heed: use read-txn-no-tls for Send read transactions

Cargo.lock

@@ -623,6 +623,7 @@ dependencies = [
  "serde",
  "tempfile",
  "thiserror",
+ "thread_local",
  "tokio",
  "tokio-util",
  "tower",

database/Cargo.toml

@@ -40,10 +40,14 @@ futures    = { workspace = true, optional = true }
 tokio        = { workspace = true, features = ["full"], optional = true }
 tokio-util   = { workspace = true, features = ["full"], optional = true }
 tower        = { workspace = true, features = ["full"], optional = true }
+thread_local = { workspace = true }
 rayon        = { workspace = true, optional = true }
 
 # Optional features.
-heed  = { version = "0.20.0-alpha.9", optional = true }
+# SAFETY: Do not remove the `read-txn-no-tls` feature.
+# Required for `heed`'s read transaction to be `Send`.
+# See <TODO: link_to_safety_commit_line> for more info.
+heed  = { version = "0.20.0-alpha.9", features = ["read-txn-no-tls"], optional = true }
 redb  = { version = "2.0.0", optional = true }
 serde = { workspace = true, optional = true }
 

database/src/backend/heed/database.rs

@@ -49,6 +49,17 @@ pub(super) struct HeedTableRw<'env, 'tx, T: Table> {
     pub(super) tx_rw: &'tx RefCell<heed::RwTxn<'env>>,
 }
 
+/// SAFETY: `cuprate_database`'s Cargo.toml enables a feature
+/// for `heed` that turns on the `MDB_NOTLS` flag for LMDB.
+///
+/// This makes read transactions `Send`, but only if that flag is enabled.
+///
+/// This is required as in `crate::service` we must put our transactions and
+/// tables inside `ThreadLocal`'s to use across multiple threads.
+///
+/// `ThreadLocal<T>` requires that `T: Send`.
+unsafe impl<T: Table> Send for HeedTableRo<'_, T> {}
+
 //---------------------------------------------------------------------------------------------------- Shared functions
 // FIXME: we cannot just deref `HeedTableRw -> HeedTableRo` and
 // call the functions since the database is held by value, so