On 10-09-09 12:38 PM, Mathieu Desnoyers wrote:
* Pierre-Marc Fournier ([email protected]) wrote:On 09/07/2010 11:43 AM, Mathieu Desnoyers wrote:* Pierre-Marc Fournier ([email protected]) wrote:On 09/06/2010 11:29 AM, Mathieu Desnoyers wrote:* Mathieu Desnoyers ([email protected]) wrote:* Pierre-Marc Fournier ([email protected]) wrote:I disagree with you Mathieu. These retvals are the same as i/o syscalls (read/write/send/recv/...)and therefore should in my opinion remain as is.Well, this function is not technically the same as i/o syscalls at all. It uses I/O syscalls, but it is not an I/O syscall per se, so the return value transformation to a more standard pattern (neg err val, 0 ok) should happen right in this function rather than to let all callers handle this. I/O syscalls use positive return values to indicate the number of bytes read/written/etc. Here, this function arbitrarily choose 1 to indicate that "something has been sent" without caring about the amount of data moved at all. So as it doesn't need the whole positive range to spell out the amount of data moved, it doesn't need to do the same special-cases that the I/O syscalls are doing. It adds a lot of error values management oddness without adding anything. So even though I agree with you that this function is close to the I/O system calls because it calls it, it is very far from the I/O syscalls semantically (we don't care about the number of bytes written), and even though we might be tempted to use the same error values as system calls for them, the fact that we just don't care about the number of bytes written combined with the fact that standardizing error value across the code makes it much easier to follow and to write just call for this change.By the way, looking at include/share.h:patient_write(), in the case where write returns 0, I think we should consider this as a success and loop again to retry write rather than consider that an error occurred. The same apply to patient_send(). See the manpages for details: write(2): RETURN VALUE On success, the number of bytes written is returned (zero indicates nothing was written). On error, -1 is returned, and errno is set appropriately. If count is zero and fd refers to a regular file, then write() may return a failure status if one of the errors below is detected. If no errors are detected, 0 will be returned without causing any other effect. If count is zero and fd refers to a file other than a regular file, the results are not specified. send(2): RETURN VALUE On success, these calls return the number of characters sent. On error, -1 is returned, and errno is set appropriately.No. 0 indicates end of streamBy end of stream, you mean this sequence ? server: (block SIGPIPE) sockwrite = socket(AF_UNIX, SOCK_STREAM, 0); addr.sun_family = AF_UNIX; strcpy(addr.sun_path, "./mysocket"); /* wait for client to create the unix socket */ sleep(2); connect(sockwrite, (struct sockaddr *)&addr, sizeof(addr));) /* let's do one write which will be consumed by the client */ write(sockwrite, "blah", sizeof("blah")); /* wait for client to close FD */ sleep(2); ret = write(sockwrite, "blah", sizeof("blah")); ( or even with ret = send(sockwrite, "blah", sizeof("blah"), MSG_NOSIGNAL); ) printf("write ret %ld errno %ld\n", ret, errno); client: (block SIGPIPE) sockserv = socket(AF_UNIX, SOCK_STREAM, 0); addr.sun_family = AF_UNIX; strcpy(addr.sun_path, "./mysocket"); bind(sockserv, (struct sockaddr *)&addr, sizeof(addr)); listen(sockserv, 1); sockread = accept(sockserv, NULL, NULL); recv(sockread, buf, sizeof(buf), 0); close(sockread); The server printf returns: "write ret -1 errno 32" So the return value is -1, with an error number: EPIPE: Broken pipeand looping on it will result in an infinite loop. You need to refer to the specific backend driver to understand these specific semantics. The read/write manpages are notorious for their non-clarity about this.This is what I just did by creating my small test program. Unless I am misunderstanding your definition of "end of stream", your assumptions about the way the unix sockets work seem incorrect.Well, just that one assumption. But I accept your explanation and my foolishness. ;-) patient_write() and patient_send() need to be fixed as you describe.OK, is Nils or David willing to take the ball ?
It's a time factor on my part. I don't think this is quite urgent so I'll try to make it as soon as I can. I know that Nils is quite busy on Trace events so I can do that to offload him.
About that, while doing this, I propose we just pass over UST code to correct and standardize the error handling and integrate your proposition below systematically. I came across a lot of code in ustctl, ustcmd and ustcomm that was not quite following the error "convention" of UST. Also, as I mention in an early post, the UST "standard" error (usterr.h) are positive value so we might also refactor that.
However, this might modify a lot of code so mistakes or missing part can easily be forgotten. It will be very important that a careful review be done. I'm willing to do that in a near future but, as I said, time factor on my part will delay this patch set.
Thanks David
Note that as the manpage says, if the file descriptor refers to a regular file, then write() can return 0, with an error number set, which could indicate that an error occured. If the file descriptor refers to a file other than a regular file, the results are unspecified. So I think the sane way to handle this would be to check if errno is 0 when write returns 0. If errno != 0, then we have an error, but if errno == 0, I'd consider that 0 bytes were written and retry. By the way, you forgot to reply to my first message above which covers the topic of semantics, so I'm re-pasting it here. (about ustcomm_send_request()) Well, this function is not technically the same as i/o syscalls at all. It uses I/O syscalls, but it is not an I/O syscall per se, so the return value transformation to a more standard pattern (neg err val, 0 ok) should happen right in this function rather than to let all callers handle this. I/O syscalls use positive return values to indicate the number of bytes read/written/etc. Here, this function arbitrarily choose 1 to indicate that "something has been sent" without caring about the amount of data moved at all.Let me copy the description of the return values here. /* * Return value: * 0: Success, but no reply because recv() returned 0 * 1: Success * -1: Error * * On error, the error message is printed, except on * ECONNRESET, which is normal when the application dies. */So as it doesn't need the whole positive range to spell out the amount of data moved, it doesn't need to do the same special-cases that the I/O syscalls are doing. It adds a lot of error values management oddness without adding anything.There are 3 cases the caller must consider when calling ustcomm_send_msg(). - An error occurred, there is a problem and it is necessary to complain (-1) - The remote connection was closed so we were not able to send the message (0) - Everything worked fine (1) Callers need to know if the connection was closed in order not to complain but at the same time take consequent measures.So even though I agree with you that this function is close to the I/O system calls because it calls it, it is very far from the I/O syscalls semantically (we don't care about the number of bytes written), and even though we might be tempted to use the same error values as system calls for them, the fact that we just don't care about the number of bytes written combined with the fact that standardizing error value across the code makes it much easier to follow and to write just call for this change.If you want to propose different semantics that still allow the caller to know which of the 3 cases happened, I'm all open.Yep, this is what I am proposing: return 0: everything is OK return -EPIPE: remote connection was closed, so we were unable to send the message. return -EWHATEVER (other than -EPIPE): An error occured. So the caller can deal with -EPIPE as it wants. How does that sound ? Thanks, MathieuThanks pmf
-- David Goulet LTTng project, DORSAL Lab. PGP/GPG : 1024D/16BD8563 BE3C 672B 9331 9796 291A 14C6 4AF7 C14B 16BD 8563 _______________________________________________ ltt-dev mailing list [email protected] http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
