test_userstring: test_find_periodic_pattern() failed on GHA Hypothesis tests on Ubuntu #110160

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1. vstinner

   added

   testsTests in the Lib/test dirTests in the Lib/test dir

   on Sep 30, 2023
2. vstinner commented on Sep 30, 2023

   

   vstinner

   on Sep 30, 2023

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   I'm not sure how Hypothesis managed to get p='' and text=''. text must always be non-empty, no?

       def test_find_periodic_pattern(self):
           """Cover the special path for periodic patterns."""
           def reference_find(p, s):
               for i in range(len(s)):
                   if s.startswith(p, i):
                       return i
               return -1
   
           rr = random.randrange
           choices = random.choices
           for _ in range(1000):
               p0 = ''.join(choices('abcde', k=rr(10))) * rr(10, 20)
               p = p0[:len(p0) - rr(10)] # pop off some characters
               left = ''.join(choices('abcdef', k=rr(2000)))
               right = ''.join(choices('abcdef', k=rr(2000)))
               text = left + p + right
               with self.subTest(p=p, text=text):
                   self.checkequal(reference_find(p, text),
                                   text, 'find', p)

   cc @sobolevn @hugovk
3. sobolevn

   self-assigned this

   on Sep 30, 2023
4. terryjreedy commented on Sep 30, 2023

   

   terryjreedy

   on Sep 30, 2023

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   rr = randrange(n) selects from 0 to n-1. choices(s, k=0) returns an empty list (verified on main). So I believe p0, p, left, right, and text can all be ''. ''.find('') == 0. I don't know what reference_find is but it apparently returns -1 in the same situation. If that is correct and not a bug, 1 should be added as the first parameter to one of the rr(2000) calls, depending on whether only text or also p needs to be non-empty.
5. sobolevn commented on Oct 1, 2023

   

   sobolevn

   on Oct 1, 2023

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   Thanks for the report, @vstinner!
   @terryjreedy thanks for the analysis.

   First of all, this is not related to Hypothesis, this test just happens to fail in this test job.

   Yes, this test is flaky by design. Here's the minimal repro that always fails:

   » git patch
   diff --git Lib/test/string_tests.py Lib/test/string_tests.py
   index 8d210b198d2..1df4147521b 100644
   --- Lib/test/string_tests.py
   +++ Lib/test/string_tests.py
   @@ -332,9 +332,9 @@ def reference_find(p, s):
            rr = random.randrange
            choices = random.choices
            for _ in range(1000):
   -            p0 = ''.join(choices('abcde', k=rr(10))) * rr(10, 20)
   -            p = p0[:len(p0) - rr(10)] # pop off some characters
   -            left = ''.join(choices('abcdef', k=rr(2000)))
   -            right = ''.join(choices('abcdef', k=rr(2000)))
   +            p0 = ''.join(choices('abcde', k=0)) * rr(10, 20)
   +            p = p0[:len(p0) - 0] # pop off some characters
   +            left = ''.join(choices('abcdef', k=0))
   +            right = ''.join(choices('abcdef', k=0))
                text = left + p + right
                with self.subTest(p=p, text=text):                                        

   Sometimes even it fails even without right = ''.join(choices('abcdef', k=0)) change. But, with this line it is always failing.

   Since randrange can return 0, it is possible to get this result. Possibly, there are other cases when this can fail.
6. sobolevn

   added a commit that references this issue on Oct 1, 2023

   pythongh-110160: Fix flaky `test_find_periodic_pattern`

   9e12a7b
7. vstinner

   added a commit that references this issue on Oct 1, 2023

   gh-110160: Fix flaky `test_find_periodic_pattern` in `string_tests` (#…

   ...

   06faa9a
8. miss-islington

   added a commit that references this issue on Oct 1, 2023

   pythongh-110160: Fix flaky `test_find_periodic_pattern` in `string_te…

   ...

   6fe6378
9. vstinner

   closed this as completedon Oct 1, 2023
10. vstinner

    added a commit that references this issue on Oct 1, 2023

    [3.11] gh-110160: Fix flaky `test_find_periodic_pattern` in `string_t…

    ...

    86084d0
11. Yhg1s

    added a commit that references this issue on Oct 2, 2023

    [3.12] gh-110160: Fix flaky `test_find_periodic_pattern` in `string_t…

    ...

    10af224
12. Glyphack

    added a commit that references this issue on Sep 2, 2024

    pythongh-110160: Fix flaky `test_find_periodic_pattern` in `string_te…

    ...

    c7bc1ee
13. hugovk

    marked Test failure in test_find_periodic_pattern in string_tests.py #105639 as a duplicate of this issue on Mar 7, 2025