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bug#30626: 26.0.91; Crash when traversing a `stream-of-directory-files'

From: Noam Postavsky
Subject: bug#30626: 26.0.91; Crash when traversing a `stream-of-directory-files'
Date: Tue, 27 Feb 2018 20:29:21 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.90 (gnu/linux)

Eli Zaretskii <address@hidden> writes:

>> From: Michael Heerdegen <address@hidden>
>> Cc: address@hidden,  address@hidden
>> Date: Tue, 27 Feb 2018 13:08:59 +0100
>> #+begin_src emacs-lisp
>> (seq-doseq (_ (stream-range 1 1000000)) nil)
>> #+end_src
>> Note that this is executed as a loop due how to streams are implemented,
>> although the definition of `seq-doseq' looks recursive.

Doesn't look recursive to me, it expands to a call to seq-do, which uses
a simple loop.

>> But it seems that gc has a problem with the large number of conses
>> created when processing that.
> What can we do instead in such cases?  Stack-overflow protection
> cannot work in GC, so you are shooting yourself in the foot by
> creating such large recursive structures.  By the time we get to GC,
> where the problem will happen, it's too late, because the memory was
> already allocated.
> Does anyone has a reasonable idea for avoiding the crash in such
> programs?

I don't have a quick answer for the general case, but I think it's a bug
in stream.el that it's creating such large structures in the first
place.  As far as I understand it, the point of streams is to handle
long lists by encoding them as

so as to avoid allocating large amounts of memory.  Is there an easy way
to find out what the large structures are, and where they are coming

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