PYTHONPYCACHEPREFIX fails when importing a module from the root ("/")

[nedbat]

BPO	38735
Nosy	@vstinner, @nedbat, @encukou
PRs	bpo-38735: Don't fail when importing from / with sys.pycache_prefix set #30456

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-11-07.12:06:03.297>
labels = []
title = 'PYTHONPYCACHEPREFIX fails when importing a module from the root ("/")'
updated_at = <Date 2022-01-07.15:59:18.551>
user = 'https://github.com/nedbat'

bugs.python.org fields:

activity = <Date 2022-01-07.15:59:18.551>
actor = 'petr.viktorin'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-11-07.12:06:03.297>
creator = 'nedbat'
dependencies = []
files = []
hgrepos = []
issue_num = 38735
keywords = ['patch']
message_count = 3.0
messages = ['356188', '356189', '409977']
nosy_count = 3.0
nosy_names = ['vstinner', 'nedbat', 'petr.viktorin']
pr_nums = ['30456']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue38735'
versions = []

[nedbat]

On the manylinux docker images, the current directory is "/".

If I create a file there and try to import it with PYTHONPYCACHEPREFIX set, I get a stack trace:

$ docker run -it --init --rm -v `pwd`:/io quay.io/pypa/manylinux1_x86_64 bash
[root@29d75403d7d1 /]# cat > foo.py
a = 1
[root@29d75403d7d1 /]# /opt/python/cp38-cp38/bin/python
Python 3.8.0 (default, Oct 30 2019, 22:13:22)
[GCC 4.8.2 20140120 (Red Hat 4.8.2-15)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import foo
>>> foo.a
1
>>>
[root@29d75403d7d1 /]# PYTHONPYCACHEPREFIX=/opt/pyc /opt/python/cp38-cp38/bin/python
Python 3.8.0 (default, Oct 30 2019, 22:13:22)
[GCC 4.8.2 20140120 (Red Hat 4.8.2-15)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 657, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 562, in module_from_spec
  File "<frozen importlib._bootstrap>", line 541, in _init_module_attrs
  File "<frozen importlib._bootstrap>", line 382, in cached
  File "<frozen importlib._bootstrap_external>", line 427, in _get_cached
  File "<frozen importlib._bootstrap_external>", line 352, in cache_from_source
IndexError: string index out of range
>>>

[vstinner]

The bug occurs on this line:

    if sys.pycache_prefix is not None:
        if not _path_isabs(head):
            head = _path_join(_os.getcwd(), head)
        if head[1] == ':' and head[0] not in path_separators: # <==== HERE
            head = head[2:]
        ...

I guess that head="/" in your case: so head[1] raises an IndexError.

[encukou]

Here's a patch, but testing it would require putting a file in the root directory. I don't dare do that from the test suite. Is one-time manual testing OK here?