Make POSIX-like writes write everything when blocking #5466
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First draft, not ready yet. @SeppoTakalo, @juhaylinen - maybe the tests your working on should be written on the assumption TCPSocket works like this? review thoughts? @geky, @bogdanm, @mikaleppanen |
SeppoTakalo
approved these changes
Nov 9, 2017
SeppoTakalo
approved these changes
Nov 10, 2017
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Looks good to me. |
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@kjbracey-arm Is this ready yet? |
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I'm happy that this can proceed now. |
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/morph build |
Build : FAILUREBuild number : 558 |
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IAR related failures |
Previously, send() was somewhat soft - it only ever made one send
call to the underlying stack, so it would typically take as much data
as would fit in the buffer, and only block if it was unable to write
anything.
This is not the intent of a POSIX socket/filehandle write. It should try
to send everything if blocking, and only send less if interrupted by a
signal:
- If the O_NONBLOCK flag is clear, write() shall block the calling
thread until the data can be accepted.
- If the O_NONBLOCK flag is set, write() shall not block the thread.
If some data can be written without blocking the thread, write()
shall write what it can and return the number of bytes written.
Otherwise, it shall return -1 and set errno to [EAGAIN].
This "send all" behaviour is of slightly limited usefulness in POSIX, as
you still usually have to worry about the interruption possibility:
- If write() is interrupted by a signal before it writes any data, it
shall return -1 with errno set to [EINTR].
- If write() is interrupted by a signal after it successfully writes
some data, it shall return the number of bytes written.
But as mbed OS does not have the possibility of signal interruption, if we
strengthen send to write everything, we can make applications' lives
easier - they can just do "send(large amount)" confident that it will
all go in one call (if no errors).
So, rework to make multiple sends to the underlying stack, blocking as
necessary, until all data is written.
This change does not apply to recv(), which is correct in only blocking until
some data is available:
- If O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].
- If O_NONBLOCK is clear, read() shall block the calling thread until some
data becomes available.
- The use of the O_NONBLOCK flag has no effect if there is some data
available.
Previously, write() was somewhat soft - it only ever made one attempt to
wait for buffer space, so it would take as much data as would fit in the
buffer in one call.
This is not the intent of a POSIX filehandle write. It should try to
send everything if blocking, and only send less if interrupted by a
signal:
- If the O_NONBLOCK flag is clear, write() shall block the calling
thread until the data can be accepted.
- If the O_NONBLOCK flag is set, write() shall not block the thread.
If some data can be written without blocking the thread, write()
shall write what it can and return the number of bytes written.
Otherwise, it shall return -1 and set errno to [EAGAIN].
This "send all" behaviour is of slightly limited usefulness in POSIX, as
you still usually have to worry about the interruption possibility:
- If write() is interrupted by a signal before it writes any data, it
shall return -1 with errno set to [EINTR].
- If write() is interrupted by a signal after it successfully writes
some data, it shall return the number of bytes written.
But as mbed OS does not have the possibility of signal interruption, if we
strengthen write to write everything, we can make applications' lives
easier - they can just do "write(large amount)" confident that it will
all go in one call (if no errors).
So, rework to make multiple writes to the buffer, blocking as necessary,
until all data is written.
This change does not apply to read(), which is correct in only blocking until
some data is available:
- If O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].
- If O_NONBLOCK is clear, read() shall block the calling thread until some
data becomes available.
- The use of the O_NONBLOCK flag has no effect if there is some data
available.
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Pointer arithmetic on void pointer corrected. |
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/morph build |
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We did some CI work last night, might have disrupted this build. Rekicking off. |
Build : SUCCESSBuild number : 577 Triggering tests/morph test |
Exporter Build : SUCCESSBuild number : 192 |
Test : SUCCESSBuild number : 393 |
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FileHandle and TCPSocket both have POSIX-like write calls. Both of them only ever write as much as currently fits into the buffer or underlying stack in one go.
It would be advantageous if we were to follow the spirit of POSIX a bit more closely and make those write calls write everything when in blocking mode - POSIX does try to write everything when blocking, and only returns early if interrupted by a signal.
As we don't have signals, we can guarantee to always write everything when blocking (if no errors), and this could make callers' lives easier.
First commit here updates TCPSocket for testing. A commit for UARTSerial.cpp and FileHandle.h will follow.
See commit message for the TCPSocket for more excruciating detail and POSIX spec quotes.