Description:
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When executing a program where the output's last chunk is 4095 bytes then the last chunk is swallowed.
The script below generates 4095 dots on Linux and counts how many system()'s output contain.
None of the dots reach the output, a patch resolving this issue is attached.
The bug could already be reproduced in 5.3.0, I did not trace it back farther than that.

Test script:
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# Execute command print 4095 dots
php -r 'system("printf .%.0s {1..4095}");'  | wc -c

Expected result:
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4095

Actual result:
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0