We used a WaitGroup to prevent a call to Peers.End from melting
snowflakes while a new one is being collected. However, calls to
WaitGroup.Add are in a race with WaitGroup.Wait. To fix this, we use a
Mutex instead.
The race condition occurs because concurrent goroutines are intermixing
reads and writes of `WebRTCPeer.closed`.
Spotted when integrating Snowflake inside OONI in
https://github.com/ooni/probe-cli/pull/373.
This fixes a race condition in which snowflakes.End() is called while
snowflakes.Collect() is in progress resulting in a write to a closed
channel. We now wait for all in-progress collections to finish and add
an extra check before proceeding with a collection.
The other interfaces in client/lib/interfaces.go exist for the purpose
of running tests, but not Snowflake. Existing code would not have worked
with other types anyway, because it does unchecked .(*WebRTCPeer)
conversions.
- Error strings are no longer capitalized nor end with punctuation
- Alias import
- Remove extraneous initilisation code (No need to provide zero value
for variables, because the compiler does that anyway)
