MockLink tweaks to avoid uncaught exceptions

CHANGELOG.md

@@ -14,6 +14,10 @@
 - `HttpLink` will now automatically strip any unused `variables` before sending queries to the GraphQL server, since those queries are very likely to fail validation, according to the [All Variables Used](https://spec.graphql.org/draft/#sec-All-Variables-Used) rule in the GraphQL specification. If you depend on the preservation of unused variables, you can restore the previous behavior by passing `includeUnusedVariables: true` to the `HttpLink` constructor (which is typically passed as `options.link` to the `ApolloClient` constructor). <br/>
   [@benjamn](https://github.com/benjamn) in [#7127](https://github.com/apollographql/apollo-client/pull/7127)
 
+- Ensure `MockLink` (used by `MockedProvider`) returns mock configuration errors (e.g. `No more mocked responses for the query ...`) through the Link's `Observable`, instead of throwing them. These errors are now available through the `error` property of a result. <br/>
+  [@hwillson](https://github.com/hwillson) in [#7110](https://github.com/apollographql/apollo-client/pull/7110)
+  > Returning mock configuration errors through the Link's `Observable` was the default behavior in Apollo Client 2.x. We changed it for 3, but the change has been problematic for those looking to migrate from 2.x to 3. We've decided to change this back with the understanding that not many people want or are relying on `MockLink`'s throwing exception approach. If you want to change this functionality, you can define custom error handling through `MockLink.setOnError`.
+
 ## Improvements
 
 - Support inheritance of type and field policies, according to `possibleTypes`. <br/>

src/__tests__/client.ts

@@ -1768,7 +1768,7 @@ describe('client', () => {
     });
 
     itAsync('fails if network request fails', (resolve, reject) => {
-      const link = mockSingleLink().setOnError(error => { throw error }); // no queries = no replies.
+      const link = mockSingleLink(); // no queries = no replies.
       const client = new ApolloClient({
         link,
         cache: new InMemoryCache({ addTypename: false }),
@@ -2028,7 +2028,7 @@ describe('client', () => {
         request: { query: mutation },
         result: { data },
         error: networkError,
-      }).setOnError(reject),
+      }),
       cache: new InMemoryCache({ addTypename: false }),
     });
 
@@ -2475,7 +2475,7 @@ describe('client', () => {
       { request: { query }, result: { data } },
       { request: { query }, error: new Error('This is an error!') },
       { request: { query }, result: { data: dataTwo } }
-    ).setOnError(reject);
+    );
     const client = new ApolloClient({
       link,
       cache: new InMemoryCache({ addTypename: false }),

src/__tests__/fetchMore.ts

@@ -620,7 +620,7 @@ describe('fetchMore on an observable query', () => {
         request: { query, variables: variablesMore },
         error: fetchMoreError,
         delay: 5,
-      }).setOnError(reject);
+      });
 
       const client = new ApolloClient({
         link,
@@ -674,7 +674,7 @@ describe('fetchMore on an observable query', () => {
         request: { query, variables: variablesMore },
         error: fetchMoreError,
         delay: 5,
-      }).setOnError(reject);
+      });
 
       let calledFetchMore = false;
 

src/__tests__/mutationResults.ts

@@ -124,7 +124,7 @@ describe('mutation results', () => {
       link: mockSingleLink({
         request: { query: queryWithTypename } as any,
         result,
-      }, ...mockedResponses).setOnError(reject),
+      }, ...mockedResponses),
       cache: new InMemoryCache({
         dataIdFromObject: (obj: any) => {
           if (obj.id && obj.__typename) {

src/__tests__/optimistic.ts

@@ -123,7 +123,7 @@ describe('optimistic mutation results', () => {
     const link = mockSingleLink({
       request: { query },
       result,
-    }, ...mockedResponses).setOnError(reject);
+    }, ...mockedResponses);
 
     const client = new ApolloClient({
       link,

src/core/__tests__/fetchPolicies.ts

@@ -65,14 +65,14 @@ const createLink = (reject: (reason: any) => any) =>
     result: { data: result },
   }).setOnError(reject);
 
-const createFailureLink = (reject: (reason: any) => any) =>
+const createFailureLink = () =>
   mockSingleLink({
     request: { query },
     error: new Error('query failed'),
   }, {
     request: { query },
     result: { data: result },
-  }).setOnError(reject);
+  });
 
 const createMutationLink = (reject: (reason: any) => any) =>
   // fetch the data
@@ -149,7 +149,7 @@ describe('network-only', () => {
     });
 
     const client = new ApolloClient({
-      link: inspector.concat(createFailureLink(reject)),
+      link: inspector.concat(createFailureLink()),
       cache: new InMemoryCache({ addTypename: false }),
     });
 
@@ -280,7 +280,7 @@ describe('no-cache', () => {
     });
 
     const client = new ApolloClient({
-      link: inspector.concat(createFailureLink(reject)),
+      link: inspector.concat(createFailureLink()),
       cache: new InMemoryCache({ addTypename: false }),
     });
 

src/link/core/ApolloLink.ts

@@ -141,11 +141,25 @@ export class ApolloLink {
     throw new InvariantError('request is not implemented');
   }
 
-  protected onError(reason: any) {
-    throw reason;
+  protected onError(
+    error: any,
+    observer: ZenObservable.Observer<FetchResult>,
+  ): false | void {
+    if (observer && observer.error) {
+      observer.error(error);
+      // Returning false indicates that observer.error does not need to be
+      // called again, since it was already called (on the previous line).
+      // Calling observer.error again would not cause any real problems,
+      // since only the first call matters, but custom onError functions
+      // might have other reasons for wanting to prevent the default
+      // behavior by returning false.
+      return false;
+    }
+    // Throw errors will be passed to observer.error.
+    throw error;
   }
 
-  public setOnError(fn: (reason: any) => any): this {
+  public setOnError(fn: ApolloLink["onError"]): this {
     this.onError = fn;
     return this;
   }