tr: clean up TaprootSpendInfo locking logic

1. `RwLock` is typically much slower than `Mutex`, so the Rust developers
   discourage its use unless you really need lots of simultaneous readers.
   We do not, we only need to unlock this mutex for tiny amounts of time locking logic

2. The `clone` implementation missed an opportunity to clone an internal
   `Arc`, instead using `Mutex::new(None)` and forcing the clone to recompute
   the spend info.

3. Minor idiomatic cleanups.

src/descriptor/tr.rs

@@ -17,7 +17,7 @@ use expression::{self, FromTree};
 use miniscript::{limits::TAPROOT_MAX_NODE_COUNT, Miniscript};
 use std::cmp::{self, max};
 use std::hash;
-use std::sync::{Arc, RwLock};
+use std::sync::{Arc, Mutex};
 use std::{fmt, str::FromStr};
 use Tap;
 use {Error, MiniscriptKey};
@@ -46,19 +46,21 @@ pub struct Tr<Pk: MiniscriptKey> {
     /// This will be [`None`] when the descriptor is not derived.
     /// This information will be cached automatically when it is required
     //
-    // It is also possible to wrap this in a further Arc<RwLock...> so that the cache
-    // is shared across clones of the this descriptor. But this (slightly) complicates
-    // the code, and it might be desirable for clone to keep separate caches
-    spend_info: RwLock<Option<Arc<TaprootSpendInfo>>>,
+    // The inner `Arc` here is because Rust does not allow us to return a reference
+    // to the contents of the `Option` from inside a `MutexGuard`. There is no outer
+    // `Arc` because when this structure is cloned, we create a whole new mutex.
+    spend_info: Mutex<Option<Arc<TaprootSpendInfo>>>,
 }
 
