Fix context cancellation racy handling

[why]
Context cancellation goroutine is not in sync with Next() method lifetime.
It leads to sql.ErrNoRows instead of context.Canceled often (easy to reproduce).
It leads to interruption of next query executed on same connection (harder to reproduce).

[how]
Do query in goroutine, wait when interruption done.

[testing]
Add unit test that reproduces error cases.

1 files changed, 59 insertions, 37 deletions

diff --git a/sqlite3.go b/sqlite3.go
index 4000173..1f0730a 100644
--- a/sqlite3.go
+++ b/sqlite3.go
@@ -328,7 +328,7 @@ type SQLiteRows struct {
 	decltype []string
 	cls      bool
 	closed   bool
-	done     chan struct{}
+	ctx      context.Context // no better alternative to pass context into Next() method
 }
 
 type functionInfo struct {
@@ -1846,22 +1846,7 @@ func (s *SQLiteStmt) query(ctx context.Context, args []namedValue) (driver.Rows,
 		decltype: nil,
 		cls:      s.cls,
 		closed:   false,
-		done:     make(chan struct{}),
-	}
-
-	if ctxdone := ctx.Done(); ctxdone != nil {
-		go func(db *C.sqlite3) {
-			select {
-			case <-ctxdone:
-				select {
-				case <-rows.done:
-				default:
-					C.sqlite3_interrupt(db)
-					rows.Close()
-				}
-			case <-rows.done:
-			}
-		}(s.c.db)
+		ctx:      ctx,
 	}
 
 	return rows, nil
@@ -1889,29 +1874,43 @@ func (s *SQLiteStmt) Exec(args []driver.Value) (driver.Result, error) {
 	return s.exec(context.Background(), list)
 }
 
+// exec executes a query that doesn't return rows. Attempts to honor context timeout.
 func (s *SQLiteStmt) exec(ctx context.Context, args []namedValue) (driver.Result, error) {
+	if ctx.Done() == nil {
+		return s.execSync(args)
+	}
+
+	type result struct {
+		r   driver.Result
+		err error
+	}
+	resultCh := make(chan result)
+	go func() {
+		r, err := s.execSync(args)
+		resultCh <- result{r, err}
+	}()
+	select {
+	case rv := <- resultCh:
+		return rv.r, rv.err
+	case <-ctx.Done():
+		select {
+		case <-resultCh: // no need to interrupt
+		default:
+			// this is still racy and can be no-op if executed between sqlite3_* calls in execSync.
+			C.sqlite3_interrupt(s.c.db)
+			<-resultCh // ensure goroutine completed
+		}
+		return nil, ctx.Err()
+	}
+}
+
+func (s *SQLiteStmt) execSync(args []namedValue) (driver.Result, error) {
 	if err := s.bind(args); err != nil {
 		C.sqlite3_reset(s.s)
 		C.sqlite3_clear_bindings(s.s)
 		return nil, err
 	}
 
-	if ctxdone := ctx.Done(); ctxdone != nil {
-		done := make(chan struct{})
-		defer close(done)
-		go func(db *C.sqlite3) {
-			select {
-			case <-done:
-			case <-ctxdone:
-				select {
-				case <-done:
-				default:
-					C.sqlite3_interrupt(db)
-				}
-			}
-		}(s.c.db)
-	}
-
 	var rowid, changes C.longlong
 	rv := C._sqlite3_step_row_internal(s.s, &rowid, &changes)
 	if rv != C.SQLITE_ROW && rv != C.SQLITE_OK && rv != C.SQLITE_DONE {
@@ -1932,9 +1931,6 @@ func (rc *SQLiteRows) Close() error {
 		return nil
 	}
 	rc.closed = true
-	if rc.done != nil {
-		close(rc.done)
-	}
 	if rc.cls {
 		rc.s.mu.Unlock()
 		return rc.s.Close()
@@ -1978,13 +1974,39 @@ func (rc *SQLiteRows) DeclTypes() []string {
 	return rc.declTypes()
 }
 
-// Next move cursor to next.
+// Next move cursor to next. Attempts to honor context timeout from QueryContext call.
 func (rc *SQLiteRows) Next(dest []driver.Value) error {
 	rc.s.mu.Lock()
 	defer rc.s.mu.Unlock()
+
 	if rc.s.closed {
 		return io.EOF
 	}
+
+	if rc.ctx.Done() == nil {
+		return rc.nextSyncLocked(dest)
+	}
+	resultCh := make(chan error)
+	go func() {
+		resultCh <- rc.nextSyncLocked(dest)
+	}()
+	select {
+	case err := <- resultCh:
+		return err
+	case <-rc.ctx.Done():
+		select {
+		case <-resultCh: // no need to interrupt
+		default:
+			// this is still racy and can be no-op if executed between sqlite3_* calls in nextSyncLocked.
+			C.sqlite3_interrupt(rc.s.c.db)
+			<-resultCh // ensure goroutine completed
+		}
+		return rc.ctx.Err()
+	}
+}
+
+// nextSyncLocked moves cursor to next; must be called with locked mutex.
+func (rc *SQLiteRows) nextSyncLocked(dest []driver.Value) error {
 	rv := C._sqlite3_step_internal(rc.s.s)
 	if rv == C.SQLITE_DONE {
 		return io.EOF