[Bug 239803] Problem with concurrent transfer of file descriptors

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239803

            Bug ID: 239803
           Summary: Problem with concurrent transfer of file descriptors
           Product: Base System
           Version: 12.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: knizhnik at garret.ru
 Attachment #206476 text/plain
         mime type:

Created attachment 206476
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=206476&action=edit
Reporduce the problem with file description corruption

Transferring file descriptors to another process without waiting confirmations
leads to unexpected behavior: descriptor is successfully transferred but
attempt to read from is reported as EOF (recv returns 0).

Attached example reproduces the problem (please build it with -pthread option).
If number of client threads is set to 1, then problem is not reproduced.
Another way to fix the problem is to wait confirmation about descriptor
delivery and close socket only after it.

It is seems to be critical that socket should not be closed before confirmation
of the transfer: if we wait confirmation but close socket before, then the
problem is still persists.

The bug is reproduced at several versions of FreeBSD including 12.0-stable and
also recent Mac-OS/X.

There was bug report with similar symptoms:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=131876

But looks likes r343784 is not fixing it.

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