cache: Make SimpleCache#watches thread safe

[snowp]

SimpleCache#watches publishes a non-thread safe member of SimpleCache without synchronization which means that changes made on other threads are not guaranteed to be visible.

This change synchronizes access to the watches method and makes a defensive copy of the underlying data. To simplify this I changed the data structure from Map<Map<>> to Table, which let's me use ImmutableTable.copyOf to copy the data more easily.

It also changes usages of #watches() internal to the class to reference the field directly to avoid the cost of a copy.

Signed-off-by: Snow Pettersen snowp@squareup.com

This method is just used without synchronization in test right now, so it's probably not a big deal, but I figured it'd make sense to keep the class completely thread safe, even if just to prevent bugs in the future when someone makes this method public and to ensure that we don't get any concurrency issues in test.

cc @nicktrav

[codecov-io]

Codecov Report

Merging #31 into master will increase coverage by 0.63%. The diff coverage is 100%.

@@             Coverage Diff             @@
##             master     #31      +/-   ##
===========================================
+ Coverage     94.97%   95.6%   +0.63%     
+ Complexity       44      43       -1     
===========================================
  Files             7       7              
  Lines           179     182       +3     
  Branches         15      13       -2     
===========================================
+ Hits            170     174       +4     
  Misses            4       4              
+ Partials          5       4       -1

Impacted Files	Coverage Δ	Complexity Δ
.../io/envoyproxy/controlplane/cache/SimpleCache.java	96.82% <100%> (+1.82%)	16 <3> (-1)	:arrow_down:

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[joeyb]

@snowp - LGTM, just needs a rebase against master.

[snowp]

rebased