Implement Sampler protocol for SessionBasedSampler

[mattxw]

We are having some crashes that are being tracked on Crashlytics. Based on the stack trace, it appears that Splunk is attempting to change the sampler during session changes. I suspect that there are spans that are referencing the old sampler while Splunk is replacing them.

This pull request eliminates the constant need to replace samplers, as everything is now handled within the SessionBasedSampler class.

Crash stack trace:

Crashed: scheduler-thread
0  libsystem_kernel.dylib         0x7578 __pthread_kill + 8
1  libsystem_pthread.dylib        0x7118 pthread_kill + 268
2  libsystem_c.dylib              0x1d178 abort + 180
3  libswiftCore.dylib             0x3b8b98 swift::fatalError(unsigned int, char const*, ...) + 134
4  libswiftCore.dylib             0x3b8bb8 swift::warningv(unsigned int, char const*, char*) + 30
5  libswiftCore.dylib             0x3bd7c8 swift_deallocPartialClassInstance + 190
6  libswiftCore.dylib             0x3bd618 _swift_release_dealloc + 56
7  libswiftCore.dylib             0x3be43c bool swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::doDecrementSlow<(swift::PerformDeinit)1>(swift::RefCountBitsT<(swift::RefCountInlinedness)1>, unsigned int) + 132
8  ******                      0x14ee5b4 specialized static SessionBasedSampler.sessionShouldSample() + 75 (TracerSharedState.swift:75)
9  ******                      0x14ea8e0 getRumSessionId(forceNewSessionId:) + 64 (Session.swift:64)
10 ******                      0x152c3bc GlobalAttributesProcessor.onStart(parentContext:span:) + 68 (InstrumentationUtils.swift:68)
11 ******                      0x152c2e8 protocol witness for SpanProcessor.onStart(parentContext:span:) in conformance GlobalAttributesProcessor + 15150240 (<compiler-generated>:15150240)
12 ******                      0x1500ee8 MultiSpanProcessor.onStart(parentContext:span:) + 37 (MultiSpanProcessor.swift:37)
13 ******                      0x15229a8 specialized static RecordEventsReadableSpan.startSpan(context:name:instrumentationScopeInfo:kind:parentContext:hasRemoteParent:spanLimits:spanProcessor:clock:resource:attributes:links:totalRecordedLinks:startTime:) + 189 (RecordEventsReadableSpan.swift:189)
14 ******                      0x1521e54 SpanBuilderSdk.startSpan() + 152 (SpanBuilderSdk.swift:152)
15 ******                      0x26e4508 startHttpSpan(request:) + 91 (NetworkInstrumentation.swift:91)
16 ******                      0x26e3754 NSURLSessionTask.splunk_swizzled_resume() + 156 (NetworkInstrumentation.swift:156)
17 ******                      0x26e3528 @objc NSURLSessionTask.splunk_swizzled_resume() + 33726176 (<compiler-generated>:33726176)
// ... private API call
29 libdispatch.dylib              0x2320 _dispatch_call_block_and_release + 32
30 libdispatch.dylib              0x3eac _dispatch_client_callout + 20
31 libdispatch.dylib              0xb534 _dispatch_lane_serial_drain + 668
32 libdispatch.dylib              0xc0a4 _dispatch_lane_invoke + 384
33 libdispatch.dylib              0x16cdc _dispatch_workloop_worker_thread + 648
34 libsystem_pthread.dylib        0xddc _pthread_wqthread + 288
35 libsystem_pthread.dylib        0xb7c start_wqthread + 8

[johnbley]

Thanks so much for this! It looks really good and cleaner than the original. @seemk are there special rules to follow for this outside contribution?

[seemk]

Thanks so much for this! It looks really good and cleaner than the original. @seemk are there special rules to follow for this outside contribution?

I think there's only https://www.splunk.com/en_us/form/contributions.html linked in our CONTRIBUTING.md

[mattxw]

Thanks so much for this! It looks really good and cleaner than the original. @seemk are there special rules to follow for this outside contribution?

I think there's only https://www.splunk.com/en_us/form/contributions.html linked in our CONTRIBUTING.md

Thanks for the information, I just submitted the form for the Contributions to Splunk. Please kindly recheck again later.