Do checked_import sequentially

[IanButterworth]

The issue at the bottom was submitted to the julia discourse https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878

This PR locks checked_import to ensure that loading is sequential in threaded use cases. The cost of the lock when the module is already loaded is negligible.

checked_import execution times in a png-loading loop:

[ Info: Precompiling ImageIO [82e4d734-157c-48bb-816b-45c225c6df19]
  0.459511 seconds (7.11 k allocations: 546.457 KiB, 4.52% compilation time)
  0.000005 seconds (1 allocation: 16 bytes)
  0.000004 seconds (1 allocation: 16 bytes)
  0.000003 seconds (1 allocation: 16 bytes)

This may slow down parallel loops with heterogenous file types, but even in that case we can't be sure that the IO packages they're loading are themselves using independent deps, so perhaps better to be safe.

---

Issue from Discourse:

using Images, FileIO
Threads.@threads for path in readdir("img"; join=true)
  load(path)
end

signal (11): Segmentation fault
in expression starting at none:1
in expression starting at none:1
unknown function (ip: 0x7fbd79b0d298)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0d294)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0c8b7)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0d766)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0c8b7)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0d766)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0d9ec)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0dafc)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0ea0a)
unknown function (ip: 0x7fbd79b0d9ec)
unknown function (ip: 0x7fbd79b12347)
unknown function (ip: 0x7fbd79b0dafc)
jl_restore_incremental at /usr/bin/../lib/libjulia.so.1 (unknown line)
unknown function (ip: 0x7fbd79b0ea0a)
unknown function (ip: 0x7fbd79b12347)
jl_restore_incremental at /usr/bin/../lib/libjulia.so.1 (unknown line)
_include_from_serialized at ./loading.jl:681
_require_search_from_serialized at ./loading.jl:782
_require_search_from_serialized at ./loading.jl:782
Errors encountered while loading "/path/img/000811.png".
All errors:Errors encountered while loading "/path/img/000001.png".

===========================================Errors encountered while loading "/path/img/001081.png".

All errors:
===========================================
All errors:
===========================================
_require at ./loading.jl:1007
_require at ./loading.jl:1007
require at ./loading.jl:928
require at ./loading.jl:928
require at ./loading.jl:923
require at ./loading.jl:923
unknown function (ip: 0x7fbd79b1cac0)
unknown function (ip: 0x7fbd79b1cac0)
unknown function (ip: 0x7fbd79b1e16e)
unknown function (ip: 0x7fbd79b1e16e)
jl_toplevel_eval_in at /usr/bin/../lib/libjulia.so.1 (unknown line)
jl_toplevel_eval_in at /usr/bin/../lib/libjulia.so.1 (unknown line)
eval at ./boot.jl:331 [inlined]
topimport at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:13
eval at ./boot.jl:331 [inlined]
topimport at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:13
checked_import at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:30
checked_import at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:30
#load#28 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:195
#load#28 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:195
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:184 [inlined]
#load#14 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
macro expansion at /path/open_images.jl:4 [inlined]
#3#threadsfor_fun at ./threadingconstructs.jl:81
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:184 [inlined]
#load#14 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
macro expansion at /path/open_images.jl:4 [inlined]
#3#threadsfor_fun at ./threadingconstructs.jl:81
#3#threadsfor_fun at ./threadingconstructs.jl:48
#3#threadsfor_fun at ./threadingconstructs.jl:48
unknown function (ip: 0x7fbd49e314ac)
unknown function (ip: 0x7fbd49e314ac)
unknown function (ip: 0x7fbd79b053b9)
unknown function (ip: 0x7fbd79b053b9)
unknown function (ip: (nil))
nknown function (ip: (nil))
Allocations: 11229080 (Pool: 11225888; Big: 3192); GC: 8
Allocations: 11229080 (Pool: 11225888; Big: 3192); GC: 8
Segmentation fault (core dumped)

[coveralls]

Coverage decreased (-0.2%) to 73.121% when pulling c292520 on ib/load_lock into 7c2c5a2 on master.

[IanButterworth]

The same fix is needed for ImageIO JuliaIO/ImageIO.jl#11

[codecov]

Codecov Report

Merging #276 into master will decrease coverage by 0.24%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
- Coverage   73.36%   73.12%   -0.25%     
==========================================
  Files           8        8              
  Lines         443      439       -4     
==========================================
- Hits          325      321       -4     
  Misses        118      118              

Impacted Files	Coverage Δ
src/loadsave.jl	86.66% <90.90%> (-0.68%)	⬇️

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[timholy]

Is there an easy test we can add?

Feel free to merge whenever, though.

[IanButterworth]

I just added a multithreaded test here, but without bringing an external IO dependency into the testing, I don't see an easy way for the module load segfault issue to be fully tested here.

This issue is however tested properly in ImageIO as of JuliaIO/ImageIO.jl#11

I think I'll merge once CI passes (travis seems to be having a bit of a hiccup)

[IanButterworth]

The original user reports that master (as well as ImageIO master) fixes this.

Should I release 1.4.4?

[timholy]

Yes please!