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BinaryHeap is not exception safe #25842

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bluss opened this issue May 27, 2015 · 5 comments · Fixed by #25856
Closed

BinaryHeap is not exception safe #25842

bluss opened this issue May 27, 2015 · 5 comments · Fixed by #25856
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T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.

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@bluss
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bluss commented May 27, 2015

BinaryHeap is using zeroed() and may not be exception safe. I.e. it is in an inconsistent state after being recovered after panic. See issue #25662 and others.

Relevant code is BinaryHeap::sift_up, sift_down_range

cc @cmr
@bluss
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bluss commented May 27, 2015

My immediate ideas are one of

  • Use PPYP and set vector length to zero temporarily. Panicing in comparison functions will leak all elements in the heap
  • Use more swaps

@bluss
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bluss commented May 27, 2015

@gankro's got the idea: https://botbot.me/mozilla/rust-internals/2015-03-24/?msg=34945483&page=6

@Gankra
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Gankra commented May 27, 2015

Long ago I suggested two other strategies:

  • A two-pass phase where you do all comparisons first, then once you've found the Final Destination you actually do the swaps (which requires no untrusted code to run).
  • A panic guard which on drop puts the "working" element back in the "uninit" location in the array

@alexcrichton
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triage: I-nominated, T-libs

@alexcrichton alexcrichton added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label May 27, 2015
@bluss
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bluss commented May 27, 2015

I'm trying gankro's panic guard idea.

bluss pushed a commit to bluss/rust that referenced this issue May 28, 2015
collections: Make BinaryHeap panic safe in sift_up / sift_down
5f44f5b 
Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

Fixes rust-lang#25842
@bluss bluss mentioned this issue May 28, 2015
Merged
bluss pushed a commit to bluss/rust that referenced this issue May 28, 2015
collections: Make BinaryHeap panic safe in sift_up / sift_down
b221853 
Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

Fixes rust-lang#25842
bluss pushed a commit to bluss/rust that referenced this issue May 28, 2015
collections: Make BinaryHeap panic safe in sift_up / sift_down
c41a5cc 
Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

Fixes rust-lang#25842
bluss pushed a commit to bluss/rust that referenced this issue May 28, 2015
collections: Make BinaryHeap panic safe in sift_up / sift_down
5249cbb 
Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

Fixes rust-lang#25842
bors added a commit that referenced this issue May 28, 2015
@bors
Auto merge of #25856 - bluss:binary-heap-hole, r=Gankro
efebe45 
collections: Make BinaryHeap panic safe in sift_up / sift_down

Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

NOTE: The BinaryHeap will still be inconsistent after a comparison fails. It will
not have the heap property. What we fix is just that elements will be valid
values.

This is actually a performance win -- the new code does not bother to write in `zeroed()`
values in the holes, it just leaves them as they were.

Net result is something like a 5% decrease in runtime for `BinaryHeap::from_vec`. This
can be further improved by using unchecked indexing (I confirmed it makes a difference,
not a surprise with the non-sequential access going on), but let's leave that for another PR.
Safety first 😉 

Fixes #25842
XMPPwocky pushed a commit to XMPPwocky/rust that referenced this issue May 29, 2015
@XMPPwocky
collections: Make BinaryHeap panic safe in sift_up / sift_down
4b74286 
Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

Fixes rust-lang#25842
alexcrichton pushed a commit to alexcrichton/rust that referenced this issue Jun 10, 2015
@alexcrichton
collections: Make BinaryHeap panic safe in sift_up / sift_down
2cc6e8f 
Use a struct called Hole that keeps track of an invalid location
in the vector and fills the hole on drop.

I include a run-pass test that the current BinaryHeap fails, and the new
one passes.

Fixes rust-lang#25842
@Qwaz Qwaz mentioned this issue Jan 7, 2021
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collections: Make BinaryHeap panic safe in sift_up / sift_down bluss/rust
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