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Queue.add is not thread/signal safe #5309

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vicuna opened this Issue Jul 6, 2011 · 3 comments

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vicuna commented Jul 6, 2011

Original bug ID: 5309
Reporter: @lefessan
Assigned to: @lefessan
Status: closed (set by @xavierleroy on 2015-12-11T18:04:34Z)
Resolution: fixed
Priority: normal
Severity: minor
Platform: all
OS: all
OS Version: all
Version: 3.12.1+dev
Fixed in version: 3.13.0+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: khooyp

Bug description

If an exception is raised in a signal after the first line of Queue.add (i.e. increasing the length counter of the queue), the queue invariants are broken and bad things can happen (segfault...).

In the additional information, I provide a new "add" function that should be thread/signal safe.

Additional information

let add x q =
if q.length = 0 then
let rec cell = {
content = x;
next = cell
} in
q.length <- q.length + 1;
q.tail <- cell
else
let tail = q.tail in
let head = tail.next in
let cell = {
content = x;
next = head
} in
q.length <- q.length + 1;
tail.next <- cell;
q.tail <- cell

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vicuna commented Jul 6, 2011

Comment author: @mmottl

Nitpicking here:

q.length <- q.length + 1;

can be replaced by:

q.length <- 1;

in the first branch.

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vicuna commented Feb 15, 2012

Comment author: @gasche

To sum up yesterday's discussion: the OCaml library is not guaranteed to be signal-safe, or rather, it is voluntarily guaranteed not to be signal-safe. Users should acknowledge that using mutable data structures expose them to signal hazards, and stick to immutable data structures or use different, specialized signal-hardened libraries.

That said, this specific patch is relatively harmless -- in particular, it does not impact clarity and readability -- and Fabrice may include it.

As a personal note: we may also write

begin if q.length = 1 then
..
else
..
end;
q.length <- q.length + 1

to avoid duplicating the common length-extension logic. Of course, it's only a dispensable detail.

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vicuna commented Feb 18, 2012

Comment author: @lefessan

Fixed in commit r12163 in SVN trunk.

@vicuna vicuna closed this Dec 11, 2015

@vicuna vicuna added the bug label Mar 20, 2019

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