Add a method of interrupting a query executing on a separate thread, fixes #407

src/lib.rs

@@ -79,7 +79,7 @@ use std::ptr;
 use std::result;
 use std::str;
 use std::sync::atomic::{AtomicBool, Ordering, ATOMIC_BOOL_INIT};
-use std::sync::{Once, ONCE_INIT};
+use std::sync::{Once, ONCE_INIT, Arc, Mutex};
 
 use cache::StatementCache;
 use error::{error_from_handle, error_from_sqlite_code};
@@ -598,6 +598,12 @@ impl Connection {
         self.db.borrow().db()
     }
 
+    /// Get access to a handle that can be used to interrupt long running queries
+    /// from another thread.
+    pub fn get_interrupt_handle(&self) -> InterruptHandle {
+        self.db.borrow().get_interrupt_handle()
+    }
+
     fn decode_result(&self, code: c_int) -> Result<()> {
         self.db.borrow_mut().decode_result(code)
     }
@@ -629,6 +635,13 @@ impl fmt::Debug for Connection {
 
 struct InnerConnection {
     db: *mut ffi::sqlite3,
+    // It's unsafe to call `sqlite3_close` while another thread is performing
+    // a `sqlite3_interrupt`, and vice versa, so we take this mutex during
+    // those functions. This protects a copy of the `db` pointer (which is
+    // cleared on closing), however the main copy, `db`, is unprotected.
+    // Otherwise, a long running query would prevent calling interrupt, as
+    // interrupt would only acquire the lock after the query's completion.
+    interrupt_lock: Arc<Mutex<*mut ffi::sqlite3>>,
     #[cfg(feature = "hooks")]
     free_commit_hook: Option<fn(*mut ::std::os::raw::c_void)>,
     #[cfg(feature = "hooks")]
@@ -810,13 +823,17 @@ To fix this, either:
 impl InnerConnection {
     #[cfg(not(feature = "hooks"))]
     fn new(db: *mut ffi::sqlite3) -> InnerConnection {
-        InnerConnection { db }
+        InnerConnection {
+            db,
+            interrupt_lock: Arc::new(Mutex::new(db)),
+        }
     }
 
     #[cfg(feature = "hooks")]
     fn new(db: *mut ffi::sqlite3) -> InnerConnection {
         InnerConnection {
-            db,
+            db: db,
+            interrupt_lock: Arc::new(Mutex::new(db)),
             free_commit_hook: None,
             free_rollback_hook: None,
             free_update_hook: None,
@@ -875,10 +892,14 @@ impl InnerConnection {
     }
 
     fn decode_result(&mut self, code: c_int) -> Result<()> {
+        InnerConnection::decode_result_raw(self.db(), code)
+    }
+
+    fn decode_result_raw(db: *mut ffi::sqlite3, code: c_int) -> Result<()> {
         if code == ffi::SQLITE_OK {
             Ok(())
         } else {
-            Err(error_from_handle(self.db(), code))
+            Err(error_from_handle(db, code))
         }
     }
 
@@ -887,16 +908,28 @@ impl InnerConnection {
             return Ok(());
         }
         self.remove_hooks();
+        let mut shared_handle = self.interrupt_lock.lock().unwrap();
+        assert!(!shared_handle.is_null(),
+            "Bug: Somehow interrupt_lock was cleared before the DB was closed");
         unsafe {
-            let r = ffi::sqlite3_close(self.db());
+            let r = ffi::sqlite3_close(self.db);
-            let r = self.decode_result(r);
+            // Need to use _raw because _guard has a reference out, and
+            // decode_result takes &mut self.
+            let r = InnerConnection::decode_result_raw(self.db, r);
             if r.is_ok() {
+                *shared_handle = ptr::null_mut();
                 self.db = ptr::null_mut();
             }
             r
         }
     }
 
+    fn get_interrupt_handle(&self) -> InterruptHandle {
+        InterruptHandle {
+            db_lock: Arc::clone(&self.interrupt_lock),
+        }
+    }
+
     fn execute_batch(&mut self, sql: &str) -> Result<()> {
         let c_sql = try!(str_to_cstring(sql));
         unsafe {
@@ -1015,6 +1048,25 @@ impl InnerConnection {
     fn remove_hooks(&mut self) {}
 }
 
