Here's the result of "./python -m test test_decimal -m test_maxcontext_exact_arith":

0:00:00 load avg: 1.33 Run tests sequentially
0:00:00 load avg: 1.33 [1/1] test_decimal
python(7835,0x11a218dc0) malloc: can't allocate region
:*** mach_vm_map(size=842105263157895168, flags: 100) failed (error code=3)
python(7835,0x11a218dc0) malloc: *** set a breakpoint in malloc_error_break to debug
python(7835,0x11a218dc0) malloc: can't allocate region
:*** mach_vm_map(size=842105263157895168, flags: 100) failed (error code=3)
python(7835,0x11a218dc0) malloc: *** set a breakpoint in malloc_error_break to debug
python(7835,0x11a218dc0) malloc: can't allocate region
:*** mach_vm_map(size=421052631578947584, flags: 100) failed (error code=3)
python(7835,0x11a218dc0) malloc: *** set a breakpoint in malloc_error_break to debug
python(7835,0x11a218dc0) malloc: can't allocate region
:*** mach_vm_map(size=421052631578947584, flags: 100) failed (error code=3)
python(7835,0x11a218dc0) malloc: *** set a breakpoint in malloc_error_break to debug

== Tests result: SUCCESS ==

1 test OK.

Total duration: 553 ms
Tests result: SUCCESS

I spent quite a time to find where this error was coming from and it's actually not an error but a dubious message from OSX. Others will surely lose time on this too, I think the best way to handle it is to use bigmemtest() with a small value on MacOS so that it's only run when '-M' is given on this platform, but always on the others.

The annoying "error" looks the same as bpo-5614, which was closed as "won't fix".

Is this the only place in the test suite now? If it is, I'm okay with
squashing the remark.

The annoying "error" looks the same as bpo-5614, which was closed as "won't fix".

Yes, it's a similar message, test_io does not display it on Catalina anymore thought.

Is this the only place in the test suite now?

As far as I know yes.

If it is, I'm okay with squashing the remark.

I'm not sure to what this means.

"squashing the remark" means suppressing the message from malloc().
I would really like it if the test could be executed by default though.

Looking at:

https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/malloc.3.html

Could you try what happens if you set this environment variable?

export MallocLogFile=/dev/null

If it works, we could build a solution around that.

.

Thanks, I found "MallocDebugReport" in man malloc (I could have thought to look into there...) that seem to do the trick:

$ MallocDebugReport=none ./python -m test test_decimal -m test_maxcontext_exact_arith

0:00:00 load avg: 1.27 Run tests sequentially
0:00:00 load avg: 1.27 [1/1] test_decimal

== Tests result: SUCCESS ==

1 test OK.

Total duration: 694 ms
Tests result: SUCCESS

There is also MallocDebugReport=crash that will "write messages to standard error only for a condition that is about to cause a crash." and effectively suppress this one.

Setting it in the test with os.putenv() does not seem to work.

I forgot to say that MallocLogFile=/dev/null did not work, that's why I tried MallocDebugReport.

Thanks for checking, it's a pity that os.putenv() does not work.

There's a global variable malloc_logger that is set from the
environment on startup:

https://opensource.apple.com/source/libmalloc/libmalloc-53.1.1/src/malloc.c

It seems to be always checked for NULL, but I can't find any
documentation that says it is guaranteed.

A nuclear option would be to set malloc_logger to NULL on Python
startup. Or write a custom malloc_logger, which *does* seem to
be supported.

Would it be possible to wrap malloc_print_configure() (https://github.com/PureDarwin/libmalloc/blob/e37056265821cd6e014ab911d9fe3b9d0da88e22/src/malloc_printf.c#L59) in a context manager that we put in test.support? or to override https://github.com/PureDarwin/libmalloc/blob/e37056265821cd6e014ab911d9fe3b9d0da88e22/src/malloc_printf.c#L36?

This would let us change this behaviour locally without masking all other errors.

I have two observations:

First, I think that the default DEBUG_WRITE_ALWAYS, which is the cause
of this issue, is not C standard conforming -- The standard does not
permit implementation defined diagnostics (global side effects!) on a
completely normal allocation failure.

