sql: fix race condition in internal executor

In the case where the finish() method was called because of a context
cancellation, the client might not have observed the context error and
instead would have assumed that execution completed successfully. The
code now prioritizes the context error if there is one.

Fixes #62948

Release note: None

pkg/sql/internal_result_channel.go

@@ -87,7 +87,7 @@ func (c *asyncIEResultChannel) firstResult(
 		return ieIteratorResult{}, true, ctx.Err()
 	case res, ok := <-c.dataCh:
 		if !ok {
-			return ieIteratorResult{}, true, nil
+			return ieIteratorResult{}, true, ctx.Err()
 		}
 		return res, false, nil
 	}
@@ -178,7 +178,7 @@ func (i *syncIEResultChannel) firstResult(
 	case <-ctx.Done():
 		return ieIteratorResult{}, true, ctx.Err()
 	case <-i.doneCh:
-		return ieIteratorResult{}, true, nil
+		return ieIteratorResult{}, true, ctx.Err()
 	case res, ok := <-i.dataCh:
 		if !ok {
 			return ieIteratorResult{}, true, nil
@@ -226,13 +226,21 @@ func (i *syncIEResultChannel) addResult(ctx context.Context, result ieIteratorRe
 	case <-ctx.Done():
 		return ctx.Err()
 	case <-i.doneCh:
+		// Prefer the context error if there is one.
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return ctxErr
+		}
 		return errSyncIEResultReaderCanceled
 	case i.dataCh <- result:
 	}
 	select {
 	case <-ctx.Done():
 		return ctx.Err()
 	case <-i.doneCh:
+		// Prefer the context error if there is one.
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return ctxErr
+		}
 		return errSyncIEResultReaderCanceled
 	case <-i.waitCh:
 		return nil