sync: add more notes about Cond behavior

Cond is difficult to use correctly (I was just bitten by it in
a production app that I inherited). While several proposals have come
up to improve or remove sync.Cond, no action has so far been taken.

Update the documentation to discourage use of sync.Cond, and point
people in the direction of preferred alternatives. I believe this will
help encourage behavior we want (less use of sync.Cond and more use of
channels), while also paving the way for, potentially, removing Cond
in a future version of the language.

Thanks very much to Bryan Mills and Sean Liao for discussion and
recommendations.

Updates #20491.
Updates #21165.

Change-Id: Ib4d0631c79d4c4d0a30027255cd43bc47cddebd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/412237
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/sync/cond.go

@@ -22,6 +22,17 @@ import (
 // In the terminology of the Go memory model, Cond arranges that
 // a call to Broadcast or Signal “synchronizes before” any Wait call
 // that it unblocks.
+//
+// For many simple use cases, users will be better off using channels than a
+// Cond (Broadcast corresponds to closing a channel, and Signal corresponds to
+// sending on a channel).
+//
+// For more on replacements for sync.Cond, see [Roberto Clapis's series on
+// advanced concurrency patterns], as well as [Bryan Mills's talk on concurrency
+// patterns].
+//
+// [Roberto Clapis's series on advanced concurrency patterns]: https://blogtitle.github.io/categories/concurrency/
+// [Bryan Mills's talk on concurrency patterns]: https://drive.google.com/file/d/1nPdvhB0PutEJzdCq5ms6UI58dp50fcAN/view
 type Cond struct {
 	noCopy noCopy
 
@@ -64,6 +75,9 @@ func (c *Cond) Wait() {
 //
 // It is allowed but not required for the caller to hold c.L
 // during the call.
+//
+// Signal() does not affect goroutine scheduling priority; if other goroutines
+// are attempting to lock c.L, they may be awoken before a "waiting" goroutine.
 func (c *Cond) Signal() {
 	c.checker.check()
 	runtime_notifyListNotifyOne(&c.notify)