Fix: Downcase all incoming headers (#154)

Downcase all incoming headers to remove case-sensitivity issues.

Previously, the Content-Type header could be duplicated if the incoming
header was anything besides downcased.

.changeset/rude-planets-kiss.md

@@ -0,0 +1,5 @@
+---
+'@apollo/datasource-rest': patch
+---
+
+Addresses duplicate content-type header bug due to upper-cased headers being forwarded. This change instead maps all headers to lowercased headers.

src/RESTDataSource.ts

@@ -454,14 +454,22 @@ export abstract class RESTDataSource {
     path: string,
     incomingRequest: DataSourceRequest = {},
   ): Promise<DataSourceFetchResult<TResult>> {
+    const downcasedHeaders: Record<string, string> = {};
+    if (incomingRequest.headers) {
+      // map incoming headers to lower-case headers
+      Object.entries(incomingRequest.headers).forEach(([key, value]) => {
+        downcasedHeaders[key.toLowerCase()] = value;
+      });
+    }
+
     const augmentedRequest: AugmentedRequest = {
       ...incomingRequest,
       // guarantee params and headers objects before calling `willSendRequest` for convenience
       params:
         incomingRequest.params instanceof URLSearchParams
           ? incomingRequest.params
           : this.urlSearchParamsFromRecord(incomingRequest.params),
-      headers: incomingRequest.headers ?? Object.create(null),
+      headers: downcasedHeaders,
     };
     // Default to GET in the case that `fetch` is called directly with no method
     // provided. Our other request methods all provide one.
@@ -481,9 +489,7 @@ export abstract class RESTDataSource {
     if (this.shouldJSONSerializeBody(augmentedRequest.body)) {
       augmentedRequest.body = JSON.stringify(augmentedRequest.body);
       // If Content-Type header has not been previously set, set to application/json
-      if (!augmentedRequest.headers) {
-        augmentedRequest.headers = { 'content-type': 'application/json' };
-      } else if (!augmentedRequest.headers['content-type']) {
+      if (!augmentedRequest.headers['content-type']) {
         augmentedRequest.headers['content-type'] = 'application/json';
       }
     }

src/__tests__/RESTDataSource.test.ts

@@ -260,6 +260,134 @@ describe('RESTDataSource', () => {
       await dataSource.getFoo('1');
     });
 
+    it('contains only one Content-Type header when Content-Type already exists is application/json', async () => {
+      const requestOptions = {
+        headers: {
+          'Content-Type': 'application/json',
+        },
+        body: { foo: 'bar' },
+      };
+      const dataSource = new (class extends RESTDataSource {
+        override baseURL = 'https://api.example.com';
+
+        postFoo() {
+          return this.post('foo', requestOptions);
+        }
+      })();
+
+      const spyOnHttpFetch = jest.spyOn(dataSource['httpCache'], 'fetch');
+
+      nock(apiUrl)
+        .post('/foo')
+        .reply(200, { foo: 'bar' }, { 'Content-Type': 'application/json' });
+
+      const data = await dataSource.postFoo();
+      expect(spyOnHttpFetch.mock.calls[0][1]).toEqual({
+        headers: { 'content-type': 'application/json' },
+        body: '{"foo":"bar"}',
+        method: 'POST',
+        params: new URLSearchParams(),
+      });
+      expect(data).toEqual({ foo: 'bar' });
+    });
+
+    it('converts uppercase-containing headers to lowercase', async () => {
+      const requestOptions = {
+        headers: {
+          'Content-Type': 'application/json',
+          'Test-Header': 'foobar',
+          'ANOTHER-TEST-HEADER': 'test2',
+        },
+        body: { foo: 'bar' },
+      };
+      const dataSource = new (class extends RESTDataSource {
+        override baseURL = 'https://api.example.com';
+
+        postFoo() {
+          return this.post('foo', requestOptions);
+        }
+      })();
+
+      const spyOnHttpFetch = jest.spyOn(dataSource['httpCache'], 'fetch');
+
+      nock(apiUrl)
+        .post('/foo')
+        .reply(200, { foo: 'bar' }, { 'Content-Type': 'application/json' });
+
+      const data = await dataSource.postFoo();
+      expect(spyOnHttpFetch.mock.calls[0][1]).toEqual({
+        headers: {
+          'content-type': 'application/json',
+          'test-header': 'foobar',
+          'another-test-header': 'test2',
+        },
+        body: '{"foo":"bar"}',
+        method: 'POST',
+        params: new URLSearchParams(),
+      });
+      expect(data).toEqual({ foo: 'bar' });
+    });
+
+    it('adds an `application/json` content-type header when none is present', async () => {
+      const requestOptions = {
+        body: { foo: 'bar' },
+      };
+      const dataSource = new (class extends RESTDataSource {
+        override baseURL = 'https://api.example.com';
+
+        postFoo() {
+          return this.post('foo', requestOptions);
+        }
+      })();
+
+      const spyOnHttpFetch = jest.spyOn(dataSource['httpCache'], 'fetch');
+
+      nock(apiUrl)
+        .post('/foo')
+        .reply(200, { foo: 'bar' }, { 'Content-Type': 'application/json' });
+
+      const data = await dataSource.postFoo();
+      expect(spyOnHttpFetch.mock.calls[0][1]).toEqual({
+        headers: {
+          'content-type': 'application/json',
+        },
+        body: '{"foo":"bar"}',
+        method: 'POST',
+        params: new URLSearchParams(),
+      });
+      expect(data).toEqual({ foo: 'bar' });
+    });
+
+    it('adds an `application/json` content header when no headers are passed in', async () => {
+      const requestOptions = {
+        body: { foo: 'bar' },
+      };
+      const dataSource = new (class extends RESTDataSource {
+        override baseURL = 'https://api.example.com';
+
+        postFoo() {
+          return this.post('foo', requestOptions);
+        }
+      })();
+
+      const spyOnHttpFetch = jest.spyOn(dataSource['httpCache'], 'fetch');
+
+      nock(apiUrl)
+        .post('/foo')
+        .reply(200, { foo: 'bar' }, { 'Content-Type': 'application/json' });
+
+      const data = await dataSource.postFoo();
+      expect(spyOnHttpFetch.mock.calls[0][1]).toEqual({
+        headers: {
+          'content-type': 'application/json',
+        },
+        body: '{"foo":"bar"}',
+        method: 'POST',
+        params: new URLSearchParams(),
+      });
+      expect(data).toEqual({ foo: 'bar' });
+    });
+
     it('serializes a request body that is an object as JSON', async () => {
       const expectedFoo = { foo: 'bar' };
       const dataSource = new (class extends RESTDataSource {