ERL-690: #Ref ordering is broken due to signed cast

[OTP-Maintainer]

Original reporter: dotsimon.
Affected version: OTP-21.0
Fixed in versions: OTP-20.3.8.5, OTP-21.0.5, OTP-19.3.6.10
Component: erts
Migrated from: https://bugs.erlang.org/browse/ERL-690

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#Ref ordering is broken in all versions since at least R13B03 (the earliest in github). It does work in R12B03.

The comparison takes the difference of two Uint32 and casts it to Sint32 which is the wrong thing to do. For example, 2200000000 - 1 is positive, but sadly (Sint32)(22000000000-1) is negative.

The scary thing here is that it can make terms "invisible" in ets or Mnesia ram_copies or disc_copies tables.

I will submit a PR with test cases and a fix for this today.

{{    W2s = [610350147,899574699,2994196869,686384822,2397690439, 923302211],
    ExtRefBase = <<131,114,0,3,100,0,3,110,64,98,3>>,
    ExtRefs = [<<ExtRefBase/binary, 1:32, W2:32, 0:32>> || W2 <- W2s],
    Refs = [binary_to_term(Bin) || Bin <- ExtRefs],

    Ets = ets:new(refbug, [ordered_set]),
    ets:insert(Ets, [{Ref,Ref} || Ref <- Refs]),
    0 = length([R || R<-ets:tab2list(Ets), ets:lookup(Ets, element(1,R)) == []]),
    ok.
}}

[OTP-Maintainer]

dotsimon. said:

Can anyone explain to me why the preformatted markdown didn't work?

[OTP-Maintainer]

anders said:

The double-brace notation only works on a single line (I think). For a block you can use {{{noformat\}}} or {{{code\}}}.

https://jira.atlassian.com/secure/WikiRendererHelpAction.jspa?section=all

{code:erlang}
W2s = [610350147,899574699,2994196869,686384822,2397690439, 923302211],
ExtRefBase = <<131,114,0,3,100,0,3,110,64,98,3>>,
ExtRefs = [<<ExtRefBase/binary, 1:32, W2:32, 0:32>> || W2 <- W2s],
Refs = [binary_to_term(Bin) || Bin <- ExtRefs],

Ets = ets:new(refbug, [ordered_set]),
ets:insert(Ets, [{Ref,Ref} || Ref <- Refs]),
0 = length([R || R <- ets:tab2list(Ets), ets:lookup(Ets, element(1,R)) == []]),
ok.
{code}

[OTP-Maintainer]

rickard said:

Thanks for fix and test-cases! These have been merged into the {{maint}} branch now.

[OTP-Maintainer]

rickard said:

Patches for OTP 19, 20, and 21 including this fix are on the way out as well.