Implement hamt fuzzer (#872)

* Implement hamt fuzzer

* Change author

ipld/hamt/fuzz/.gitignore

@@ -0,0 +1,5 @@
+
+target
+corpus
+artifacts
+Cargo.lock

ipld/hamt/fuzz/Cargo.toml

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+
+[package]
+name = "ipld_hamt-fuzz"
+version = "0.0.0"
+authors = ["ChainSafe Systems <info@chainsafe.io>"]
+publish = false
+edition = "2018"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.3"
+arbitrary = { version = "0.4", features = ["derive"] }
+ahash = "0.5"
+db = { path = "../../../node/db" }
+
+[dependencies.ipld_hamt]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+name = "hamt_fuzz"
+path = "fuzz_targets/hamt_fuzz.rs"
+test = false
+doc = false

ipld/hamt/fuzz/fuzz_targets/hamt_fuzz.rs

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+#![no_main]
+use arbitrary::Arbitrary;
+use ipld_hamt::Hamt;
+use libfuzzer_sys::fuzz_target;
+
+#[derive(Debug, Arbitrary)]
+struct Operation {
+    key: u64,
+    method: Method,
+}
+
+#[derive(Debug, Arbitrary)]
+enum Method {
+    Insert(u64),
+    Remove,
+    Get,
+}
+
+fuzz_target!(|data: (u8, Vec<Operation>)| {
+    let (flush_rate, operations) = data;
+    let db = db::MemoryDB::default();
+    let mut hamt = Hamt::<_, _, _>::new_with_bit_width(&db, 5);
+    let mut elements = ahash::AHashMap::new();
+
+    let flush_rate = (flush_rate as usize).saturating_add(5);
+    for (i, Operation { key, method }) in operations.into_iter().enumerate() {
+        if i % flush_rate == 0 {
+            // Periodic flushing of Hamt to fuzz blockstore usage also
+            hamt.flush().unwrap();
+        }
+
+        match method {
+            Method::Insert(v) => {
+                elements.insert(key, v);
+                hamt.set(key, v).unwrap();
+            }
+            Method::Remove => {
+                let el = elements.remove(&key);
+                let hamt_deleted = hamt.delete(&key).unwrap().map(|(_, v)| v);
+                assert_eq!(hamt_deleted, el);
+            }
+            Method::Get => {
+                let ev = elements.get(&key);
+                let av = hamt.get(&key).unwrap();
+                assert_eq!(av, ev);
+            }
+        }
+    }
+});