Gurobi.free_env segfaults with low probability

[rdeits]

Running the following:

using Gurobi, JuMP

while true
    env = Gurobi.Env()
    m = Model(solver=GurobiSolver(env, OutputFlag=0))
    @variable m x
    @objective m Min x^2
    solve(m)
    Gurobi.free_env(env)
end

causes Julia to segfault after a few seconds with:

*** Error in `julia': corrupted double-linked list: 0x0000000003735d20 ***

gdb shows the following:

*** Error in `/usr/local/bin/julia': corrupted double-linked list: 0x0000000002f025f0 ***

Program received signal SIGABRT, Aborted.
0x00007ffff6d24c37 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56      ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  0x00007ffff6d24c37 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x00007ffff6d28028 in __GI_abort () at abort.c:89
#2  0x00007ffff6d612a4 in __libc_message (do_abort=do_abort@entry=1, 
    fmt=fmt@entry=0x7ffff6e6f6b0 "*** Error in `%s': %s: 0x%s ***\n") at ../sysdeps/posix/libc_fatal.c:175
#3  0x00007ffff6d6cb02 in malloc_printerr (ptr=0x2f025f0, str=0x7ffff6e6b7e4 "corrupted double-linked list", 
    action=1) at malloc.c:4996
#4  malloc_consolidate (av=av@entry=0x7ffff70ac760 <main_arena>) at malloc.c:4157
#5  0x00007ffff6d6d45d in _int_free (av=0x7ffff70ac760 <main_arena>, p=<optimized out>, have_lock=0)
    at malloc.c:4057
#6  0x00007fffcda0c1be in PRIVATE00000000000620d1 ()
   from /home/rdeits/Applications/gurobi750/linux64/lib/libgurobi75.so
#7  0x00007fffcda0addc in GRBfreemodel () from /home/rdeits/Applications/gurobi750/linux64/lib/libgurobi75.so

I'm running the following Julia version:

julia> versioninfo()
Julia Version 0.6.0
Commit 9036443 (2017-06-19 13:05 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-3920XM CPU @ 2.90GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Sandybridge)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, ivybridge)

and the following Gurobi version:

julia> Gurobi.version
v"7.5.0"

julia> Pkg.installed("Gurobi")
v"0.3.3+"

I'm not entirely sure that this is a Gurobi.jl issue, but I have noticed that just constructing and freeing environments seems to work fine. That is, the following does not crash:

while true
  env = Gurobi.Env()
  Gurobi.free_env(env)
end

[odow]

I can reproduce this on Windows.

However putting in short sleep seems to fix the problem (at least with sleep(0.1). I still crashed it with sleep(0.01)). Looks like Gurobi is trying to free the env before it has been released from the solve or something.

using Gurobi, JuMP

while true
    env = Gurobi.Env()
    m = Model(solver=GurobiSolver(env, OutputFlag=0))
    @variable m x
    @objective m Min x^2
    solve(m)
    sleep(0.1)
    Gurobi.free_env(env)
end

It just took a really long time.

[rdeits]

By the way, I'm still getting this from time to time. The only reliable solution has been to never ever free a Gurobi environment.

[mlubin]

Related: JuliaLang/julia#3067

[odow]

master now implements a finalizer for env. Env will only be freed if all of the models which use it have also been freed.

If you are using Gurobi.Optimizer, never use GRBfreeenv. Instead, use finalize(env).

The current suggested version of the original MWE is:

using Gurobi, JuMP

while true
    env = Gurobi.Env()
    model = Model(() -> Gurobi.Optimizer(env))
    set_silent(model)
    @variable(model, x)
    @objective(model, Min, x^2)
    optimize!(model)
    # To ensure `env` is finalized, you must also finalize any models using env.
    # If you use `Gurobi.Optimizer`, the order of finalizing `.model` and `env` 
    # doesn't matter. If you use the C API directly, `.model` _MUST_ be 
    # finalized first.
    finalize(backend(model).optimizer.model)
    finalize(env)
end