[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Benjamin Peterson]

Change by Benjamin Peterson :


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resolution:  -> duplicate
stage: patch review -> resolved
status: open -> closed
superseder:  -> never enable lchmod on Linux

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Giampaolo Rodola']

Change by Giampaolo Rodola' :


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nosy: +giampaolo.rodola

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[STINNER Victor]

STINNER Victor  added the comment:

> if I'm reading the manpage correctly: `readlink` tells the filename that the 
> symlink points to.  lchmod is concerned with setting the `stat` on the link 
> itself (which only some platforms actually support)

Oops, my bad. Ignore my comment :-)

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Anthony Sottile]

Anthony Sottile  added the comment:

if I'm reading the manpage correctly: `readlink` tells the filename that the 
symlink points to.  lchmod is concerned with setting the `stat` on the link 
itself (which only some platforms actually support)

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[STINNER Victor]

STINNER Victor  added the comment:

Dummy question. If lchmod() is not supported by the libc, would it be possible 
to reimplement it using readlink()?

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Anthony Sottile]

Change by Anthony Sottile :


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pull_requests: +4684

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Antoine Pitrou]

Antoine Pitrou  added the comment:

I don't have any strong opinion, but I think doing the check at runtime is 
better.

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[STINNER Victor]

STINNER Victor  added the comment:

If lchmod() fails with [Errno 95] Not supported, IMHO chmod(path, mode, 
follow_symlinks=False) should behaves as lchmod() doesn't exist and falls back 
to the next case, as if HAVE_LCHMOD wasn't defined, but implement such falls 
back at runtime.

So the "broken" lchmod() issue should be fixed a multiple places:

* os.lchmod()
* os.chmod()
* shutil.copymode()
* pathlib.Path.lchmod()

Oh wow, this issue impacts much more functions than what I expected.

Maybe the compromise of a configure check avoids to overengineer this issue :-)

Antoine: What do you think? Should we check if lchmod() works in configure (as 
already implemented in the PR 4267), or implement a runtime check as I proposed.


pathlib has an interesting long comment:

# Some platforms don't support lchmod().  Often the function exists
# anyway, as a stub that always returns ENOSUP or perhaps EOPNOTSUPP.
# (No, I don't know why that's a good design.)  ./configure will detect
# this and reject it--so HAVE_LCHMOD still won't be defined on such
# platforms.  This is Very Helpful.
#
# However, sometimes platforms without a working lchmod() *do* have
# fchmodat().  (Examples: Linux kernel 3.2 with glibc 2.15,
# OpenIndiana 3.x.)  And fchmodat() has a flag that theoretically makes
# it behave like lchmod().  So in theory it would be a suitable
# replacement for lchmod().  But when lchmod() doesn't work, fchmodat()'s
# flag doesn't work *either*.  Sadly ./configure isn't sophisticated
# enough to detect this condition--it only determines whether or not
# fchmodat() minimally works.
#
# Therefore we simply ignore fchmodat() when deciding whether or not
# os.chmod supports follow_symlinks.  Just checking lchmod() is
# sufficient.  After all--if you have a working fchmodat(), your
# lchmod() almost certainly works too.
#
# _add("HAVE_FCHMODAT",   "chmod")
_add("HAVE_FCHOWNAT",   "chown")

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[STINNER Victor]

STINNER Victor  added the comment:

I would prefer to catch os.lchmod() exception and reminds that lchmod() doesn't 
work. Search for _O_TMPFILE_WORKS in Lib/tempfile.py for a Python example of 
code trying to use a function, or falls back on something else.

(I gave other examples for C code in a comment on PR 4267.)

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nosy: +haypo

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Anthony Sottile]

Change by Anthony Sottile :


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keywords: +patch
pull_requests: +4230
stage:  -> patch review

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Anthony Sottile]

Anthony Sottile  added the comment:

Here's one idea for a patch (inspired by the rest of the function):

```
diff --git a/Lib/shutil.py b/Lib/shutil.py
index 464ee91..2099289 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -213,6 +213,13 @@ def copystat(src, dst, *, follow_symlinks=True):
 # symlink.  give up, suppress the error.
 # (which is what shutil always did in this circumstance.)
 pass
+except OSError as why:
+# lchmod on alpine will raise this with symlinks: #31940
+for err in 'EOPNOTSUPP', 'ENOTSUP':
+if hasattr(errno, err) and why.errno == getattr(errno, err):
+break
+else:
+raise
 if hasattr(st, 'st_flags'):
 try:
 lookup("chflags")(dst, st.st_flags, follow_symlinks=follow)
```

However lchmod seems to be just broken on alpine so the tests continue to fail 
(however my usecase is fixed)

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[issue31940] copystat on symlinks fails for alpine -- faulty lchmod implementation?

[Anthony Sottile]

New submission from Anthony Sottile :

Fortunately, this can be reproduced with the testsuite:

```
==
ERROR: test_copystat_symlinks (__main__.TestShutil)
--
Traceback (most recent call last):
  File "/usr/lib/python3.6/test/test_shutil.py", line 366, in 
test_copystat_symlinks
os.lchmod(src_link, stat.S_IRWXO)
OSError: [Errno 95] Not supported: '/tmp/tmplfli9msi/baz'

```

My simplest reproduction involves docker:

```dockerfile
FROM alpine
RUN apk update && apk add curl python3

RUN mkdir foo && ln -s /dev/null foo/bar

CMD [ \
"python3", "-c", \
"import shutil; shutil.copytree('foo', 'bar', symlinks=True)" \
]
```

```
Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/lib/python3.6/shutil.py", line 359, in copytree
raise Error(errors)
shutil.Error: [('foo/bar', 'bar/bar', "[Errno 95] Not supported: 'bar/bar'")]
```


By looking at pyconfig, I get the following:

```
/ # grep -E '(HAVE_FCHMODAT|HAVE_LCHMOD)' /usr/include/python3.6m/pyconfig.h 
#define HAVE_FCHMODAT 1
#define HAVE_LCHMOD 1
```

But it seems lchmod is actually nonfunctional in this case.

I think the fix is to augment `configure` to detect faulty `lchmod` and not set 
`HAVE_LCHMOD`?  I'm not terribly familiar with the autotools pipeline but 
that's where I'm going to take a stab at it!

I'm originally finding this issue via 
https://github.com/pre-commit/pre-commit/issues/655

--
components: Build, Library (Lib), Tests
messages: 305527
nosy: Anthony Sottile
priority: normal
severity: normal
status: open
title: copystat on symlinks fails for alpine -- faulty lchmod implementation?
versions: Python 3.6, Python 3.7, Python 3.8

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