert and I get a different error:

maajid@maajid:~/chromium/src$ python tools/code_coverage/coverage.py unit_tests -b out/Coverage -o out/Report -c '/usr/bin/python testing/xvfb.py out/Coverage/unit_tests --gtest_filter=CryptohomeUtilTest.*' -f chrome/browser/ui/webui/chromeos/ -f chromeos/
[2018-02-07 14:14:33,938] Building ['unit_tests']
ninja: Entering directory `out/Coverage'
ninja: no work to do.
[2018-02-07 14:14:36,746] Running command: "/usr/bin/python testing/xvfb.py out/Coverage/unit_tests --gtest_filter=CryptohomeUtilTest.*", the output is redirected to "out/Report/unit_tests_output.txt"
[2018-02-07 14:14:38,014] Creating the coverage profile data file
out/Report/unit_tests.9092094292374385578_3.profraw: jmemmgr.c:alloc_small: Function basic block count change detected (counter mismatch)
Make sure that all profile data to be merged is generated from the same binary.
[2018-02-07 14:14:57,661] Code coverage profile data is created as: out/Report/coverage.profdata
[2018-02-07 14:14:57,730] Generating code coverage report in html (this can take a while depending on size of target!)
error: /usr/bin/python: Failed to load coverage: No coverage data found
error: Could not load coverage information
Traceback (most recent call last):
  File "tools/code_coverage/coverage.py", line 1110, in <module>
    sys.exit(Main())
  File "tools/code_coverage/coverage.py", line 1080, in Main
    binary_paths, profdata_file_path, absolute_filter_paths)
  File "tools/code_coverage/coverage.py", line 849, in _GeneratePerFileCoverageSummary
    json_output = json.loads(subprocess.check_output(subprocess_cmd))
  File "/usr/lib/python2.7/subprocess.py", line 219, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['/usr/local/google/home/maajid/chromium/src/third_party/llvm-build/Release+Asserts/bin/llvm-cov', 'export', '-summary-only', '-instr-profile=out/Report/coverage.profdata', '/usr/bin/python', '/usr/local/google/home/maajid/chromium/src/chrome/browser/ui/webui/chromeos/', '/usr/local/google/home/maajid/chromium/src/chromeos/']' returned non-zero exit status 1

Thanks for testing it out maajid@! I'm at a convergence this week, I'll take a look at this error next week.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/545db3293cc1f6425fa68d87d18758977d9e1d38

commit 545db3293cc1f6425fa68d87d18758977d9e1d38
Author: Yuke Liao <liaoyuke@chromium.org>
Date: Thu Feb 15 17:12:01 2018

[Coverage] Support running tests using xvfb

This CL supports running tests using xvfb.py in the command.

Bug:  809377 
Change-Id: I3d8462209b947b7e9f655b378d705b89112773b5
Reviewed-on: https://chromium-review.googlesource.com/915303
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537054}
[modify] https://crrev.com/545db3293cc1f6425fa68d87d18758977d9e1d38/tools/code_coverage/coverage.py

Awesome, it works. Thanks!