fix(sqlite): run sqlite3_reset() in StatementWorker

this avoids possible race conditions without using a mutex

sqlx-core/src/sqlite/connection/executor.rs

@@ -4,7 +4,7 @@ use crate::error::Error;
 use crate::executor::{Execute, Executor};
 use crate::logger::QueryLogger;
 use crate::sqlite::connection::describe::describe;
-use crate::sqlite::statement::{StatementHandle, VirtualStatement};
+use crate::sqlite::statement::{StatementHandle, StatementWorker, VirtualStatement};
 use crate::sqlite::{
     Sqlite, SqliteArguments, SqliteConnection, SqliteQueryResult, SqliteRow, SqliteStatement,
     SqliteTypeInfo,
@@ -16,7 +16,8 @@ use libsqlite3_sys::sqlite3_last_insert_rowid;
 use std::borrow::Cow;
 use std::sync::Arc;
 
-fn prepare<'a>(
+async fn prepare<'a>(
+    worker: &mut StatementWorker,
     statements: &'a mut StatementCache<VirtualStatement>,
     statement: &'a mut Option<VirtualStatement>,
     query: &str,
@@ -39,7 +40,7 @@ fn prepare<'a>(
     if exists {
         // as this statement has been executed before, we reset before continuing
         // this also causes any rows that are from the statement to be inflated
-        statement.reset();
+        statement.reset(worker).await?;
     }
 
     Ok(statement)
@@ -61,21 +62,25 @@ fn bind(
 
 /// A structure holding sqlite statement handle and resetting the
 /// statement when it is dropped.
-struct StatementResetter {
+struct StatementResetter<'a> {
     handle: Arc<StatementHandle>,
+    worker: &'a mut StatementWorker,
 }
 
-impl StatementResetter {
-    fn new(handle: &Arc<StatementHandle>) -> Self {
+impl<'a> StatementResetter<'a> {
+    fn new(worker: &'a mut StatementWorker, handle: &Arc<StatementHandle>) -> Self {
         Self {
+            worker,
             handle: Arc::clone(handle),
         }
     }
 }
 
-impl Drop for StatementResetter {
+impl Drop for StatementResetter<'_> {
     fn drop(&mut self) {
-        self.handle.reset();
+        // this method is designed to eagerly send the reset command
+        // so we don't need to await or spawn it
+        let _ = self.worker.reset(&self.handle);
     }
 }
 
@@ -105,7 +110,7 @@ impl<'c> Executor<'c> for &'c mut SqliteConnection {
             } = self;
 
             // prepare statement object (or checkout from cache)
-            let stmt = prepare(statements, statement, sql, persistent)?;
+            let stmt = prepare(worker, statements, statement, sql, persistent).await?;
 
             // keep track of how many arguments we have bound
             let mut num_arguments = 0;
@@ -115,7 +120,7 @@ impl<'c> Executor<'c> for &'c mut SqliteConnection {
                 // is dropped. `StatementResetter` will reliably reset the
                 // statement even if the stream returned from `fetch_many`
                 // is dropped early.
-                let _resetter = StatementResetter::new(stmt);
+                let resetter = StatementResetter::new(worker, stmt);
 
                 // bind values to the statement
                 num_arguments += bind(stmt, &arguments, num_arguments)?;
@@ -127,7 +132,7 @@ impl<'c> Executor<'c> for &'c mut SqliteConnection {
 
                     // invoke [sqlite3_step] on the dedicated worker thread
                     // this will move us forward one row or finish the statement
-                    let s = worker.step(stmt).await?;
+                    let s = resetter.worker.step(stmt).await?;
 
                     match s {
                         Either::Left(changes) => {
@@ -190,7 +195,7 @@ impl<'c> Executor<'c> for &'c mut SqliteConnection {
             } = self;
 
             // prepare statement object (or checkout from cache)
-            let virtual_stmt = prepare(statements, statement, sql, persistent)?;
+            let virtual_stmt = prepare(worker, statements, statement, sql, persistent).await?;
 
             // keep track of how many arguments we have bound
             let mut num_arguments = 0;
@@ -218,7 +223,7 @@ impl<'c> Executor<'c> for &'c mut SqliteConnection {
 
                         logger.increment_rows();
 
-                        virtual_stmt.reset();
+                        virtual_stmt.reset(worker).await?;
                         return Ok(Some(row));
                     }
                 }
@@ -240,11 +245,12 @@ impl<'c> Executor<'c> for &'c mut SqliteConnection {
                 handle: ref mut conn,
                 ref mut statements,
                 ref mut statement,
+                ref mut worker,
                 ..
             } = self;
 
