DOC: internal: commit notes about polling states and flags
Some detailed observations were made on polling general and POLLHUP
more specifically, they can be useful later.
diff --git a/doc/internals/notes-polling.txt b/doc/internals/notes-polling.txt
new file mode 100644
index 0000000..e7741a6
--- /dev/null
+++ b/doc/internals/notes-polling.txt
@@ -0,0 +1,192 @@
+2019-09-03
+
+u8 fd.state;
+u8 fd.ev;
+
+
+ev = one of :
+ #define FD_POLL_IN 0x01
+ #define FD_POLL_PRI 0x02
+ #define FD_POLL_OUT 0x04
+ #define FD_POLL_ERR 0x08
+ #define FD_POLL_HUP 0x10
+
+Could we instead have :
+
+ FD_WAIT_IN 0x01
+ FD_WAIT_OUT 0x02
+ FD_WAIT_PRI 0x04
+ FD_SEEN_HUP 0x08
+ FD_SEEN_HUP 0x10
+ FD_WAIT_CON 0x20 <<= shouldn't this be in the connection itself in fact ?
+
+=> not needed, covered by the state instead.
+
+What is missing though is :
+ - FD_DATA_PENDING -- overlaps with READY_R, OK if passed by pollers only
+ - FD_EOI_PENDING
+ - FD_ERR_PENDING
+ - FD_EOI
+ - FD_SHW
+ - FD_ERR
+
+fd_update_events() could do that :
+
+ if ((fd_data_pending|fd_eoi_pending|fd_err_pending) && !(fd_err|fd_eoi))
+ may_recv()
+
+ if (fd_send_ok && !(fd_err|fd_shw))
+ may_send()
+
+ if (fd_err)
+ wake()
+
+the poller could do that :
+ HUP+OUT => always indicates a failed connect(), it should not lack ERR. Is this err_pending ?
+
+ ERR HUP OUT IN
+ 0 0 0 0 => nothing
+ 0 0 0 1 => FD_DATA_PENDING
+ 0 0 1 0 => FD_SEND_OK
+ 0 0 1 1 => FD_DATA_PENDING|FD_SEND_OK
+ 0 1 0 0 => FD_EOI (|FD_SHW)
+ 0 1 0 1 => FD_DATA_PENDING|FD_EOI_PENDING (|FD_SHW)
+ 0 1 1 0 => FD_EOI |FD_ERR (|FD_SHW)
+ 0 1 1 1 => FD_EOI_PENDING (|FD_ERR_PENDING) |FD_DATA_PENDING (|FD_SHW)
+ 1 X 0 0 => FD_ERR | FD_EOI (|FD_SHW)
+ 1 X X 1 => FD_ERR_PENDING | FD_EOI_PENDING | FD_DATA_PENDING (|FD_SHW)
+ 1 X 1 0 => FD_ERR | FD_EOI (|FD_SHW)
+
+ OUT+HUP,OUT+HUP+ERR => FD_ERR
+
+This reorders to:
+
+ IN ERR HUP OUT
+ 0 0 0 0 => nothing
+ 0 0 0 1 => FD_SEND_OK
+ 0 0 1 0 => FD_EOI (|FD_SHW)
+
+ 0 X 1 1 => FD_ERR | FD_EOI (|FD_SHW)
+ 0 1 X 0 => FD_ERR | FD_EOI (|FD_SHW)
+ 0 1 X 1 => FD_ERR | FD_EOI (|FD_SHW)
+
+ 1 0 0 0 => FD_DATA_PENDING
+ 1 0 0 1 => FD_DATA_PENDING|FD_SEND_OK
+ 1 0 1 0 => FD_DATA_PENDING|FD_EOI_PENDING (|FD_SHW)
+ 1 0 1 1 => FD_EOI_PENDING (|FD_ERR_PENDING) |FD_DATA_PENDING (|FD_SHW)
+ 1 1 X X => FD_ERR_PENDING | FD_EOI_PENDING | FD_DATA_PENDING (|FD_SHW)
+
+Regarding "|SHW", it's normally useless since it will already have been done,
+except on connect() error where this indicates there's no need for SHW.
+
+FD_EOI and FD_SHW could be part of the state (FD_EV_SHUT_R, FD_EV_SHUT_W).
+Then all states having these bit and another one would be transient and need
+to resync. We could then have "fd_shut_recv" and "fd_shut_send" to turn these
+states.