 impl<Pk: MiniscriptKey> Clone for Tr<Pk> {
     fn clone(&self) -> Self {
+        // When cloning, construct a new Mutex so that distinct clones don't
+        // cause blocking between each other. We clone only the internal `Arc`,
+        // so the clone is always cheap (in both time and space)
         Self {
             internal_key: self.internal_key.clone(),
             tree: self.tree.clone(),
-            // Cloning creates a new instance of RwLock
-            spend_info: RwLock::new(None),
+            spend_info: Mutex::new(self.spend_info.lock().expect("Lock poisoned").clone()),
         }
     }
 }
@@ -169,7 +171,7 @@ impl<Pk: MiniscriptKey> Tr<Pk> {
             Ok(Self {
                 internal_key,
                 tree,
-                spend_info: RwLock::new(None),
+                spend_info: Mutex::new(None),
             })
         } else {
             Err(Error::MaxRecursiveDepthExceeded)
@@ -212,64 +214,54 @@ impl<Pk: MiniscriptKey> Tr<Pk> {
     {
         // If the value is already cache, read it
         // read only panics if the lock is poisoned (meaning other thread having a lock panicked)
-        let spend_info = self
-            .spend_info
-            .read()
-            .expect("Lock poisoned")
-            .as_ref()
-            .map(Arc::clone);
-
-        match spend_info {
-            Some(spend_info) => spend_info,
-            None => {
-                // Get a new secp context
-                // This would be cheap operation after static context support from upstream
-                let secp = secp256k1::Secp256k1::verification_only();
-                // Key spend path with no merkle root
-                let data = if self.tree.is_none() {
-                    TaprootSpendInfo::new_key_spend(
-                        &secp,
-                        self.internal_key.to_x_only_pubkey(),
-                        None,
-                    )
-                } else {
-                    let mut builder = TaprootBuilder::new();
-                    for (depth, ms) in self.iter_scripts() {
-                        let script = ms.encode();
-                        builder = builder
-                            .add_leaf(depth, script)
-                            .expect("Computing spend data on a valid Tree should always succeed");
+        let read_lock = self.spend_info.lock().expect("Lock poisoned");
+        if let Some(ref spend_info) = *read_lock {
+            return spend_info.clone();
+        }
+        drop(read_lock);
+
+        // Get a new secp context
+        // This would be cheap operation after static context support from upstream
+        let secp = secp256k1::Secp256k1::verification_only();
+        // Key spend path with no merkle root
+        let data = if self.tree.is_none() {
+            TaprootSpendInfo::new_key_spend(&secp, self.internal_key.to_x_only_pubkey(), None)
+        } else {
+            let mut builder = TaprootBuilder::new();
+            for (depth, ms) in self.iter_scripts() {
+                let script = ms.encode();
+                builder = builder
+                    .add_leaf(depth, script)
+                    .expect("Computing spend data on a valid Tree should always succeed");
+            }
+            // Assert builder cannot error here because we have a well formed descriptor
+            match builder.finalize(&secp, self.internal_key.to_x_only_pubkey()) {
+                Ok(data) => data,
+                Err(e) => match e {
+                    TaprootBuilderError::InvalidMerkleTreeDepth(_) => {
+                        unreachable!("Depth checked in struct construction")
+                    }
+                    TaprootBuilderError::NodeNotInDfsOrder => {
+                        unreachable!("Insertion is called in DFS order")
+                    }
+                    TaprootBuilderError::OverCompleteTree => {
+                        unreachable!("Taptree is a well formed tree")
                     }
-                    // Assert builder cannot error here because we have a well formed descriptor
-                    match builder.finalize(&secp, self.internal_key.to_x_only_pubkey()) {
-                        Ok(data) => data,
-                        Err(e) => match e {
-                            TaprootBuilderError::InvalidMerkleTreeDepth(_) => {
-                                unreachable!("Depth checked in struct construction")
-                            }
-                            TaprootBuilderError::NodeNotInDfsOrder => {
-                                unreachable!("Insertion is called in DFS order")
-                            }
-                            TaprootBuilderError::OverCompleteTree => {
-                                unreachable!("Taptree is a well formed tree")
-                            }
-                            TaprootBuilderError::InvalidInternalKey(_) => {
-                                unreachable!("Internal key checked for validity")
-                            }
-                            TaprootBuilderError::IncompleteTree => {
-                                unreachable!("Taptree is a well formed tree")
-                            }
-                            TaprootBuilderError::EmptyTree => {
-                                unreachable!("Taptree is a well formed tree with atleast 1 element")
-                            }
-                        },
+                    TaprootBuilderError::InvalidInternalKey(_) => {
+                        unreachable!("Internal key checked for validity")
                     }
-                };
-                let spend_info = Arc::new(data);
-                *self.spend_info.write().expect("Lock poisoned") = Some(Arc::clone(&spend_info));
-                spend_info
+                    TaprootBuilderError::IncompleteTree => {
+                        unreachable!("Taptree is a well formed tree")
+                    }
+                    TaprootBuilderError::EmptyTree => {
+                        unreachable!("Taptree is a well formed tree with atleast 1 element")
+                    }
+                },
             }
-        }
+        };
+        let spend_info = Arc::new(data);
+        *self.spend_info.lock().expect("Lock poisoned") = Some(spend_info.clone());
+        spend_info
     }
 }
 
@@ -387,7 +379,7 @@ where
                     Ok(Tr {
                         internal_key: expression::terminal(key, Pk::from_str)?,
                         tree: None,
-                        spend_info: RwLock::new(None),
+                        spend_info: Mutex::new(None),
                     })
                 }
                 2 => {
@@ -403,7 +395,7 @@ where
                     Ok(Tr {
                         internal_key: expression::terminal(key, Pk::from_str)?,
                         tree: Some(ret),
-                        spend_info: RwLock::new(None),
+                        spend_info: Mutex::new(None),
                     })
                 }
                 _ => {
@@ -668,7 +660,7 @@ impl<P: MiniscriptKey, Q: MiniscriptKey> TranslatePk<P, Q> for Tr<P> {
                 Some(tree) => Some(tree.translate_helper(&mut translatefpk, &mut translatefpkh)?),
                 None => None,
             },
-            spend_info: RwLock::new(None),
+            spend_info: Mutex::new(None),
         };
         Ok(translate_desc)
     }