             // prepare statement object (or checkout from cache)
-            let statement = prepare(statements, statement, sql, true)?;
+            let statement = prepare(worker, statements, statement, sql, true).await?;
 
             let mut parameters = 0;
             let mut columns = None;

sqlx-core/src/sqlite/statement/handle.rs

@@ -1,13 +1,11 @@
-use either::Either;
 use std::ffi::c_void;
 use std::ffi::CStr;
-use std::hint;
+
 use std::os::raw::{c_char, c_int};
 use std::ptr;
 use std::ptr::NonNull;
 use std::slice::from_raw_parts;
 use std::str::{from_utf8, from_utf8_unchecked};
-use std::sync::atomic::{AtomicU8, Ordering};
 
 use libsqlite3_sys::{
     sqlite3, sqlite3_bind_blob64, sqlite3_bind_double, sqlite3_bind_int, sqlite3_bind_int64,
@@ -27,7 +25,7 @@ use crate::sqlite::type_info::DataType;
 use crate::sqlite::{SqliteError, SqliteTypeInfo};
 
 #[derive(Debug)]
-pub(crate) struct StatementHandle(NonNull<sqlite3_stmt>, Lock);
+pub(crate) struct StatementHandle(NonNull<sqlite3_stmt>);
 
 // access to SQLite3 statement handles are safe to send and share between threads
 // as long as the `sqlite3_step` call is serialized.
@@ -37,7 +35,11 @@ unsafe impl Sync for StatementHandle {}
 
 impl StatementHandle {
     pub(super) fn new(ptr: NonNull<sqlite3_stmt>) -> Self {
-        Self(ptr, Lock::new())
+        Self(ptr)
+    }
+
+    pub(crate) fn as_ptr(&self) -> *mut sqlite3_stmt {
+        self.0.as_ptr()
     }
 
     #[inline]
@@ -288,41 +290,13 @@ impl StatementHandle {
         Ok(from_utf8(self.column_blob(index))?)
     }
 
-    pub(crate) fn step(&self) -> Result<Either<u64, ()>, Error> {
-        self.1.enter_step();
-
-        let status = unsafe { sqlite3_step(self.0.as_ptr()) };
-        let result = match status {
-            SQLITE_ROW => Ok(Either::Right(())),
-            SQLITE_DONE => Ok(Either::Left(self.changes())),
-            _ => Err(self.last_error().into()),
-        };
-
-        if self.1.exit_step() {
-            unsafe { sqlite3_reset(self.0.as_ptr()) };
-            self.1.exit_reset();
-        }
-
-        result
-    }
-
-    pub(crate) fn reset(&self) {
-        if !self.1.enter_reset() {
-            // reset or step already in progress
-            return;
-        }
-
-        unsafe { sqlite3_reset(self.0.as_ptr()) };
-
-        self.1.exit_reset();
-    }
-
     pub(crate) fn clear_bindings(&self) {
         unsafe { sqlite3_clear_bindings(self.0.as_ptr()) };
     }
 }
 impl Drop for StatementHandle {
     fn drop(&mut self) {
+        // SAFETY: we have exclusive access to the `StatementHandle` here
         unsafe {
             // https://sqlite.org/c3ref/finalize.html
             let status = sqlite3_finalize(self.0.as_ptr());
@@ -338,44 +312,3 @@ impl Drop for StatementHandle {
         }
     }
 }
-
-const RESET: u8 = 0b0000_0001;
-const STEP: u8 = 0b0000_0010;
-
-// Lock to synchronize calls to `step` and `reset`.
-#[derive(Debug)]
-struct Lock(AtomicU8);
-
-impl Lock {
-    fn new() -> Self {
-        Self(AtomicU8::new(0))
-    }
-
-    // If this returns `true` reset can be performed, otherwise reset must be delayed until the
-    // current step finishes and `exit_step` is called.
-    fn enter_reset(&self) -> bool {
-        self.0.fetch_or(RESET, Ordering::Acquire) == 0
-    }
-
-    fn exit_reset(&self) {
-        self.0.fetch_and(!RESET, Ordering::Release);
-    }
-
-    fn enter_step(&self) {
-        // NOTE: spin loop should be fine here as we are only waiting for a `reset` to finish which
-        // should be quick.
-        while self
-            .0
-            .compare_exchange(0, STEP, Ordering::Acquire, Ordering::Relaxed)
-            .is_err()
-        {
-            hint::spin_loop();
-        }
-    }
-
-    // If this returns `true` it means a previous attempt to reset was delayed and must be
-    // performed now (followed by `exit_reset`).
-    fn exit_step(&self) -> bool {
-        self.0.fetch_and(!STEP, Ordering::Release) & RESET != 0
-    }
-}

sqlx-core/src/sqlite/statement/virtual.rs

@@ -3,7 +3,7 @@
 use crate::error::Error;
 use crate::ext::ustr::UStr;
 use crate::sqlite::connection::ConnectionHandle;
-use crate::sqlite::statement::StatementHandle;
+use crate::sqlite::statement::{StatementHandle, StatementWorker};
 use crate::sqlite::{SqliteColumn, SqliteError, SqliteRow, SqliteValue};
 use crate::HashMap;
 use bytes::{Buf, Bytes};
@@ -176,7 +176,7 @@ impl VirtualStatement {
         )))
     }
 
