chore(turbopack): Update dashmap from 5.x to 6.x (#72433)

Dashmap 6.x claims fairly large performance improvements:

> This release contains performance optimizations, most notably 10-40% gains on Apple Silicon but also 5-10% gains when measured in Intel Sapphire Rapids.

-- https://github.com/xacrimon/dashmap/releases/tag/v6.0.0

I don't think this means too much for us because while `turbo-tasks` uses dashmap pretty heavily, we spend more time actually doing computations than in `turbo-tasks`, so it's a fraction of a fraction.

Still, here's a fairly unscientific (noisy environment, inside a VM) benchmark on my macbook using a microbenchmark that's designed to hit `turbo-tasks` extremely hard (much more so than would happen in realistic scenarios):

```
$ TURBOPACK_BENCH_STRESS=yes cargo bench -p turbo-tasks-memory -- fibonacci/200 
   Compiling turbo-tasks-memory v0.1.0 (/home/bgw.linux/next.js/turbopack/crates/turbo-tasks-memory)
    Finished `bench` profile [optimized] target(s) in 2.80s
     Running benches/mod.rs (target/release/deps/mod-7b9a423ac1e10dce)
Gnuplot not found, using plotters backend
Benchmarking turbo_tasks_memory_stress/fibonacci/200: Warming up for 3.0000 s
Warning: Unable to complete 2000 samples in 5.0s. You may wish to increase target time to 91.9s, or reduce sample count to 100.
turbo_tasks_memory_stress/fibonacci/200
                        time:   [33.008 ms 33.108 ms 33.208 ms]
                        thrpt:  [605.31 Kelem/s 607.13 Kelem/s 608.97 Kelem/s]
                 change:
                        time:   [-2.0573% -1.6309% -1.2221%] (p = 0.00 < 0.05)
                        thrpt:  [+1.2373% +1.6580% +2.1005%]
                        Performance has improved.
Found 44 outliers among 2000 measurements (2.20%)
  2 (0.10%) low mild
  38 (1.90%) high mild
  4 (0.20%) high severe
```

Any potential improvement to realistic topline numbers are going to be <1% and thus too small to measure (especially since I don't have a low-noise way of measuring Apple Silicon where most of the benefits supposedly are, only x86-64).

Closes PACK-3411

Cargo.toml

@@ -136,7 +136,7 @@ console = "0.15.5"
 console-subscriber = "0.1.8"
 criterion = "0.5.1"
 crossbeam-channel = "0.5.8"
-dashmap = "5.4.0"
+dashmap = "6.1.0"
 dhat = { version = "0.3.2" }
 dialoguer = "0.10.3"
 dunce = "1.0.3"

turbopack/crates/turbo-tasks-backend/Cargo.toml

@@ -25,7 +25,7 @@ auto-hash-map = { workspace = true }
 byteorder = "1.5.0"
 dashmap = { workspace = true, features = ["raw-api"]}
 either = { workspace = true }
-hashbrown = { workspace = true }
+hashbrown = { workspace = true, features = ["raw"] }
 indexmap = { workspace = true }
 lmdb-rkv = "0.14.0"
 once_cell = { workspace = true }

turbopack/crates/turbo-tasks-backend/src/backend/storage.rs

@@ -359,7 +359,7 @@ where
     T: KeyValuePair,
     T::Key: Indexed,
 {
-    inner: RefMut<'a, K, InnerStorage<T>, BuildHasherDefault<FxHasher>>,
+    inner: RefMut<'a, K, InnerStorage<T>>,
 }
 
 impl<K, T> Deref for StorageWriteGuard<'_, K, T>

turbopack/crates/turbo-tasks-backend/src/utils/dash_map_multi.rs

@@ -5,29 +5,26 @@ use std::{
 };
 
 use dashmap::{DashMap, RwLockWriteGuard, SharedValue};
-use hashbrown::{hash_map, HashMap};
+use hashbrown::raw::{Bucket, RawTable};
 
-pub enum RefMut<'a, K, V, S>
-where
-    S: BuildHasher,
-{
-    Base(dashmap::mapref::one::RefMut<'a, K, V, S>),
+type RwLockWriteTableGuard<'a, K, V> = RwLockWriteGuard<'a, RawTable<(K, SharedValue<V>)>>;
+
+pub enum RefMut<'a, K, V> {
+    Base(dashmap::mapref::one::RefMut<'a, K, V>),
     Simple {
-        _guard: RwLockWriteGuard<'a, HashMap<K, SharedValue<V>, S>>,
-        key: *const K,
-        value: *mut V,
+        _guard: RwLockWriteTableGuard<'a, K, V>,
+        bucket: Bucket<(K, SharedValue<V>)>,
     },
     Shared {
-        _guard: Arc<RwLockWriteGuard<'a, HashMap<K, SharedValue<V>, S>>>,
-        key: *const K,
-        value: *mut V,
+        _guard: Arc<RwLockWriteTableGuard<'a, K, V>>,
+        bucket: Bucket<(K, SharedValue<V>)>,
     },
 }
 
