Fix cascading cancellations from components (#503)

* Fix cascading cancellations from components

wonka@^4.0.0 has some improved behaviour that fixed
some edge cases and bugs. As it turned out urql was
relying on one of them.
The switchMap in the hooks would cancel the last
query and start the next. However, this cancellation
cascades not only up to executeQuery to end and
teardown the operation, it also flows up through the
exchange pipeline, ending the entire thing like an
electrical surge.

This can be prevented by publishing the results
chain and making sure it can never be interrupted.

* Fix pollInterval test

* Add scheduler dependency to silence peer dependency warning

package.json

@@ -127,7 +127,6 @@
     "rollup-plugin-replace": "^2.2.0",
     "rollup-plugin-terser": "^5.1.3",
     "rollup-plugin-typescript2": "^0.25.3",
-    "scheduler": "^0.18.0",
     "terser": "^4.6.2",
     "ts-jest": "^24.3.0",
     "typescript": "^3.7.4"
@@ -137,6 +136,7 @@
     "react": ">= 16.8.0"
   },
   "dependencies": {
+    "scheduler": ">= 0.16.0",
     "react-wonka": "^2.0.0",
     "wonka": "^4.0.5"
   }

src/client.test.ts

@@ -195,10 +195,12 @@ describe('executeQuery', () => {
 
     expect(receivedOps.length).toEqual(1);
     jest.advanceTimersByTime(200);
-    expect(receivedOps.length).toEqual(3);
+    expect(receivedOps.length).toEqual(5);
     expect(receivedOps[0].operationName).toEqual('query');
-    expect(receivedOps[1].operationName).toEqual('query');
+    expect(receivedOps[1].operationName).toEqual('teardown');
     expect(receivedOps[2].operationName).toEqual('query');
+    expect(receivedOps[3].operationName).toEqual('teardown');
+    expect(receivedOps[4].operationName).toEqual('query');
     unsubscribe();
   });
 });

src/client.ts

@@ -28,6 +28,7 @@ import {
   interval,
   fromValue,
   switchMap,
+  publish,
 } from 'wonka';
 
 import {
@@ -135,6 +136,10 @@ export class Client {
         forward: fallbackExchangeIO,
       })(this.operations$)
     );
+
+    // Prevent the `results$` exchange pipeline from being closed by active
+    // cancellations cascading up from components
+    pipe(this.results$, publish);
   }
 
   private createOperationContext = (

yarn.lock

@@ -5137,7 +5137,7 @@ sax@^1.2.4:
   resolved "https://registry.yarnpkg.com/sax/-/sax-1.2.4.tgz#2816234e2378bddc4e5354fab5caa895df7100d9"
   integrity sha512-NqVDv9TpANUjFm0N8uM5GxL36UgKi9/atZw+x7YFnQ8ckwFGKrl4xX4yWtrey3UJm5nP1kUbnYgLopqWNSRhWw==
 
-scheduler@^0.18.0:
+"scheduler@>= 0.16.0", scheduler@^0.18.0:
   version "0.18.0"
   resolved "https://registry.yarnpkg.com/scheduler/-/scheduler-0.18.0.tgz#5901ad6659bc1d8f3fdaf36eb7a67b0d6746b1c4"
   integrity sha512-agTSHR1Nbfi6ulI0kYNK0203joW2Y5W4po4l+v03tOoiJKpTBbxpNhWDvqc/4IcOw+KLmSiQLTasZ4cab2/UWQ==