So I would feel justified to make DEBUG_WRITE_ON_CRASH the default
on Python startup, even if that means that we force that on extension
modules.

That would also address annoyances like in msg327446. Speaking of
which, do you also get that diagnostic in cases like these?

>>> [0] * 10000000000000000
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
MemoryError

FreeBSD has a similar reporting mechanism, but the default is not
DEBUG_WRITE_ALWAYS. I suspect DEBUG_WRITE_ALWAYS is there for the
convenience of Apple developers dealing with user reports.

Second, we *could* write such a context manager and it would probably
be okay *for the test suite*, but in general changing a static variable
that is not thread safe at runtime in a library should be avoided.

I don't have OS X at all though. Ronald, what do you think?

I have two observations:

First, I think that the default DEBUG_WRITE_ALWAYS, which is the cause
of this issue, is not C standard conforming -- The standard does not
permit implementation defined diagnostics (global side effects!) on a
completely normal allocation failure.

That makes sense

> So I would feel justified to make DEBUG_WRITE_ON_CRASH the default
> on Python startup, even if that means that we force that on extension
> modules.
> 
> That would also address annoyances like in msg327446. Speaking of
> which, do you also get that diagnostic in cases like these?
> 
> 
> >>> [0] * 10000000000000000
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> MemoryError
>

It gives:

>>> [0] * 10000000000000000
python3(36633,0x110c08dc0) malloc: can't allocate region
:*** mach_vm_map(size=80000000000004096, flags: 100) failed (error code=3)
python3(36633,0x110c08dc0) malloc: *** set a breakpoint in malloc_error_break to debug
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
MemoryError

Not very nice.

FreeBSD has a similar reporting mechanism, but the default is not
DEBUG_WRITE_ALWAYS. I suspect DEBUG_WRITE_ALWAYS is there for the
convenience of Apple developers dealing with user reports.

Second, we *could* write such a context manager and it would probably
be okay *for the test suite*, but in general changing a static variable
that is not thread safe at runtime in a library should be avoided.

What about overriding the default to DEBUG_WRITE_ON_CRASH if MallocDebugReport
is not set? This would be more standard compliant and still let the user
override it if he whishes so.

I don't have OS X at all though. Ronald, what do you think?

What about overriding the default to DEBUG_WRITE_ON_CRASH if MallocDebugReport is not set?

That sounds like a very good compromise!

Ok, I will try to do this in the coming days. Is adding this kind of super ugly, super specific code to pymain_init() ok?

I would probably put all that code in a separate function like
init_darwin_libc(), and call init_darwin_libc() in pymain_init().

The real fun will start when we figure out which OS X versions
support this feature. I hope we have a buildbot for each. :-)

Changing the title to get more input from others.

Ok, I will try to do this in the coming days. Is adding this kind of super ugly, super specific code to pymain_init() ok?

Maybe _PyPreConfig_Write() would be a better place.

There is a very little risk that memory allocation done before _PyPreConfig_Write() (i.e. in _PyPreConfig_Read()) fail with out of memory.

Would it be possible to wrap malloc_print_configure() (https://github.com/PureDarwin/libmalloc/blob/e37056265821cd6e014ab911d9fe3b9d0da88e22/src/malloc_printf.c#L59) in a context manager that we put in test.support? or to override https://github.com/PureDarwin/libmalloc/blob/e37056265821cd6e014ab911d9fe3b9d0da88e22/src/malloc_printf.c#L36?

I don't see much benefit of having a feature which only works on macOS.

You can put a hook on memory allocations using PyMem_SetAllocator() to trigger any action you want when any memory allocation done by Python fails.

But is it really an use case to log any memory allocation failure? Python is very likely to raise a MemoryError exception in this case. It's well defined, no?

But is it really an use case to log any memory allocation failure? Python is very likely to raise a MemoryError exception in this case. It's well defined, no?

Yes, we are not trying to log the allocation failure, we are just trying to suppress a dubious error message that occurs specifically on MacOS.

