ssh: fix spinloop in mux SendRequest drain on closed channel
The drain loop added in 4e7a738 ("ssh: fix deadlock on unexpected
global responses") receives from m.globalResponses without checking
the comma-ok flag. Once mux.loop closes globalResponses on connection
teardown, the loop spins forever on the zero value, pinning a CPU
core for every concurrent SendRequest caller.
Use the comma-ok idiom to detect the closed channel and break out.
Change-Id: Icf0a7419fe5b206b7a4f70429fa9f25d0b5af731
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/783681
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Neal Patel <neal@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
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