INTEGRATION_WeakPluginPtr test fails

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-plugin/issues/7)) by Juan Oxoby (Bitbucket: [Juan Oxoby](https://bitbucket.org/%7B7d3743fc-bfd8-4e48-a53b-9cc0a264f66d%7D/)).**

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# Prerequisites

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* Checked that there is not already an Ignition package that provides the described functionality: [https://ignitionrobotics.org/libs](https://ignitionrobotics.org/libs)

# Description

`INTEGRATION_WeakPluginPtr` is failing on my machine.

# Steps to Reproduce

1. Run `INTEGRATION_WeakPluginPtr`

**Expected behavior:**

PASSED

**Actual behavior:**

```
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from WeakPluginPtr
[ RUN ] WeakPluginPtr.Lifecycle
/home/juan/ignition_workspace/src/ign-plugin/test/integration/WeakPluginPtr.cc:59: Failure
Expected equality of these values:
nullptr
Which is: NULL
dlHandle
Which is: 0x55fdb805a330
[ FAILED ] WeakPluginPtr.Lifecycle (0 ms)
[ RUN ] WeakPluginPtr.CopyMove
[ OK ] WeakPluginPtr.CopyMove (0 ms)
[----------] 2 tests from WeakPluginPtr (0 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (0 ms total)
[ PASSED ] 1 test.
[ FAILED ] 1 test, listed below:
[ FAILED ] WeakPluginPtr.Lifecycle
```

**Reproduces how often:**

Always

# Versions

```
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic

$ gcc --version
gcc (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```

# Additional Information

The error points to the macro `CHECK_FOR_LIBRARY` defined on `integration/utils.hh`

There’s a comment there saying:

> /////////////////////////////////////////////////
> // Note \(MXG\): According to some online discussions, there is no guarantee
> // that a correct number of calls to dlclose\(void\*\) will actually unload the
> // shared library. In fact, there is no guarantee that a dynamically loaded
> // library from dlopen will ever be unloaded until the program is terminated.
> // This may cause dlopen\(~, RTLD\_NOLOAD\) to return a non-null handle even if
> // we are managing the handles correctly. If the test for
> // EXPECT\_EQ\(nullptr, dlHandle\) is found to fail occasionally, we should
> // consider removing it because it may be unreliable. At the very least, if
> // it fails very infrequently, then we can safely consider the failures to be
> // false negatives and may want to consider relaxing this test.

Avoiding the check `EXPECT_EQ(nullptr, dlHandle)` in the macro `CHECK_FOR_LIBRARY` makes the test pass.

Start with integration/utils.hh and the CHECK_FOR_LIBRARY macro, then run INTEGRATION_WeakPluginPtr to reproduce the failure in test/integration/WeakPluginPtr.cc. Review the EXPECT_EQ(nullptr, dlHandle) check against the documented dlopen/dlclose behavior; done means the test no longer reports this unreliable failure while retaining the intended lifecycle coverage.

Written by the indexing model from the issue text.