Add release notes for #1930

Author: carver

newsfragments/1929.bugfix.rst

@@ -0,0 +1,14 @@
+A number of fixes related to checkpoints and persisting old headers, especially handling
+when we try to persist headers that don't match the checkpoints.
+
+  - A new exception :class:`~eth.exceptions.CheckpointsMustBeCanonical` raised when persisting a
+    header that is not linked to a previously-saved checkpoint.
+    (note: we now explicitly save checkpoints)
+  - More broadly, any block persist that would cause the checkpoint to be decanonicalized will
+    raise the :class:`~eth.exceptions.CheckpointsMustBeCanonical`.
+  - Re-insert gaps in the chain when a checkpoint and (parent or child) header do not link
+  - De-canonicalize all children of orphans. (Previously, only decanonicalized headers with block
+    numbers that matched the new canonical headers)
+  - Added some new hypothesis tests to get more confidence that we covered most cases
+  - When filling a gap, if there's an existing child that is not a checkpoint and doesn't link to
+    the parent, then the parent block wins, and the child block is de-canonicalized (and gap added).