* ethdb: remove Set
Set deadlocks immediately and isn't part of the Database interface.
* trie: add Err to Iterator
This is useful for testing because the underlying NodeIterator doesn't
need to be kept in a separate variable just to get the error.
* trie: add LeafKey to iterator, panic when not at leaf
LeafKey is useful for callers that can't interpret Path.
* trie: retry failed seek/peek in iterator Next
Instead of failing iteration irrecoverably, make it so Next retries the
pending seek or peek every time.
Smaller changes in this commit make this easier to test:
* The iterator previously returned from Next on encountering a hash
node. This caused it to visit the same path twice.
* Path returned nibbles with terminator symbol for valueNode attached
to fullNode, but removed it for valueNode attached to shortNode. Now
the terminator is always present. This makes Path unique to each node
and simplifies Leaf.
* trie: add Path to MissingNodeError
The light client trie iterator needs to know the path of the node that's
missing so it can retrieve a proof for it. NodeIterator.Path is not
sufficient because it is updated when the node is resolved and actually
visited by the iterator.
Also remove unused fields. They were added a long time ago before we
knew which fields would be needed for the light client.
The 'step' method is split into two parts, 'peek' and 'push'. peek
returns the next state but doesn't make it current.
The end of iteration was previously tracked by setting 'trie' to nil.
End of iteration is now tracked using the 'iteratorEnd' error, which is
slightly cleaner and requires less code.
Make it so each iterator has exactly one public constructor:
- NodeIterators can be created through a method.
- Iterators can be created through NewIterator on any NodeIterator.
This PR implements a differenceIterator, which allows iterating over trie nodes
that exist in one trie but not in another. This is a prerequisite for most GC
strategies, in order to find obsolete nodes.
