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Some detailed observations were made on polling general and POLLHUP more specifically, they can be useful later.
193 lines
6.5 KiB
Plaintext
193 lines
6.5 KiB
Plaintext
2019-09-03
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u8 fd.state;
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u8 fd.ev;
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ev = one of :
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#define FD_POLL_IN 0x01
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#define FD_POLL_PRI 0x02
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#define FD_POLL_OUT 0x04
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#define FD_POLL_ERR 0x08
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#define FD_POLL_HUP 0x10
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Could we instead have :
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FD_WAIT_IN 0x01
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FD_WAIT_OUT 0x02
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FD_WAIT_PRI 0x04
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FD_SEEN_HUP 0x08
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FD_SEEN_HUP 0x10
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FD_WAIT_CON 0x20 <<= shouldn't this be in the connection itself in fact ?
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=> not needed, covered by the state instead.
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What is missing though is :
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- FD_DATA_PENDING -- overlaps with READY_R, OK if passed by pollers only
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- FD_EOI_PENDING
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- FD_ERR_PENDING
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- FD_EOI
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- FD_SHW
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- FD_ERR
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fd_update_events() could do that :
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if ((fd_data_pending|fd_eoi_pending|fd_err_pending) && !(fd_err|fd_eoi))
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may_recv()
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if (fd_send_ok && !(fd_err|fd_shw))
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may_send()
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if (fd_err)
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wake()
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the poller could do that :
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HUP+OUT => always indicates a failed connect(), it should not lack ERR. Is this err_pending ?
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ERR HUP OUT IN
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0 0 0 0 => nothing
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0 0 0 1 => FD_DATA_PENDING
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0 0 1 0 => FD_SEND_OK
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0 0 1 1 => FD_DATA_PENDING|FD_SEND_OK
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0 1 0 0 => FD_EOI (|FD_SHW)
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0 1 0 1 => FD_DATA_PENDING|FD_EOI_PENDING (|FD_SHW)
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0 1 1 0 => FD_EOI |FD_ERR (|FD_SHW)
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0 1 1 1 => FD_EOI_PENDING (|FD_ERR_PENDING) |FD_DATA_PENDING (|FD_SHW)
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1 X 0 0 => FD_ERR | FD_EOI (|FD_SHW)
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1 X X 1 => FD_ERR_PENDING | FD_EOI_PENDING | FD_DATA_PENDING (|FD_SHW)
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1 X 1 0 => FD_ERR | FD_EOI (|FD_SHW)
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OUT+HUP,OUT+HUP+ERR => FD_ERR
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This reorders to:
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IN ERR HUP OUT
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0 0 0 0 => nothing
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0 0 0 1 => FD_SEND_OK
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0 0 1 0 => FD_EOI (|FD_SHW)
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0 X 1 1 => FD_ERR | FD_EOI (|FD_SHW)
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0 1 X 0 => FD_ERR | FD_EOI (|FD_SHW)
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0 1 X 1 => FD_ERR | FD_EOI (|FD_SHW)
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1 0 0 0 => FD_DATA_PENDING
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1 0 0 1 => FD_DATA_PENDING|FD_SEND_OK
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1 0 1 0 => FD_DATA_PENDING|FD_EOI_PENDING (|FD_SHW)
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1 0 1 1 => FD_EOI_PENDING (|FD_ERR_PENDING) |FD_DATA_PENDING (|FD_SHW)
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1 1 X X => FD_ERR_PENDING | FD_EOI_PENDING | FD_DATA_PENDING (|FD_SHW)
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Regarding "|SHW", it's normally useless since it will already have been done,
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except on connect() error where this indicates there's no need for SHW.
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FD_EOI and FD_SHW could be part of the state (FD_EV_SHUT_R, FD_EV_SHUT_W).
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Then all states having these bit and another one would be transient and need
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to resync. We could then have "fd_shut_recv" and "fd_shut_send" to turn these
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states.
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The FD's ev then only needs to update EOI_PENDING, ERR_PENDING, ERR, DATA_PENDING.