+/// Allows interrupting a long-running computation.
+pub struct InterruptHandle {
+    db_lock: Arc<Mutex<*mut ffi::sqlite3>>,
+}
+
+unsafe impl Send for InterruptHandle {}
+unsafe impl Sync for InterruptHandle {}
+
+impl InterruptHandle {
+    /// Interrupt the query currently executing on another thread. This will
+    /// cause that query to fail with a `SQLITE3_INTERRUPT` error.
+    pub fn interrupt(&self) {
+        let db_handle = self.db_lock.lock().unwrap();
+        if !db_handle.is_null() {
+            unsafe { ffi::sqlite3_interrupt(*db_handle) }
+        }
+    }
+}
+
 impl Drop for InnerConnection {
     #[allow(unused_must_use)]
     fn drop(&mut self) {
@@ -1057,6 +1109,12 @@ mod test {
     #[allow(dead_code, unconditional_recursion)]
     fn ensure_send<T: Send>() {
         ensure_send::<Connection>();
+        ensure_send::<InterruptHandle>();
+    }
+
+    #[allow(dead_code, unconditional_recursion)]
+    fn ensure_sync<T: Sync>() {
+        ensure_sync::<InterruptHandle>();
     }
 
     pub fn checked_memory_handle() -> Connection {
@@ -1472,6 +1530,85 @@ mod test {
         assert!(version().contains(&format!("{}.{}.{}", major, minor, patch)));
     }
 
+    #[test]
+    fn test_interrupt() {
+        use std::thread;
+        use std::time::Duration;
+        let tries = 15;
+        // Sadly, this is inherently finnicky. Even if sqlite gets the
+        // interrupt, it isn't guaranteed to stop. In practice, 15 tends to be
+        // more than enough so long as we move everything we can outside of
+        // the thread body, but it still is unfortunate.
+        for i in 0..tries {
+            let db = checked_memory_handle();
+            db.execute_batch("CREATE TABLE dummy(id)").unwrap();
+            let interrupt_handle = db.get_interrupt_handle();
+            // generate an arbitrary query which will be very slow to execute.
+            let sql = format!("{};",
+                (0..100_000).into_iter()
+                    .map(|i| format!("INSERT INTO dummy(id) VALUES({})", i))
+                    .collect::<Vec<_>>()
+                    .join(";\n"));
+
+            // Do this on the main thread to minimize the amount of time spent
+            // when interrupt won't do anything (because we haven't started
+            // executing the query).
+            let c_sql = str_to_cstring(&sql).unwrap();
+
+            let joiner = thread::spawn(move || {
+                unsafe {
+                    let raw_db = db.db.borrow().db;
+                    let r = ffi::sqlite3_exec(
+                        raw_db,
+                        c_sql.as_ptr(),
+                        None,
+                        ptr::null_mut(),
+                        ptr::null_mut(),
+                    );
+                    db.decode_result(r)
+                }
+            });
+
+            // Try a few times to make sure we don't catch it too early.
+            interrupt_handle.interrupt();
+            for &delay in &[10, 100, 1000] {
+                thread::sleep(Duration::from_millis(delay));
+                interrupt_handle.interrupt();
+            }
+            let result = joiner.join().unwrap();
+
+            if i != tries - 1 && !result.is_err() {
+                continue;
+            }
+
+            match result.unwrap_err() {
+                Error::SqliteFailure(err, _) => {
+                    assert_eq!(err.code, ErrorCode::OperationInterrupted);
+                    return;
+                }
+                err => {
+                    panic!("Unexpected error {}", err);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn test_interrupt_close() {
+        let db = checked_memory_handle();
+        let handle = db.get_interrupt_handle();
+        handle.interrupt();
+        db.close().unwrap();
+        handle.interrupt();
+
+        // Look at it's internals to see if we cleared it out properly.
+        let db_guard = handle.db_lock.lock().unwrap();
+        assert!(db_guard.is_null());
+        // It would be nice to test that we properly handle close/interrupt
+        // running at the same time, but it seems impossible to do with any
+        // degree of reliability.
+    }
+
     mod query_and_then_tests {
         extern crate libsqlite3_sys as ffi;
         use super::*;