Why does the peer socket still return readable when the select is closed?

Why does the peer socket have been closed and the select still returns readable?

Apr.03,2021

the EOF that may be returned

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b31504-2bd91.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b31504-2bd91.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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