From 5ba9345c63a1afcbf2a06183ff00ead3bdf9e717 Mon Sep 17 00:00:00 2001 From: Tyler Goodlet Date: Thu, 30 Dec 2021 14:29:57 -0500 Subject: [PATCH] Add `try_read()` to shm mod --- piker/data/_sharedmem.py | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) diff --git a/piker/data/_sharedmem.py b/piker/data/_sharedmem.py index 9c515ce3..c741ba1c 100644 --- a/piker/data/_sharedmem.py +++ b/piker/data/_sharedmem.py @@ -505,3 +505,35 @@ def maybe_open_shm_array( # to fail if a block has been allocated # on the OS by someone else. return open_shm_array(key=key, dtype=dtype, **kwargs), True + + +def try_read( + array: np.ndarray + +) -> Optional[np.ndarray]: + ''' + Try to read the last row from a shared mem array or ``None`` + if the array read returns a zero-length array result. + + Can be used to check for backfilling race conditions where an array + is currently being (re-)written by a writer actor but the reader is + unaware and reads during the window where the first and last indexes + are being updated. + + ''' + try: + return array[-1] + except IndexError: + # XXX: race condition with backfilling shm. + # + # the underlying issue is that a backfill (aka prepend) and subsequent + # shm array first/last index update could result in an empty array + # read here since the indices may be updated in such a way that + # a read delivers an empty array (though it seems like we + # *should* be able to prevent that?). also, as and alt and + # something we need anyway, maybe there should be some kind of + # signal that a prepend is taking place and this consumer can + # respond (eg. redrawing graphics) accordingly. + + # the array read was emtpy + return None