Re: [Xen-devel] [PATCH v2 2/2] libxl: Add new "notify-only" childproc mode

[George Dunlap <george.dunlap@citrix.com>]

On 1/9/20 12:12 PM, George Dunlap wrote:
> libxl needs to be able to know when processes it forks have completed.
> 
> At the moment, libxl has two basic mode (with some variations).  In
> one mode -- libxl_sigchld_owner_libxl* -- libxl sets up its own
> SIGCHLD signal handler, and also handles the event loop that allows
> libxl to safely block until the child in question is finished (using a
> self-pipe for the SIGCHLD handler to notify the waiters that it's time
> to look for reaped children).
> 
> In the other mode, libxl does not set up the SIGCHLD handler, nor does
> it do anything with processing the event loop; it expects the library
> caller to handle the event loop itself.
> 
> The golang runtime manages its own processes, and thus must use
> SIGCHLD itself; and it has an easy way for other users to get SIGCHLD
> notifications.  However, because its event loop is hidden away behind
> abstractions, it's not easy to hook into; and there's no need -- the
> golang runtime assumes that C function calls may block, and handles
> everything behind the scenes.
> 
> Introduce a new mode, libxl_sigchld_owner_notify, in which libxl sets
> up the SIGCHLD event handling machinery, but relies on the caller to
> tell it when a SIGCHLD happens.
> 
> Call these two actions "notify" (for the self-pipe notification
> machinery) and "handler" (for the actual SIGCHLD handler).
> 
> Provide a new external function, libxl_childproc_sigchld_notify(), for
> library users to call when a SIGCHLD happens.  Have this call
> sigchld_handler().
> 
> Rename chldmode_ours() to chldmode_notify(), and use it to determine
> whether to set up the notification chain.
> 
> When setting up the notification chain, do everything except setting
> up the signal handler in "notify-only" mode.
> 
> defer_sigchld() and release_sigchld() do two things: they modify the
> signal handler, and grab and release locks.  Refactor these so that
> they grab and release the locks correctly in "notify-only" mode, but
> don't tweak the signal handler unless it's been set up.
> 
> With the golang bindings ported to use this change, domain creation
> works.
> 
> NB an alternate approach would be to make libxl_sigchld_owner_mainloop
> *always* set up and tear down the self-pipe notification mechanisms,
> and then simply expose libxl_childproc_sigchld_notify().  However,
> this would entail grabbing a libxl-wide global lock (across all libxl
> ctx's) twice on every fork.  This should be avoided for callers which
> don't need it.
> 
> Signed-off-by: George Dunlap <george.dunlap@citrix.com>

FAOD, with the fixes in your other series, I consider this patch to now
be moot.

 - George

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