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With this said, these are not exactly polling states either, as err/eoi/shw are
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orthogonal to the other states and are required to update them so that the polling
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state really is DISABLED in the end. So we need more of an operational status for
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the FD containing EOI_PENDING, EOI, ERR_PENDING, ERR, SHW, CLO?. These could be
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classified in 3 categories: read:(OPEN, EOI_PENDING, EOI); write:(OPEN,SHW),
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ctrl:(OPEN,ERR_PENDING,ERR,CLO). That would be 2 bits for R, 1 for W, 2 for ctrl
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or total 5 vs 6 for individual ones, but would be harder to manipulate.
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Proposal:
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- rename fdtab[].state to "polling_state"
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- rename fdtab[].ev to "status"
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Note: POLLHUP is also reported is a listen() socket has gone in shutdown()
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TEMPORARILY! Thus we may not always consider this as a final error.
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Work hypothesis:
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SHUT RDY ACT
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0 0 0 => disabled
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0 0 1 => active
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0 1 0 => stopped
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0 1 1 => ready
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1 0 0 => final shut
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1 0 1 => shut pending without data
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1 1 0 => shut pending, stopped
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1 1 1 => shut pending
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PB: we can land into final shut if one thread disables the FD while another
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one that was waiting on it reports it as shut. Theorically it should be
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implicitly ready though, since reported. But if no data is reported, it
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will be reportedly shut only. And no event will be reported then. This
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might still make sense since it's not active, thus we don't want events.
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But it will not be enabled later either in this case so the shut really
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risks not to be properly reported. The issue is that there's no difference
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between a shut coming from the bottom and a shut coming from the top, and
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we need an event to report activity here. Or we may consider that a poller
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never leaves a final shut by itself (100) and always reports it as
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shut+stop (thus ready) if it was not active. Alternately, if active is
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disabled, shut should possibly be ignored, then a poller cannot report
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shut. But shut+stopped seems the most suitable as it corresponds to
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disabled->stopped transition.
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Now let's add ERR. ERR necessarily implies SHUT as there doesn't seem to be a
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valid case of ERR pending without shut pending.
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ERR SHUT RDY ACT
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0 0 0 0 => disabled
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0 0 0 1 => active
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0 0 1 0 => stopped
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0 0 1 1 => ready
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0 1 0 0 => final shut, no error
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0 1 0 1 => shut pending without data
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0 1 1 0 => shut pending, stopped
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0 1 1 1 => shut pending
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1 0 X X => invalid
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1 1 0 0 => final shut, error encountered
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1 1 0 1 => error pending without data
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1 1 1 0 => error pending after data, stopped
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1 1 1 1 => error pending
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So the algorithm for the poller is:
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- if (shutdown_pending or error) reported and ACT==0,
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report SHUT|RDY or SHUT|ERR|RDY
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For read handlers :
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- if (!(flags & (RDY|ACT)))
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return
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- if (ready)
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try_to_read
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- if (err)
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report error
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- if (shut)
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read0
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For write handlers:
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- if (!(flags & (RDY|ACT)))
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return
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- if (err||shut)
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report error
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- if (ready)
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try_to_write
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For listeners:
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- if (!(flags & (RDY|ACT)))
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return
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- if (err||shut)
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pause
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- if (ready)
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try_to_accept
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Kqueue reports events differently, it says EV_EOF() on READ or WRITE, that
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we currently map to FD_POLL_HUP and FD_POLL_ERR. Thus kqueue reports only
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POLLRDHUP and not POLLHUP, so for now a direct mapping of POLLHUP to
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FD_POLL_HUP does NOT imply write closed with kqueue while it does for others.
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Other approach, use the {RD,WR}_{ERR,SHUT,RDY} flags to build a composite
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status in each poller and pass this to fd_update_events(). We normally
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have enough to be precise, and this latter will rework the events.
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FIXME: Normally on KQUEUE we're supposed to look at kev[].fflags to get the error
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on EV_EOF() on read or write.
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