-    pub(crate) fn reset(&mut self) {
+    pub(crate) async fn reset(&mut self, worker: &mut StatementWorker) -> Result<(), Error> {
         self.index = 0;
 
         for (i, handle) in self.handles.iter().enumerate() {
@@ -185,9 +185,11 @@ impl VirtualStatement {
             // Reset A Prepared Statement Object
             // https://www.sqlite.org/c3ref/reset.html
             // https://www.sqlite.org/c3ref/clear_bindings.html
-            handle.reset();
+            worker.reset(handle).await?;
             handle.clear_bindings();
         }
+
+        Ok(())
     }
 }
 

sqlx-core/src/sqlite/statement/worker.rs

@@ -6,6 +6,9 @@ use futures_channel::oneshot;
 use std::sync::{Arc, Weak};
 use std::thread;
 
+use libsqlite3_sys::{sqlite3_reset, sqlite3_step, SQLITE_DONE, SQLITE_ROW};
+use std::future::Future;
+
 // Each SQLite connection has a dedicated thread.
 
 // TODO: Tweak this so that we can use a thread pool per pool of SQLite3 connections to reduce
@@ -21,6 +24,10 @@ enum StatementWorkerCommand {
         statement: Weak<StatementHandle>,
         tx: oneshot::Sender<Result<Either<u64, ()>, Error>>,
     },
+    Reset {
+        statement: Weak<StatementHandle>,
+        tx: oneshot::Sender<()>,
+    },
 }
 
 impl StatementWorker {
@@ -31,13 +38,37 @@ impl StatementWorker {
             for cmd in rx {
                 match cmd {
                     StatementWorkerCommand::Step { statement, tx } => {
-                        let resp = if let Some(statement) = statement.upgrade() {
-                            statement.step()
+                        let statement = if let Some(statement) = statement.upgrade() {
+                            statement
                         } else {
-                            // Statement is already finalized.
-                            Err(Error::WorkerCrashed)
+                            // statement is already finalized, the sender shouldn't be expecting a response
+                            continue;
+                        };
+
+                        // SAFETY: only the `StatementWorker` calls this function
+                        let status = unsafe { sqlite3_step(statement.as_ptr()) };
+                        let result = match status {
+                            SQLITE_ROW => Ok(Either::Right(())),
+                            SQLITE_DONE => Ok(Either::Left(statement.changes())),
+                            _ => Err(statement.last_error().into()),
                         };
-                        let _ = tx.send(resp);
+
+                        let _ = tx.send(result);
+                    }
+                    StatementWorkerCommand::Reset { statement, tx } => {
+                        if let Some(statement) = statement.upgrade() {
+                            // SAFETY: this must be the only place we call `sqlite3_reset`
+                            unsafe { sqlite3_reset(statement.as_ptr()) };
+
+                            // `sqlite3_reset()` always returns either `SQLITE_OK`
+                            // or the last error code for the statement,
+                            // which should have already been handled;
+                            // so it's assumed the return value is safe to ignore.
+                            //
+                            // https://www.sqlite.org/c3ref/reset.html
+
+                            let _ = tx.send(());
+                        }
                     }
                 }
             }
@@ -61,4 +92,34 @@ impl StatementWorker {
 
         rx.await.map_err(|_| Error::WorkerCrashed)?
     }
+
+    /// Send a command to the worker to execute `sqlite3_reset()` next.
+    ///
+    /// This method is written to execute the sending of the command eagerly so
+    /// you do not need to await the returned future unless you want to.
+    ///
+    /// The only error is `WorkerCrashed` as `sqlite3_reset()` returns the last error
+    /// in the statement execution which should have already been handled from `step()`.
+    pub(crate) fn reset(
+        &mut self,
+        statement: &Arc<StatementHandle>,
+    ) -> impl Future<Output = Result<(), Error>> {
+        // execute the sending eagerly so we don't need to spawn the future
+        let (tx, rx) = oneshot::channel();
+
+        let send_res = self
+            .tx
+            .send(StatementWorkerCommand::Reset {
+                statement: Arc::downgrade(statement),
+                tx,
+            })
+            .map_err(|_| Error::WorkerCrashed);
+
+        async move {
+            send_res?;
+
+            // wait for the response
+            rx.await.map_err(|_| Error::WorkerCrashed)
+        }
+    }
 }