Fix deadlock if steady_clock::now() returns the same value twice (#132)

* Fix deadlock if steady_clock::now() returns the same value twice

`Span::isFinished()` returns true iff the span's duration is zero.
So, if `steady_clock::now()` returns the same value at span creation as
at `Finish()` (unlikely, but possible, especially in virtualized
environments), the following scenario can happen:

- `Span::FinishWithOptions` sets `_duration` to zero
- and then calls `reportSpan`, which calls `RemoteReporter::report`
- which acquires the reporter mutex and
- adds the span to the reporter's `_queue`

Later:
- `RemoteReporter::sweepQueue` acquires the reporter mutex
- `RemoteReporter::sweepQueue` makes a copy of the span
(https://github.com/jaegertracing/jaeger-client-cpp/blob/7e9a135b362c47f7e4d42e0693b05b91a70e6888/src/jaegertracing/reporters/RemoteReporter.cpp#L95)
- The span is serialized in `RemoteReporter::sendSpan`
- The span (the copy made above) needs to be destroyed because the block in `sweepQueue` ends.
- The span's destructor is called, which calls `FinishWithOptions` again
- ... which sees that `_duration` is zero, so `isFinished()` returns
false:
https://github.com/jaegertracing/jaeger-client-cpp/blob/7e9a135b362c47f7e4d42e0693b05b91a70e6888/src/jaegertracing/Span.cpp#L94
- so it tries to call `reportSpan` again
- which tries to acquire the reporter's mutex
- but we're holding the lock in `sweepQueue`, so
- deadlock.

Relevant stack trace:
```
(gdb) bt
(gdb) bt
 #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
 #1  0x00007fb09c722023 in __GI___pthread_mutex_lock (mutex=0x7fb088b1c1c0) at ../nptl/pthread_mutex_lock.c:78
 #2  0x00007fb08860987b in jaegertracing::reporters::RemoteReporter::report(jaegertracing::Span const&) () from /usr/local/lib/libjaegertracing_plugin.so
 #3  0x00007fb0885b6506 in jaegertracing::Span::FinishWithOptions(opentracing::v2::FinishSpanOptions const&) [clone .localalias.361] () from /usr/local/lib/libjaegertracing_plugin.so
 #4  0x00007fb08860849a in jaegertracing::Span::~Span() [clone .constprop.253] () from /usr/local/lib/libjaegertracing_plugin.so
 #5  0x00007fb08860a8fa in jaegertracing::reporters::RemoteReporter::sweepQueue() () from /usr/local/lib/libjaegertracing_plugin.so
 #6  0x00007fb0886f2f5f in execute_native_thread_routine () from /usr/local/lib/libjaegertracing_plugin.so
 #7  0x00007fb09c71f6db in start_thread (arg=0x7fb087574700) at pthread_create.c:463
 #8  0x00007fb0989ae88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
```

Fixed by enforcing a minimum span duration of 1 `SteadyClock` tick.

Signed-off-by: Tudor Bosman <[email protected]>

* fix typo

Signed-off-by: Tudor Bosman <[email protected]>

Author: tudor

src/jaegertracing/Span.cpp

@@ -96,6 +96,12 @@ void Span::FinishWithOptions(
             return;
         }
         _duration = finishTimeSteady - _startTimeSteady;
+        if (_duration <= SteadyClock::duration()) {
+            // Enforce minimum duration of 1 tick (1ns on Linux),
+            // so isFinished() returns true
+            _duration = SteadyClock::duration(1);
+        }
+
         tracer = _tracer;
 
         std::copy(finishSpanOptions.log_records.begin(),