But possibly the bigger benefit: no heap allocation for many objects at all. If you can prove single ownership, you can stack allocate it (or "inline it" if in a struct/object).
Just as an aside - in Python, it surprised me that 'with' is _not_ a destructor. The object created will remain in-scope after the 'with' body, but the '__exit__' method will be called. Which for the canonical example of file-handling, only really closes the file.
RAII is actually an antipattern in Python if you actually care about resources being released.
with open(path) as file:
result = func(file.read())
another_func(result)
However:
with open(path) as file:
...
if file.mode == 'r':
print("Huh? file is still a thing?") def foo():
if False:
x = 123
print("x is visible out here:", x)
x = "is global out here"
def foo():
print("all good:", x)
def bar():
print("not good:", x)
if False:
x = "now x is local, even before this declaration"IIRC python3 fixed scope leaks in list comprehensions, does anyone know why they didn't fix this one as well? Or does it have actual uses in practice?
The motivation is especially opaque, because though CPython does run __exit__ immediately, it isn't guaranteed to do so by the standard. So __exit__ will happen at some point, like __del__ with the GC, but it won't actually call __del__, which makes GC'ing the value more complicated as __del__ can't safely run until __exit__ does.
I _think_ the motivation is something to do with __exit__ being allowed to throw an exception that can be caught, whereas __del__ isn't allowed to do that, but I don't find it that convincing. And I've never seen an example where the introduced variable is ever referenced afterwards in Python's documentation (including in the PEP itself).
with context() as foo:
leak = foo
So maybe the reason is that it is not really worth the trouble.
* Well, not variables in generator expressions. There might be a few other exceptions.
And it isn't as if using languages without static analysis is still considered a best practice in the 21st century.
So in summary, "Please just do better". Not helpful as a video game meme, and really no better for programming.
An example of a clippy lint that I like in clippy:
If you try to transmute() &[u8] into a &str clippy will point out that you should probably from_utf8_unchecked() -- this lint results in code that does the same thing but is clearer for maintainers about what we intended. If our reaction to the lint is "But it isn't UTF-8" then we just got an important red flag, but that's not why the lint is there.
On the other hand, clippy lints for truncating casts and I want those gone from the language so the warnings ought to at least live in the compiler not clippy. I would like to see a future Edition outlaw these casts entirely so that you need to write a truncating cast explicitly if that's what you needed.
FYI: As I understand, escape analysis is frequently used to either (a) avoid allocating on the heap, or (b) auto-magically free-ing a malloc'd object from a tight scope.
https://en.m.wikipedia.org/wiki/List_of_Java_virtual_machine...
As it stands is mostly done at runtime and only for a).
Otherwise you are better with try-resources, and for something like malloced memory there are off-GC heap APIs.
Also Valhalla will eventually land, although it is getting long by now.
Locks are entirely a memory construct. You can trivially write one, e.g. Java locks are written entirely in java. The files descriptors (incl. socket) - they should not be relied to be GC'd by default, they do require their own lifecycle. For instance: c# Disposable/ java's try with resource.
>no heap allocation for many objects at all.
This has been already the case for many a year - reference escape analysis, ref. escape elission, trace compilation.
Historically, in concurrent software your only option was to either use these OS allocated locks or build a spin lock from atomic operations (and thus burn CPU every time you're waiting for a lock). The OS allocated locks, such as from sem_open() or create_sem() must later be properly released or you're leaking a finite OS resource that will only be reclaimed on process exit or in some cases only at reboot.
Today your operating system probably offers a lightweight "futex" style locking primitive, in which so long as it's not locked no OS resources are reserved it's just memory. But, you're not guaranteed that all your locks are these lightweight structures and even if you were you need to clean up properly because when they are locked the OS is using resources so you need to unlock properly or you'll blow up.
The locks that result in thread awaiting are still in-memory, of course. They are just OS provided primitives. Yet again, a well behaved/designed lock should be non-contested for the most calls (easily 99.9%+). In that case there is no mode-switch to get to the kernel. A contested lock that goes to the kernel is a context switch.
In the end there is no need to do anything special about locks as they can implemented in the user-space unlike the file descriptors.
If you get semaphores from create_sem() you will need to give them back, you can't just say "This shouldn't be an OS resource" and throw it away, exactly like a file descriptor it's your responsibility to dispose of it properly.
Speaking of which, depending on the OS, your file descriptors might also just be implemented in user space. They're just integers, it's no big deal, of course you must still properly open() and close() files, the fact it's "implemented in user space" does not magically mean it isn't your problem to clean up properly.
Likewise, if you opened a sqlite database, you must close it properly, you can't just say "Eh, it's only userspace, it will be fine" and throw away the memory without closing properly.
I don't quite get the continued argument - the created locks in the user space have 'zero' extra dependency on the kernel. In a GC setup they would be an object no different than a mere string (likely less memory as well). They can be GC'd at any time as long as there are no references to them.
If you wish to see an example: java.util.concurrent package. ReentrantLock[0] is a classic lock, and there is even a Semaphore in the same package (technically not in the very same). Originated in 2004 with JMM and the availability of CAS. You can create as many as you choose - no kernel mode switch would occur.