+
+The FD's ev then only needs to update EOI_PENDING, ERR_PENDING, ERR, DATA_PENDING.
+With this said, these are not exactly polling states either, as err/eoi/shw are
+orthogonal to the other states and are required to update them so that the polling
+state really is DISABLED in the end. So we need more of an operational status for
+the FD containing EOI_PENDING, EOI, ERR_PENDING, ERR, SHW, CLO?. These could be
+classified in 3 categories: read:(OPEN, EOI_PENDING, EOI); write:(OPEN,SHW),
+ctrl:(OPEN,ERR_PENDING,ERR,CLO). That would be 2 bits for R, 1 for W, 2 for ctrl
+or total 5 vs 6 for individual ones, but would be harder to manipulate.
+
+Proposal:
+ - rename fdtab[].state to "polling_state"
+ - rename fdtab[].ev to "status"
+
+Note: POLLHUP is also reported is a listen() socket has gone in shutdown()
+TEMPORARILY! Thus we may not always consider this as a final error.
+
+
+Work hypothesis:
+
+SHUT RDY ACT
+ 0 0 0 => disabled
+ 0 0 1 => active
+ 0 1 0 => stopped
+ 0 1 1 => ready
+ 1 0 0 => final shut
+ 1 0 1 => shut pending without data
+ 1 1 0 => shut pending, stopped
+ 1 1 1 => shut pending
+
+PB: we can land into final shut if one thread disables the FD while another
+ one that was waiting on it reports it as shut. Theorically it should be
+ implicitly ready though, since reported. But if no data is reported, it
+ will be reportedly shut only. And no event will be reported then. This
+ might still make sense since it's not active, thus we don't want events.
+ But it will not be enabled later either in this case so the shut really
+ risks not to be properly reported. The issue is that there's no difference
+ between a shut coming from the bottom and a shut coming from the top, and
+ we need an event to report activity here. Or we may consider that a poller
+ never leaves a final shut by itself (100) and always reports it as
+ shut+stop (thus ready) if it was not active. Alternately, if active is
+ disabled, shut should possibly be ignored, then a poller cannot report
+ shut. But shut+stopped seems the most suitable as it corresponds to
+ disabled->stopped transition.
+
+Now let's add ERR. ERR necessarily implies SHUT as there doesn't seem to be a
+valid case of ERR pending without shut pending.
+
+ERR SHUT RDY ACT
+ 0 0 0 0 => disabled
+ 0 0 0 1 => active
+ 0 0 1 0 => stopped
+ 0 0 1 1 => ready
+
+ 0 1 0 0 => final shut, no error
+ 0 1 0 1 => shut pending without data
+ 0 1 1 0 => shut pending, stopped
+ 0 1 1 1 => shut pending
+
+ 1 0 X X => invalid
+
+ 1 1 0 0 => final shut, error encountered
+ 1 1 0 1 => error pending without data
+ 1 1 1 0 => error pending after data, stopped
+ 1 1 1 1 => error pending
+
+So the algorithm for the poller is:
+ - if (shutdown_pending or error) reported and ACT==0,
+ report SHUT|RDY or SHUT|ERR|RDY
+
+For read handlers :
+ - if (!(flags & (RDY|ACT)))
+ return
+ - if (ready)
+ try_to_read
+ - if (err)
+ report error
+ - if (shut)
+ read0
+
+For write handlers:
+ - if (!(flags & (RDY|ACT)))
+ return
+ - if (err||shut)
+ report error
+ - if (ready)
+ try_to_write
+
+For listeners:
+ - if (!(flags & (RDY|ACT)))
+ return
+ - if (err||shut)
+ pause
+ - if (ready)
+ try_to_accept
+
+Kqueue reports events differently, it says EV_EOF() on READ or WRITE, that
+we currently map to FD_POLL_HUP and FD_POLL_ERR. Thus kqueue reports only
+POLLRDHUP and not POLLHUP, so for now a direct mapping of POLLHUP to
+FD_POLL_HUP does NOT imply write closed with kqueue while it does for others.
+
+Other approach, use the {RD,WR}_{ERR,SHUT,RDY} flags to build a composite
+status in each poller and pass this to fd_update_events(). We normally
+have enough to be precise, and this latter will rework the events.
+
+FIXME: Normally on KQUEUE we're supposed to look at kev[].fflags to get the error
+on EV_EOF() on read or write.