I tried to build CPython out of the source tree with a customized read-only source tree (chmod -R -w). I got an error when running tests: tests want to write into a build/ directory... in the source tree.

Attached patch fixes this issue.

Ah, there is one and only one failing test:

======================================================================
ERROR: test_synopsis_sourceless (test.test_pydoc.PydocDocTest)
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Traceback (most recent call last):
  File "/home/haypo/prog/python/ro.master/Lib/test/test_pydoc.py", line 586, in test_synopsis_sourceless
    synopsis = pydoc.synopsis(filename)
  File "/home/haypo/prog/python/ro.master/Lib/pydoc.py", line 253, in synopsis
    mtime = os.stat(filename).st_mtime
FileNotFoundError: [Errno 2] No such file or directory: '/home/haypo/prog/python/ro.master/Lib/__pycache__/os.cpython-37.pyc'

test_synopsis_sourceless() should create a script, build it and test the generated .pyc file. So we control how the .pyc file is created.

New changeset f7e07840d725f2ebb469129cb5e6574ce61725ef by Victor Stinner in branch 'master':
bpo-30284: Fix regrtest for out of tree build (#1481)
https://github.com/python/cpython/commit/f7e07840d725f2ebb469129cb5e6574ce61725ef

I don't think that the test_pydoc failure is very important. I'm not interested to fix it right now.

The main blocker issue was regrtest and it's now fixed in the master branch.

I don't recall any user complaining about issues with read-only source tree, so I don't think that it's worth it to backport the fix. If I'm wrong, please comment the issue ;-)

New changeset a0ccc54e6dffacf9e7c06f2a3e9056d2d35d21eb by Victor Stinner in branch '3.6':
Synchronize libregrtest from master to 3.6 (#2244)
https://github.com/python/cpython/commit/a0ccc54e6dffacf9e7c06f2a3e9056d2d35d21eb