For the record I do no object that proper resource managed is needed - incl. sqldatabase close. The latter does use a file descriptor any time you establish a connection. However, locks need no special support from the kernel. Also in GC'd setup file descriptors would be GC'd in pre (or post) morterm via some finalization mechanism, however relying on the GC to perform resource management is a terrible practice.
[0]: https://github.com/openjdk/jdk/blob/master/src/java.base/sha...
I keep telling you that, in fact, these primitives can literally be OS resources such as the semaphores from the BeOS system call create_sem() or the POSIX sem_open or Windows CreateSemaphore.
Your insistence that these don't have a dependency on the kernel, that they're just memory to be garbage collected is wrong, they aren't, this won't work.
You propose the Java ReentrantLock class as an example, but, that's just a thin wrapper for the underlying Java runtime parking mechanism, so, sure it's just a Java object no big deal, but the actual implementation (which is not in the file you just linked) requires an OS primitive that you've not examined at all, that OS primitive is being managed by the runtime (via the "Unsafe" interface) and is excluded from your garbage collection entirely.
In essence you get to "just" garbage collect these locks because they're only a superficial Java language shadow of the real mechanism, which you aren't allowed to touch and can't garbage collect.
This is most definitely false. The methods needed are the compare-and-set + LockSupport/park (parkNanos) and LockSupport.unpark. The former is a standard CPU instruction w/o any special magic about and the latter is setting the current thread to sleep and being awaken.
As for examine: I know how j.u.c works down the OS calls and generated assembly. Unsafe is a class (not an interface), albeit mostly resulting in compiler magic.
The use of unsafe is solely for: LockSupport::park/unpark and AbstractQueuedSynchronizer::compareAndSet. ReentrantLock is NOT any thing wrapper.
And here's the big reveal. It's actually calling the OS for "setting the current thread to sleep and being awaken" ie it's just using the OS locks and those are hidden from you so that you can't garbage collect them.
> Unsafe is a class (not an interface), albeit mostly resulting in compiler magic.
Unfortunate choice of language, I didn't mean a Java interface, but an interface in the broader sense. I assure you that "setting the current thread to sleep" is not "compiler magic", it's just using the OS locks, on a modern system it will presumably use a lightweight lock but on older systems it'll be something more heavyweight like the semaphores we saw earlier.
The claim that LockSupport, which is calling the OS is not a wrapper is bogus.
But all this is a distraction, even on systems where old semaphore APIs are now rarely needed they still exist and if you hold one you are still responsible for cleaning it up properly even if you think you know better.
Those are not any part of the developer capability to manage, aside starting the thread (and locking on it). The mutex is released with the thread, itself; Technically hotspot impl java does not ever releases them back to the OS but keeps a free-list. The thread is a lot larger OS resource as well. No new OS resources are exhausted by the OS b/c of the extra locking.
>The claim that LockSupport, which is calling the OS is not a wrapper is bogus.
Of course it does call the OS; it happens on the dormant part (w/o an OS call it'd a busy spin), or if a thread needs to be awaken. I said not a 'thin' wrapper. All the locking and queueing mechanism is outside the OS. The major distinction is there is only a single OS primitive per Thread NOT per lock. (Edit: now I see I have spelled thin as 'thing' which is a definite mistake on my part)
/*
* This is the platform-specific implementation underpinning
* the ParkEvent class, which itself underpins Java-level monitor
* operations. See park.hpp for details.
* These event objects are type-stable and immortal - we never delete them.
* Events are associated with a thread for the lifetime of the thread.
*/ // ParkEvents are type-stable and immortal.
//
// Lifecycle: Once a ParkEvent is associated with a thread that ParkEvent remains
// associated with the thread for the thread's entire lifetime - the relationship is
// stable. A thread will be associated at most one ParkEvent. When the thread
// expires, the ParkEvent moves to the EventFreeList.
Edit: My point has always been that locks/monitors/semaphores/etc. dont need any special care in a GC-setup as they can be implemented purely in the language itself, and without special care to release them, and manage them similar to file descriptors. The need to block (become dormant) obviously requires OS calls and support but that part doesn't have to scale with the amount of locks/semaphores/latches/etc. created.
That does make a lot more sense. I agree that what's going on here is not merely a thin wrapper.
> My point has always been that locks/monitors/semaphores/etc. dont need any special care in a GC-setup as they can be implemented purely in the language itself
And my point was always that you can only depend upon this if you in fact are always using the "implemented purely in the language itself" mechanisms, while plenty of others exist and you might need them - but you can't just treat them as garbage safely.
So overall I think we were mostly talking at cross purposes.
Actually looking at the Java stuff reminded me of m_ou_se's thread about how big boost::shared_mutex is (it's 312 bytes). Most people who need one of these synchronization primitives only really need the one that costs 4 bytes, but it's hard to keep saying "No" to people who want to add features. They're always confident that everybody will love their feature, at the cost of a little more memory, and it spirals out of control.
On most systems the pthread_mutex_t you mentioned is already 40 bytes. A "nightly" Rust can do a 32-byte array with a mutex to protect it, all inside 40 bytes.