perf: calculate the main chain height more efficiently (#237)

The Bitcoin testnet's metrics endpoint ran out of cycles because it had
a large number of unstable blocks, and computing the main chain height
under those conditions was very expensive.

This commit makes that computation more efficient. Prior to this change,
the `get_metrics` benchmark took `163_059_419` cycles. Now it takes
`8_102_095`.

canister/src/state.rs

@@ -177,7 +177,8 @@ pub fn ingest_stable_blocks_into_utxoset(state: &mut State) -> bool {
 }
 
 pub fn main_chain_height(state: &State) -> Height {
-    unstable_blocks::get_main_chain(&state.unstable_blocks).len() as u32 + state.utxos.next_height()
+    unstable_blocks::get_main_chain_length(&state.unstable_blocks) as u32
+        + state.utxos.next_height()
         - 1
 }
 

canister/src/unstable_blocks.rs

@@ -228,6 +228,22 @@ pub fn get_main_chain(blocks: &UnstableBlocks) -> BlockChain {
     main_chain
 }
 
+/// Returns the length of the "main chain".
+/// See `get_main_chain` for what defines a main chain.
+pub fn get_main_chain_length(blocks: &UnstableBlocks) -> usize {
+    let blocks_by_height = blocks.blocks_with_depths_by_heights();
+
+    // Traverse the heights in reverse order. The highest height with a single block corresponds to
+    // the tip of the main chain.
+    for height in (0..blocks_by_height.len()).rev() {
+        if blocks_by_height[height].len() == 1 {
+            return height + 1;
+        }
+    }
+
+    unreachable!("There must be at least one height with exactly one block.");
+}
+
 pub fn get_blocks(blocks: &UnstableBlocks) -> Vec<&Block> {
     blocktree::blockchains(&blocks.tree)
         .into_iter()