bidi streams have racy read caused by goroutine that closes over local variable

[jhump]

In the code generated for a bidi streaming endpoint, a closure named handleSend captures a variable named err from the enclosing function. This function is then executed in its own goroutine. That means there are racy reads and writes between that goroutine and the one that spawned it (which is also using err as a local variable in later statements). There is no attempt at synchronization of any kind between the two goroutines.

When I run do go test -race with a test that involves a bidi stream through grpc-gateway, it triggers the race detector which causes the test to always fail. And it's pretty consistent.

And the fix is easy: it appears to be an accidental closure -- a typo that uses = instead of :=. There is no need for the two goroutines to share this variable.

You can find a fix here: #575