NAME
docs/pdds/draft/pdd24_events.pod - Parrot Events
ABSTRACT
This document defines the requirements and implementation strategy for Parrot's event subsystem.
VERSION
$Revision: 18563 $
DESCRIPTION
Description of the subject.
DEFINITIONS
Definitions of important terms. (optional)
IMPLEMENTATION
[Excerpt from Perl 6 and Parrot Essentials to seed discussion.]
An event is a notification that something has happened: the user has manipulated a GUI element, an I/O request has completed, a signal has been triggered, or a timer has expired. Most systems these days have an event handler (often two or three, which is something of a problem), because handling events is so fundamental to modern GUI programming. Unfortunately, the event handling system is not integrated, or poorly integrated, with the I/O system, leading to nasty code and unpleasant workarounds to try and make a program responsive to network, file, and GUI events simultaneously. Parrot presents a unified event handling system, integrated with its I/O system, which makes it possible to write cross-platform programs that work well in a complex environment.
Parrot's events are fairly simple. An event has an event type, some event data, an event handler, and a priority. Each thread has an event queue, and when an event happens it's put into the right thread's queue (or the default thread queue in those cases where we can't tell which thread an event was destined for) to wait for something to process it.
Any operation that would potentially block drains the event queue while it waits, as do a number of the cleanup opcodes that Parrot uses to tidy up on scope exit. Parrot doesn't check each opcode for an outstanding event for pure performance reasons, as that check gets expensive quickly. Still, Parrot generally ensures timely event handling, and events shouldn't sit in a queue for more than a few milliseconds unless event handling has been explicitly disabled.
When Parrot does extract an event from the event queue, it calls that event's event handler, if it has one. If an event doesn't have a handler, Parrot instead looks for a generic handler for the event type and calls it instead. If for some reason there's no handler for the event type, Parrot falls back to the generic event handler, which throws an exception when it gets an event it doesn't know how to handle. You can override the generic event handler if you want Parrot to do something else with unhandled events, perhaps silently discarding them instead.
Because events are handled in mainline code, they don't have the restrictions commonly associated with interrupt-level code. It's safe and acceptable for an event handler to throw an exception, allocate memory, or manipulate thread or global state safely. Event handlers can even acquire locks if they need to, though it's not a good idea to have an event handler blocking on lock acquisition.
Parrot uses the priority on events for two purposes. First, the priority is used to order the events in the event queue. Events for a particular priority are handled in a FIFO manner, but higher-priority events are always handled before lower-priority events. Parrot also allows a user program or event handler to set a minimum event priority that it will handle. If an event with a priority lower than the current minimum arrives, it won't be handled, instead sitting in the queue until the minimum priority level is dropped. This allows an event handler that's dealing with a high-priority event to ignore lower-priority events.
User code generally doesn't need to deal with prioritized events, so programmers should adjust event priorities with care. Adjusting the default priority of an event, or adjusting the current minimum priority level, is a rare occurrence. It's almost always a mistake to change them, but the capability is there for those rare occasions where it's the correct thing to do.
Signals
Signals are a special form of event, based on the Unix signal mechanism. Parrot presents them as mildly special, as a remnant of Perl's Unix heritage, but under the hood they're not treated any differently from any other event.
The Unix signaling mechanism is something of a mash, having been extended and worked on over the years by a small legion of undergrad programmers. At this point, signals can be divided into two categories, those that are fatal, and those that aren't.
Fatal signals are things like SIGKILL, which unconditionally kills a process, or SIGSEGV, which indicates that the process has tried to access memory that isn't part of your process. There's no good way for Parrot to catch these signals, so they remain fatal and will kill your process. On some systems it's possible to catch some of the fatal signals, but Parrot code itself operates at too high a level for a user program to do anything with them--they must be handled with special-purpose code written in C or some other low-level language. Parrot itself may catch them in special circumstances for its own use, but that's an implementation detail that isn't exposed to a user program.
Non-fatal signals are things like SIGCHLD, indicating that a child process has died, or SIGINT, indicating that the user has hit ^C on the keyboard. Parrot turns these signals into events and puts them in the event queue. Your program's event handler for the signal will be called as soon as Parrot gets to the event in the queue, and your code can do what it needs to with it.
SIGALRM, the timer expiration signal, is treated specially by Parrot. Generated by an expiring alarm() system call, this signal is normally used to provide timeouts for system calls that would otherwise block forever, which is very useful. The big downside to this is that on most systems there can only be one outstanding alarm() request, and while you can get around this somewhat with the setitimer call (which allows up to three pending alarms) it's still quite limited.
Since Parrot's IO system is fully asynchronous and never blocks--even what looks like a blocking request still drains the event queue--the alarm signal isn't needed for this. Parrot instead grabs SIGALRM for its own use, and provides a fully generic timer system which allows any number of timer events, each with their own callback functions and private data, to be outstanding.
ATTACHMENTS
None.
FOOTNOTES
None.
REFERENCES
None.