feat: comprehensive fuzzing for BTreeMap (#143)

Adds a fuzz test that can run indefinitely comparing the results of a
stable BTreeMap to a standard BTreeMap.

I ended up using `proptest` rather than the conventional `cargo fuzz`
for two reasons:

1. `cargo fuzz` requires nightly rust.
2. With proptest, we can have a short version of the fuzz test that runs
as part of CI (which is included in this commit).

src/btreemap.rs

@@ -1197,7 +1197,7 @@ mod test {
     use std::cell::RefCell;
     use std::rc::Rc;
 
-    fn make_memory() -> Rc<RefCell<Vec<u8>>> {
+    pub(crate) fn make_memory() -> Rc<RefCell<Vec<u8>>> {
         Rc::new(RefCell::new(Vec::new()))
     }
 

src/btreemap/proptests.rs

@@ -1,17 +1,92 @@
 use crate::{
-    btreemap::test::{b, btree_test},
+    btreemap::{
+        test::{b, btree_test, make_memory},
+        BTreeMap,
+    },
     storable::Blob,
+    Memory,
 };
 use proptest::collection::btree_set as pset;
 use proptest::collection::vec as pvec;
 use proptest::prelude::*;
-use std::collections::BTreeSet;
+use std::collections::{BTreeMap as StdBTreeMap, BTreeSet};
 use test_strategy::proptest;
 
+#[derive(Debug, Clone)]
+enum Operation {
+    Insert { key: Vec<u8>, value: Vec<u8> },
+    Iter { from: usize, len: usize },
+    Get(usize),
+    Remove(usize),
+}
+
+// A custom strategy that gives unequal weights to the different operations.
+// Note that `Insert` has a higher weight than `Remove` so that, on average, BTreeMaps
+// are growing in size the more operations are executed.
+fn operation_strategy() -> impl Strategy<Value = Operation> {
+    prop_oneof![
+        3 => (any::<Vec<u8>>(), any::<Vec<u8>>())
+            .prop_map(|(key, value)| Operation::Insert { key, value }),
+        1 => (any::<usize>(), any::<usize>())
+            .prop_map(|(from, len)| Operation::Iter { from, len }),
+        2 => (any::<usize>()).prop_map(Operation::Get),
+        1 => (any::<usize>()).prop_map(Operation::Remove),
+    ]
+}
+
 fn arb_blob() -> impl Strategy<Value = Blob<10>> {
     pvec(0..u8::MAX, 0..10).prop_map(|v| Blob::<10>::try_from(v.as_slice()).unwrap())
 }
 
+// Runs a comprehensive test for the major stable BTreeMap operations.
+// Results are validated against a standard BTreeMap.
+#[proptest(cases = 10)]
+fn comprehensive(#[strategy(pvec(operation_strategy(), 100..5_000))] ops: Vec<Operation>) {
+    let mem = make_memory();
+    let mut btree = BTreeMap::new(mem);
+    let mut std_btree = StdBTreeMap::new();
+
+    // Execute all the operations, validating that the stable btreemap behaves similarly to a std
+    // btreemap.
+    for op in ops.into_iter() {
+        execute_operation(&mut std_btree, &mut btree, op);
+    }
+}
+
+// A comprehensive fuzz test that runs until it's explicitly terminated. To run:
+//
+// ```
+// cargo t comprehensive_fuzz -- --ignored --nocapture 2> comprehensive_fuzz.log
+// ```
+//
+// comprehensive_fuzz.log contains all the operations to help triage a failure.
+#[test]
+#[ignore]
+fn comprehensive_fuzz() {
+    use proptest::strategy::ValueTree;
+    use proptest::test_runner::TestRunner;
+    let mut runner = TestRunner::default();
+
+    let mem = make_memory();
+    let mut btree = BTreeMap::new(mem);
+    let mut std_btree = StdBTreeMap::new();
+
+    let mut i = 0;
+
+    loop {
+        let operation = operation_strategy()
+            .new_tree(&mut runner)
+            .unwrap()
+            .current();
+        execute_operation(&mut std_btree, &mut btree, operation);
+        i += 1;
+        if i % 1000 == 0 {
+            println!("=== Step {i} ===");
+            println!("=== BTree Size: {}", btree.len());
+        }
+    }
+}
+
 #[proptest(cases = 10)]
 fn insert(#[strategy(pset(arb_blob(), 1000..10_000))] keys: BTreeSet<Blob<10>>) {
     btree_test(|mut btree| {
@@ -61,3 +136,65 @@ fn map_upper_bound_iter(#[strategy(pvec(0u64..u64::MAX -1 , 10..100))] keys: Vec
         Ok(())
     });
 }
+
+// Given an operation, executes it on the given stable btreemap and standard btreemap, verifying
+// that the result of the operation is equal in both btrees.
+fn execute_operation<M: Memory>(
+    std_btree: &mut StdBTreeMap<Vec<u8>, Vec<u8>>,
+    btree: &mut BTreeMap<Vec<u8>, Vec<u8>, M>,
+    op: Operation,
+) {
+    match op {
+        Operation::Insert { key, value } => {
+            let std_res = std_btree.insert(key.clone(), value.clone());
+
+            eprintln!("Insert({}, {})", hex::encode(&key), hex::encode(&value));
+            let res = btree.insert(key, value);
+            assert_eq!(std_res, res);
+        }
+        Operation::Iter { from, len } => {
+            assert_eq!(std_btree.len(), btree.len() as usize);
+            if std_btree.is_empty() {
+                return;
+            }
+
+            let from = from % std_btree.len();
+            let len = len % std_btree.len();
+
+            eprintln!("Iterate({}, {})", from, len);
+            let std_iter = std_btree.iter().skip(from).take(len);
+            let stable_iter = btree.iter().skip(from).take(len);
+            for ((k1, v1), (k2, v2)) in std_iter.zip(stable_iter) {
+                assert_eq!(k1, &k2);
+                assert_eq!(v1, &v2);
+            }
+        }
+        Operation::Get(idx) => {
+            assert_eq!(std_btree.len(), btree.len() as usize);
+            if std_btree.is_empty() {
+                return;
+            }
+            let idx = idx % std_btree.len();
+
+            if let Some((k, v)) = btree.iter().skip(idx).take(1).next() {
+                eprintln!("Get({})", hex::encode(&k));
+                assert_eq!(std_btree.get(&k), Some(&v));
+                assert_eq!(btree.get(&k), Some(v));
+            }
+        }
+        Operation::Remove(idx) => {
+            assert_eq!(std_btree.len(), btree.len() as usize);
+            if std_btree.is_empty() {
+                return;
+            }
+
+            let idx = idx % std_btree.len();
+
+            if let Some((k, v)) = btree.iter().skip(idx).take(1).next() {
+                eprintln!("Remove({})", hex::encode(&k));
+                assert_eq!(std_btree.remove(&k), Some(v.clone()));
+                assert_eq!(btree.remove(&k), Some(v));
+            }
+        }
+    };
+}