TomTJarosz opened a new pull request, #39314:
URL: https://github.com/apache/arrow/pull/39314
### Rationale for this change
See:
```
cdef inline bint _have_pandas_internal(self):
if not self._tried_importing_pandas:
self._check_import(raise_=False)
return self._have_pandas
```
The method `_check_import`:
1) sets `_tried_importing_pandas`
2) does some things
3) sets `_have_pandas` to true (if we indeed do have pandas)
Suppose thread 1 calls `_have_pandas_internal`. If thread 1 is at step 2
while thread 2 calls `_have_pandas_internal`, `_have_pandas_internal` may
incorrectly return False for thread 2 as thread 1 has set
`_tried_importing_pandas` to true, but has not yet (but will) set
`_have_pandas` to True. `_have_pandas_internal` will return True for thread 1.
After my fix, `_have_pandas_internal` will not return an incorrect value in
the scenario described above. It would instead result in a redundant, but (I
believe) harmless, invocation of `_check_import`.
### What changes are included in this PR?
Changes ordering of "trying to import pandas" and "recording that pandas
import has been tried"
### Are these changes tested?
yes, see test committed
### Are there any user-facing changes?
This PR resolves a user-facing race condition
https://github.com/apache/arrow/issues/39313
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