2006-08-31
SIGCHLD
versus Python: a problem of semantics
In the process of looking at my program's code again to write the last entry, I think I may have solved the mystery of how my impossible exception gets generated.
My program does a lot of forking and thus cleanups of now-dead children. The code that it generally dies on is:
def _delip(pid, ip): del ipmap[ip][pid] if len(ipmap[ip]) == 0: del ipmap[ip]
It takes a KeyError on the len(ipmap[ip])
operation and goes down.
(Because of previous fun, the main
thread forks all the children and waits for them, so this kills the
entire program.)
Clearly there is some concurrency problem, but my problem with the
exception was that I've never seen where it could come from. The main
thread is the only thread that adds or removes things from the ipmap
dictionary, and the SIGCHLD
handler that reaps children is only active
when the thread is idling in select()
(partly to avoid just this
sort of concurrency issue).
To avoid various problems and just create sanity, Unix SIGCHLD
handlers are not reentrant; even if more children die, you won't receive
a second SIGCHLD
until you return from the signal handler. (This is
an interesting source of bugs if you bail out of the signal handler
without telling the kernel, and is one reason for the existence of
siglongjmp()
.)
And in thinking about all of this I came to a horrible realization:
those are Unix semantics, not Python semantics. Python does not
run your Python-level SIGCHLD
handler from the actual C level signal
handler; it runs them from the regular bytecode interpreter. All the C
level SIGCHLD
handler does is set a flag telling the interpreter to
run your SIGCHLD
handler at the next bytecode, where it gets treated
pretty much as an ordinary function call.
This would neatly explain my mysterious exceptions. When there are two
connections from an IP address and both of them die in short succession,
if we are extremely unlucky the SIGCHLD
for the second will be
processed between _delip
's first and second lines and delete the
ipmap[ip]
dictionary entry out from underneath the first.
I personally believe that this is a bug in the CPython interpreter, but even if I can persuade the Python people of this, I still need to come up with a Python-level workaround for the mean time (ideally one that doesn't involve too much code reorganization).
How dd
does blocking
For a conceptually simple program, dd
has a number of dark corners.
One of them (at least for me) is how it deals with input and output
block sizes, and how the various blocking arguments change things
around.
ibs=
sets the input block size, the size of theread()
s thatdd
will make. Since you can get partial reads in various situations, this is really the maximum size thatdd
will ever read at once.obs=
sets the output block size and makesdd
'reblock' output;dd
will accumulate input until it can write a full sized output block (except at EOF, where it may write a final partial block).bs=
sets the (maximum) IO block size for both reads and writes, but it turns off reblocking; ifdd
gets a partial read, it will immediately write that partial block.
Because of the reblocking or lack thereof, 'ibs=N obs=N
' is subtly
different from 'bs=N
'. The former will accumulate multiple partial
reads together in order to write N bytes, while the latter won't.
(On top of this is the 'conv=sync
' option, which pads partial reads.)
So if you're reading from a network or a pipe but want to write in large
efficient blocks, you want to use obs
, not bs
(and you probably want
to use ibs
too, because otherwise you'll be doing a lot of 512 byte
reads, which are kind of inefficient).