I'm not sure if it is worthwhile to tweak CPython to work around this misfeature of libc on macOS (and likely iOS). Is this something that causes issues in real code, or just in the test suite? Looking at this particular test is seems to use a context that is not likely to occur in real code (due to using a lot of memory)

Note that malloc_print_configure is not a public API, using that can cause problems for folks that embed Python in apps that are shipped in the app store.

No, it does not cause real issues. Adding this feature would just suppress chatty diagnostics like this one, which are a bit unfriendly:

>>> [0] * 10000000000000000
python3(36633,0x110c08dc0) malloc: can't allocate region
:*** mach_vm_map(size=80000000000004096, flags: 100) failed (error code=3)
python3(36633,0x110c08dc0) malloc: *** set a breakpoint in malloc_error_break to debug
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
MemoryError

So yes, users do occasionally see it outside the test suite.

But if malloc_print_configure() is not a public API, it should probably not be used.

And indeed, with the new decimal MAX_PREC feature, you should see
the misguided diagnostic as well on OS X:

>>> from decimal import *
>>> 
>>> c = getcontext()
>>> c.prec = MAX_PREC
>>> Decimal(4).sqrt()
Decimal('2')
>>> 

So it is real code, but extremely rarely used.

I've filed a report with Apple about this (FB7731971), even though I expect that this behaviour will not change. That's something we should have done years ago.

BTW. The link to malloc_print_configure that Victor shared seems to indicate that the default for MallocDebugReport should be "none", instead of "stderr". The code on opensource.apple.com conforms this (https://opensource.apple.com/source/libmalloc/libmalloc-283.60.1/src/malloc_printf.c.auto.html). Apparently Apple uses a slightly different code when building macOS :-(

Note that this annoying message is only printed when the proces runs out of address space, which requires allocating an insane amount of memory (the first warning line in the initial message of this bug report tries to allocate 872 petabyte).

If this is something we want to avoid its probably easier to switch to a allocator that won't avoids calling malloc(3) on large enough allocations (for example return NULL for any allocation attempt above 4TB, which is comfortably above the max amount of memory in a Mac Pro). This theoretically limits memory allocations, but in practice the system will be unusable with far smaller allocations (assuming the allocated memory is actually used).

The first warning line in the initial message of this bug report tries to allocate 872 petabyte.

Yes, that's for systems like Linux where you never know what overallocation is going to permit and subsequently freeze the system.

Special casing malloc() for OS X/large allocations sounds like a good plan.

From my pr, "This is a bandaid to help people save time debugging this non-bug. I don't think this solution is very strong; I'm just hoping to move the discussion forward on the bpo, and hoping for a hint on how to suppress these warnings all together or come up with some other optimal solution.

I applied the function in test_decimal because that test consistently generates these malloc warnings on my system."

I'm still trying to find a true solution. Can anyone help to point me in a better direction?

Alternatively, my latest PR implements what @ronaldoussoren suggested: capping OS X memory allocations at 4TB.

New changeset 15d3c14 by Jack DeVries in branch 'main':
bpo-40928: notify users running test_decimal on macOS of malloc warnings (GH-26783)
15d3c14

New changeset 693a661 by Łukasz Langa in branch '3.9':
[3.9] bpo-40928: notify users running test_decimal on macOS of malloc warnings (GH-26783) (GH-27629)
693a661

New changeset a5d9963 by Miss Islington (bot) in branch '3.10':
bpo-40928: notify users running test_decimal on macOS of malloc warnings (GH-26783)
a5d9963

Given that:

• this is only surfacing in test_decimal on macOS;
• the real solution to modify malloc has potential performance implications; and
• silencing the spew isn't possible from within the test module

I elected to accept Jack's patch to notify macOS users running regression tests of the harmless malloc spew. I don't think there's anything else to do here.

Thanks all for your effort in investigating this!

the real solution to modify malloc has potential performance implications

No, it does not. This has been in obmalloc.c for at least 8 years:

    if (size > (size_t)PY_SSIZE_T_MAX)
        return NULL;

PY_SSIZE_T_MAX could be changed to a lower value for MacOS, using a zero cost ifdef.