-unsafe impl<K: Eq + Hash + Sync, V: Sync, S: BuildHasher> Send for RefMut<'_, K, V, S> {}
-unsafe impl<K: Eq + Hash + Sync, V: Sync, S: BuildHasher> Sync for RefMut<'_, K, V, S> {}
+unsafe impl<K: Eq + Hash + Sync, V: Sync> Send for RefMut<'_, K, V> {}
+unsafe impl<K: Eq + Hash + Sync, V: Sync> Sync for RefMut<'_, K, V> {}
 
-impl<K: Eq + Hash, V, S: BuildHasher> RefMut<'_, K, V, S> {
+impl<K: Eq + Hash, V> RefMut<'_, K, V> {
     pub fn value(&self) -> &V {
         self.pair().1
     }
@@ -39,94 +36,123 @@ impl<K: Eq + Hash, V, S: BuildHasher> RefMut<'_, K, V, S> {
     pub fn pair(&self) -> (&K, &V) {
         match self {
             RefMut::Base(r) => r.pair(),
-            &RefMut::Simple { key, value, .. } => unsafe { (&*key, &*value) },
-            &RefMut::Shared { key, value, .. } => unsafe { (&*key, &*value) },
+            RefMut::Simple { bucket, .. } | RefMut::Shared { bucket, .. } => {
+                // SAFETY:
+                // - The bucket is still valid, as we're holding a write guard on the shard
+                // - These bucket pointers are convertable to references
+                //
+                // https://doc.rust-lang.org/std/ptr/index.html#pointer-to-reference-conversion
+                let entry = unsafe { bucket.as_ref() };
+                (&entry.0, entry.1.get())
+            }
         }
     }
 
     pub fn pair_mut(&mut self) -> (&K, &mut V) {
         match self {
             RefMut::Base(r) => r.pair_mut(),
-            &mut RefMut::Simple { key, value, .. } => unsafe { (&*key, &mut *value) },
-            &mut RefMut::Shared { key, value, .. } => unsafe { (&*key, &mut *value) },
+            RefMut::Simple { bucket, .. } | RefMut::Shared { bucket, .. } => {
+                // SAFETY: Same as above in `pair`, plus aliasing is prevented via:
+                // 1. The lifetime of `&mut self`.
+                // 2. `Simple` values come from separate shards (no aliasing possible)
+                // 3. `Shared` values are asserted in `get_multiple_mut` to have unique pointers
+                let entry = unsafe { bucket.as_mut() };
+                (&entry.0, entry.1.get_mut())
+            }
         }
     }
 }
 
-impl<K: Eq + Hash, V, S: BuildHasher> Deref for RefMut<'_, K, V, S> {
+impl<K: Eq + Hash, V> Deref for RefMut<'_, K, V> {
     type Target = V;
 
     fn deref(&self) -> &V {
         self.value()
     }
 }
 
-impl<K: Eq + Hash, V, S: BuildHasher> DerefMut for RefMut<'_, K, V, S> {
+impl<K: Eq + Hash, V> DerefMut for RefMut<'_, K, V> {
     fn deref_mut(&mut self) -> &mut V {
         self.value_mut()
     }
 }
 
-impl<'a, K, V, S> From<dashmap::mapref::one::RefMut<'a, K, V, S>> for RefMut<'a, K, V, S>
+impl<'a, K, V> From<dashmap::mapref::one::RefMut<'a, K, V>> for RefMut<'a, K, V>
 where
     K: Hash + Eq,
-    S: BuildHasher,
 {
-    fn from(r: dashmap::mapref::one::RefMut<'a, K, V, S>) -> Self {
+    fn from(r: dashmap::mapref::one::RefMut<'a, K, V>) -> Self {
         RefMut::Base(r)
     }
 }
 
