feature(dynamic-context): Execute function for context

Provides the ability to do post-AST validation (e.g. security) as well as to build the context dynamically with a user object or other information.
Prettify and test fix
added test for context function support

src/__tests__/http-test.js

@@ -1903,5 +1903,36 @@ describe('test harness', () => {
         );
       });
     });
+
+    describe('Custom context capabiility', () => {
+      it('allows context to be a function', async () => {
+        const app = server();
+
+        get(
+          app,
+          urlString(),
+          graphqlHTTP({
+            schema: TestSchema,
+            context: () => Promise.resolve('testValue'),
+          }),
+        );
+
+        const response = await request(app).get(
+          urlString({
+            operationName: 'TestQuery',
+            query: `
+              query TestQuery { context }
+            `,
+          }),
+        );
+
+        expect(response.status).to.equal(200);
+        expect(JSON.parse(response.text)).to.deep.equal({
+          data: {
+            context: 'testValue',
+          },
+        });
+      });
+    });
   });
 });

src/index.js

@@ -247,21 +247,45 @@ function graphqlHTTP(options: Options): Middleware {
             );
           }
         }
-        // Perform the execution, reporting any errors creating the context.
-        try {
-          return execute(
-            schema,
-            documentAST,
-            rootValue,
-            context,
-            variables,
-            operationName,
-          );
-        } catch (contextError) {
-          // Return 400: Bad Request if any execution context errors exist.
-          response.statusCode = 400;
-          return { errors: [contextError] };
+
+        let preExecute;
+        if (typeof context === 'function') {
+          preExecute = Promise.resolve()
+            .then(() =>
+              context(
+                request,
+                response,
+                schema,
+                documentAST,
+                variables,
+                operationName,
+              ),
+            )
+            .catch(error => {
+              // If an error was caught, report the httpError status, or 500.
+              response.statusCode = error.status || 500;
+              return { errors: [error] };
+            });
+        } else {
+          preExecute = Promise.resolve(context);
         }
+        // Perform the execution, reporting any errors creating the context.
+        return preExecute
+          .then(dynamicContext => {
+            return execute(
+              schema,
+              documentAST,
+              rootValue,
+              dynamicContext,
+              variables,
+              operationName,
+            );
+          })
+          .catch(contextError => {
+            // Return 400: Bad Request if any execution context errors exist.
+            response.statusCode = 400;
+            return { errors: [contextError] };
+          });
       })
       .then(result => {
         // Collect and apply any metadata extensions if a function was provided.