Add missing librt on linux via NATS_EXTRA_LIB var

[film42]

I was working on loading cnats via the ffi gem with jruby and noticed that I was having issues loading libnats because...

libnats.so: undefined symbol: clock_gettime.

This loads just fine under MRI.

I noticed we set the NATS_EXTRA_LIB var in the main CMakeLists.txt and that we use it for tests, but we we're not using it in src/CMakeLists.txt. Adding it back fixes the problem.

Here's a failing job in travis: https://travis-ci.org/abrandoned/protobuf-nats/jobs/218995507#L761

[film42]

Btw, here's the same jruby job pointed at my cnats branch: https://travis-ci.org/abrandoned/protobuf-nats/jobs/219008594

[kozlovic]

Thanks for your contribution!

@abrandoned actually added NATS_BUILD_WITHOUT_CLOCK_MONOTONIC in src/CMakefile.txt to avoid this problem I believe. To be fair, this whole thing need revisiting because I used defined CLOCK_MONOTONIC but then only use CLOCK_REALTIME.

If the real issue was that rt was missing, I am also questioning if adding it in building nats library is the right thing to do. Maybe it should be added to the application linking to libnats.so? (like I do for the test and examples).

Let me experiment a bit and get back to you.

PS: At any rate, not sure it is doing anything to add it to nats_static.

[film42]

Oh yeah, I totally forgot about NATS_BUILD_WITHOUT_CLOCK_MONOTONIC! And btw, you're right about nats_static.

[kozlovic]

Yes, but let me think more about it. I think that what @abrandoned did was actually making such that libnats would not use clock_gettime at all. I think that your fix would be better. Let me experiment and I will get back to you.

[kozlovic]

@film42 So I have verified that even if we add "rt" to nats target, the examples still need to be linked with "rt". Without adding "rt" in nats, you can see from Travis that the build of the library works fine.

So I wonder if adding "rt" to your build (without changing anything to libnats) would not be enough. That being said, it seems that it would not cause a problem adding it to libnats? And if so, maybe we could remove the CMake flag that we added to disable use of clock_gettime. Could you please experiment and see if:

• adding "rt" in your build is enough
• if not and you add it to libnats, remove NATS_BUILD_WITHOUT_CLOCK_MONOTONIC and see if @abrandoned issue is no longer relevant?

Thanks again!

[film42]

@kozlovic I found that I can set my own cmake global vars with master branch, so I might not need this PR after all:

cmake .. -DCMAKE_SHARED_LINKER_FLAGS="-lrt"
make VERBOSE=1

[ 37%] Linking C shared library libnats.so
...
/usr/bin/cc  -fPIC   -std=c99 -pedantic -pthread  ... -lrt ...

So this might be good enough for me since it seems to be specific to loading in the JVM.

@abrandoned are working on the same thing, so we might not end up using NATS_BUILD_WITHOUT_CLOCK_MONOTONIC. So we can remove this and add the librt to the shared object if you prefer. Might be better to do this?

EDIT: I'm running travis with cmake global vars to make sure this is correct.
EDIT 2: While this seems to be adding the linker flags, something is still wrong. Travis is failing.

[film42]

@kozlovic I ran travis with NATS_BUILD_WITHOUT_CLOCK_MONOTONIC and it passed, and ran with the fix in this branch and it worked, so I went ahead and removed NATS_BUILD_WITHOUT_CLOCK_MONOTONIC.

[kozlovic]

Thank you for your contribution!