Add support for fair locks

This adds support for a "fair" variant of the lock.  When there are
multiple entities within a single process that are blocked waiting
for the lock the fair lock will ensure that they acquire the lock
in FIFO order.

For now at least, when fair locks are in use we don't support
the "semaphores" argument.

If external locks are enabled, the inter-process ordering will be
determined by the underlying OS lock ordering and process scheduling.

Change-Id: I37577becff4978bf643c65fa9bc2d78d342ea35a

1 files changed, 15 insertions, 0 deletions

diff --git a/releasenotes/notes/add-option-for-fair-locks-b6d660e97683cec6.yaml b/releasenotes/notes/add-option-for-fair-locks-b6d660e97683cec6.yaml
new file mode 100644
index 0000000..ad1cd4b
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+++ b/releasenotes/notes/add-option-for-fair-locks-b6d660e97683cec6.yaml
@@ -0,0 +1,15 @@
+---
+prelude: >
+    This release includes optional support for fair locks.  When fair locks
+    are specified, blocking waiters will acquire the lock in the order that
+    they blocked.
+features:
+  - |
+    We now have optional support for ``fair`` locks.  When fair locks are
+    specified, blocking waiters will acquire the lock in the order that they
+    blocked.  This can be useful to ensure that existing blocked waiters do
+    not wait indefinitely in the face of large numbers of new attempts to
+    acquire the lock.  When specifying locks as both ``external`` and ``fair``,
+    the ordering *within* a given process will be fair, but the ordering
+    *between* processes will be determined by the behaviour of the underlying
+    OS.