-pub fn get_multiple_mut<K, V, S>(
-    map: &DashMap<K, V, S>,
+pub fn get_multiple_mut<K, V>(
+    map: &DashMap<K, V, impl BuildHasher + Clone>,
     key1: K,
     key2: K,
     insert_with: impl Fn() -> V,
-) -> (RefMut<'_, K, V, S>, RefMut<'_, K, V, S>)
+) -> (RefMut<'_, K, V>, RefMut<'_, K, V>)
 where
     K: Hash + Eq + Clone,
-    S: BuildHasher + Clone,
 {
-    let s1 = map.determine_map(&key1);
-    let s2 = map.determine_map(&key2);
+    let hasher = map.hasher();
+    let hash_entry = |entry: &(K, _)| hasher.hash_one(&entry.0);
+    let h1 = hasher.hash_one(&key1);
+    let h2 = hasher.hash_one(&key2);
+
+    // Use `determine_shard` instead of `determine_map` to avoid extra rehashing.
+    // This u64 -> usize conversion also happens internally within DashMap using `as usize`.
+    // See: `DashMap::hash_usize`
+    let s1 = map.determine_shard(h1 as usize);
+    let s2 = map.determine_shard(h2 as usize);
+
+    let eq1 = |other: &(K, _)| key1.eq(&other.0);
+    let eq2 = |other: &(K, _)| key2.eq(&other.0);
+
     let shards = map.shards();
     if s1 == s2 {
         let mut guard = shards[s1].write();
-        let e1 = guard
-            .raw_entry_mut()
-            .from_key(&key1)
-            .or_insert_with(|| (key1.clone(), SharedValue::new(insert_with())));
-        let mut key1_ptr = e1.0 as *const K;
-        let mut value1_ptr = e1.1.get_mut() as *mut V;
-        let key2_ptr;
-        let value2_ptr;
-        match guard.raw_entry_mut().from_key(&key2) {
-            hash_map::RawEntryMut::Occupied(e) => {
-                let e2 = e.into_key_value();
-                key2_ptr = e2.0 as *const K;
-                value2_ptr = e2.1.get_mut() as *mut V;
-            }
-            hash_map::RawEntryMut::Vacant(e) => {
-                let e2 = e.insert(key2.clone(), SharedValue::new(insert_with()));
-                key2_ptr = e2.0 as *const K;
-                value2_ptr = e2.1.get_mut() as *mut V;
-                // inserting a new entry might invalidate the pointers of the first entry
-                let e1 = guard.get_key_value_mut(&key1).unwrap();
-                key1_ptr = e1.0 as *const K;
-                value1_ptr = e1.1.get_mut() as *mut V;
-            }
-        }
+
+        // we need to call `find_or_find_insert_slot` to avoid overwriting existing entries, but we
+        // can't use the returned bucket until after we get `bucket2` (below)
+        let _ = guard
+            .find_or_find_insert_slot(h1, eq1, hash_entry)
+            .unwrap_or_else(|slot| unsafe {
+                // SAFETY: This slot was previously returned by `find_or_find_insert_slot`, and no
+                // mutation of the table has occured since that call.
+                guard.insert_in_slot(h1, slot, (key1.clone(), SharedValue::new(insert_with())))
+            });
+
+        let bucket2 = guard
+            .find_or_find_insert_slot(h2, eq2, hash_entry)
+            .unwrap_or_else(|slot| unsafe {
+                // SAFETY: See previous call above
+                guard.insert_in_slot(h2, slot, (key2.clone(), SharedValue::new(insert_with())))
+            });
+
+        // Getting `bucket2` might invalidate the bucket pointer of the first entry, *even if no
+        // insert happens* as `RawTable::find_or_find_insert_slot` will *sometimes* resize the
+        // table, as it unconditionally reserves space for a potential insertion.
+        let bucket1 = guard.find(h1, eq1).expect(
+            "failed to find bucket of previously inserted item, is the hash or eq implementation \
+             incorrect?",
+        );
+
+        // this assertion is needed for memory safety reasons
+        assert!(
+            !std::ptr::eq(bucket1.as_ptr(), bucket2.as_ptr()),
+            "`get_multiple_mut` was called with equal keys, which breaks mutable referencing rules"
+        );
+
         let guard = Arc::new(guard);
         (
             RefMut::Shared {
                 _guard: guard.clone(),
-                key: key1_ptr,
-                value: value1_ptr,
+                bucket: bucket1,
             },
             RefMut::Shared {
                 _guard: guard,
-                key: key2_ptr,
-                value: value2_ptr,
+                bucket: bucket2,
             },
         )
     } else {
@@ -144,28 +170,28 @@ where
                 }
             }
         };
-        let e1 = guard1
-            .raw_entry_mut()
-            .from_key(&key1)
-            .or_insert_with(|| (key1, SharedValue::new(insert_with())));
-        let key1 = e1.0 as *const K;
-        let value1 = e1.1.get_mut() as *mut V;
-        let e2 = guard2
-            .raw_entry_mut()
-            .from_key(&key2)
-            .or_insert_with(|| (key2, SharedValue::new(insert_with())));
-        let key2 = e2.0 as *const K;
-        let value2 = e2.1.get_mut() as *mut V;
+
+        let bucket1 = guard1
+            .find_or_find_insert_slot(h1, eq1, hash_entry)
+            .unwrap_or_else(|slot| unsafe {
+                // SAFETY: See first insert_in_slot call
+                guard1.insert_in_slot(h1, slot, (key1.clone(), SharedValue::new(insert_with())))
+            });
+        let bucket2 = guard2
+            .find_or_find_insert_slot(h2, eq2, hash_entry)
+            .unwrap_or_else(|slot| unsafe {
+                // SAFETY: See first insert_in_slot call
+                guard2.insert_in_slot(h2, slot, (key2.clone(), SharedValue::new(insert_with())))
+            });
+
         (
             RefMut::Simple {
                 _guard: guard1,
-                key: key1,
-                value: value1,
+                bucket: bucket1,
             },
             RefMut::Simple {
                 _guard: guard2,
-                key: key2,
-                value: value2,
+                bucket: bucket2,
             },
         )
     }