how to do large-ish IPC transfers

[Brad Bishop]

at 6:30 AM, Alexander Amelkin <a.amelkin at yadro.com> wrote:

> 11.09.2019 21:31, Vernon Mauery wrote:
>> On 11-Sep-2019 01:49 PM, Brad Bishop wrote:
>>> Hi everyone
>>>
>>> I have an application or two that need to collect and save ~ 16KiB of
>>> debugging data.  The applications have various triggers but they are very
>>> infrequent when deployed (on the order of months/years).  In the lab it
>>> would happen much more frequently.  It isn’t really debugging information
>>> for the developer of the application but rather for customer service
>>> personnel debugging broken components on a deployed server, after the  
>>> event
>>> (and state) that caused the data to be collected.
>>>
>>> The applications generating this data will not be the same applications  
>>> that
>>> publish it (the webserver), so there is a need to transfer it using IPC.
>>>
>>> 16KiB seemed a little big for a DBus interface, and we’d have to ascii
>>> encode all that into a string.  Some other ideas were:
>>>
>>> 1 - passing a file descriptor over dbus
>> This would probably be the best, given our architecture. It would allow,
>> at some point in the future, to have the sender have different
>> permissions on the file and allow access to a less-privileged process.
>
> What is a 'file descriptor' you're referring to here? The Linux file  
> descriptor
> (the integer you get from open()) is process-specific and doesn't keep its
> meaning in another process' context.

On Linux you can pass file descriptors (the ones returned by open) over  
unix sockets.  The implementation of the ‘h’ DBus type relies on that.