[Bug 204812] pty read blocks after select, if ^S is written in between

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=204812

            Bug ID: 204812
           Summary: pty read blocks after select, if ^S is written in
                    between
           Product: Base System
           Version: 10.2-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: cgull at glup.org

The pts driver has an issue with breaking the promise made by select() that a
read will not block, if there is a write of ^S to the pty between the select()
and the read().

The reproduction for this is very simple:

  script /dev/null yes

Type ^S.

Type ^Q.  Nothing happens, because script is hung on a blocking read.  The read
will never complete, because script is hung and your ^Q will never be
delivered.

I originally discovered this while working on mosh (net/mosh), but script
blocks in the same way.

The sequence of events that causes this:

* The pty slave has software flow control enabled.

* select() returns read status for the pty master fd.

* Something write()s ^S to the pty master.

* A read() is started on the pty master, with the expectation that it will not
block.  But it does block, and since the tty is stopped, it will never unblock
without outside intervention.

I hesitate to call this a bug, because the state of the pty *is* changed by
that write, and the fix for this issue is not clear.  But it violates POLA,
other kernels (Linux, OpenBSD) do not behave this way, and the consequences are
serious.

I see this issue on FreeBSD 10.2, 9.1, and 7.2.  An old 4.10 box doesn't have
this issue.

I originally discovered this bug while working on Mosh.  It's not a new bug
there.  I've now implemented pty packet mode, and the problem exists there too,
even though packet mode should always be able to avoid breaking this promise. 
See

https://github.com/mobile-shell/mosh/issues/692
https://github.com/cgull/mosh/tree/pty-packet

I don't understand why nobody has run into this issue before though, either on
the FreeBSD or on the Mosh side.  (Mosh has this problem on FreeBSD and OS X.)

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