Use ring buffer to store breadcrumbs

[fractalwrench]

Goal

Breadcrumbs are stored in a ConcurrentLinkedQueue and then trimmed as they are added. This was identified as an area where performance could potentially be improved as over an application's lifecycle thousands of breadcrumbs might be added to Bugsnag.

Changeset

BreadcrumbState has been altered to use a ring buffer instead of a ConcurrentLinkedQueue. Every time a breadcrumb is added an index is incremented by one, and the breadcrumb is stored in a backing array. If the number of breadcrumbs logged is greater than config.maxBreadcrumbs then the implementation wraps around to zero and overwrites the oldest breadcrumb added.

When an error is captured breadcrumbs must be added to an Event in the order in which they were captured. Previously this was a case of transforming a Queue into a List but using a ring buffer means that the breadcrumbs can be out of order. The solution chosen here is to create two sublists based on the position of the most recent breadcrumb and then combine them together.

Performance improvements

Adding 1000 breadcrumbs to the store took ~9ms with these changes, and ~56ms without, a difference of ~45ms.

The BreadcrumbState#copy() implementation has added on ~0.5ms to the Bugsnag.notify() call.

Tradeoffs of this changeset

There are a couple of tradeoffs made with this changeset:

• BreadcrumbState#copy() does not sort elements or use synchronisation, so in rare cases this could lead to some breadcrumbs being ordered incorrectly. Sorting the collection was profiled as being expensive, but this is something that could potentially be added in if needed.
• This increases the time taken to execute Bugsnag.notify(), by the equivalent of adding ~11 breadcrumbs. This feels like a reasonable trade off as breadcrumbs should typically be added much more frequently than Bugsnag.notify() is called.

Testing

Updated existing test coverage to match new method signatures, and added additional unit test cases for the ring buffer wrapping round. Additionally added a benchmark test for copying breadcrumbs (which happens whenever an event is generated).

[lemnik]

LGTM

[fractalwrench]

@kstenerud would you mind giving a secondary review for this changeset, as it has altered the core breadcrumb functionality?