[Cases] Notify assignees when assigned to a case (#144391)

## Summary

The PR adds the ability to notify users by email when assigned to a
case. A user is:
- Not notified if he/she assigns themselves
- Notified if added as an assignee to a case
- Not notified if removed from a case

I did not add integration tests due to the complexity of simulating an
email server. I added unit test coverage. If integration test coverage
is needed we can add the tests on another PR.

Depends on: #143303
Fixes: #142307

## Email screenshot

<img width="361" alt="Screenshot 2022-11-07 at 1 27 13 PM"
src="https://user-images.githubusercontent.com/7871006/200299356-52c08515-4d43-49d6-bd47-3797b52f97e5.png">

@shanisagiv1 @lcawl What do you think about the content of the email
(see screenshot)?

## Testing

1. Put the following in your `kibana.yml`:

```
notifications.connectors.default.email: 'mail-dev'

xpack.actions.preconfigured:
  mail-dev:
    name: preconfigured-email-notification-maildev
    actionTypeId: .email
    config:
      service: other
      from: mlr-test-sink@elastic.co
      host: localhost
      port: 1025
      secure: false
      hasAuth: false
```

2. Install [`maildev`](https://www.npmjs.com/package/maildev): `npm
install -g maildev`
3. Run `maildev`: `maildev`
4. Open MailDev's web interface (http://0.0.0.0:1080/)
5. Create a case and assign users. You should see the emails in the
MailDev inbox.
6. Update the assignees of a case. You should see the emails in the
MailDev inbox.

Note: If you assign yourself you should not see an email. If you delete
an assignee you should not see an email.

### Checklist

Delete any items that are not applicable to this PR.

- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)


### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

## Release notes

Notify users by email when assigned to a case

x-pack/plugins/cases/kibana.json

@@ -31,7 +31,8 @@
      "triggersActionsUi",
      "management",
      "spaces",
-     "security"
+     "security",
+     "notifications"
   ],
   "requiredBundles": [
     "savedObjects"

x-pack/plugins/cases/server/attachment_framework/mocks.ts

@@ -14,8 +14,13 @@ import type {
   CommentRequestPersistableStateType,
 } from '../../common/api';
 import { ExternalReferenceStorageType } from '../../common/api';
+import { ExternalReferenceAttachmentTypeRegistry } from './external_reference_registry';
 import { PersistableStateAttachmentTypeRegistry } from './persistable_state_registry';
-import type { PersistableStateAttachmentTypeSetup, PersistableStateAttachmentState } from './types';
+import type {
+  PersistableStateAttachmentTypeSetup,
+  PersistableStateAttachmentState,
+  ExternalReferenceAttachmentType,
+} from './types';
 
 export const getPersistableAttachment = (): PersistableStateAttachmentTypeSetup => ({
   id: '.test',
@@ -42,6 +47,10 @@ export const getPersistableAttachment = (): PersistableStateAttachmentTypeSetup
   }),
 });
 
+export const getExternalReferenceAttachment = (): ExternalReferenceAttachmentType => ({
+  id: '.test',
+});
+
 export const externalReferenceAttachmentSO = {
   type: CommentType.externalReference as const,
   externalReferenceId: 'my-id',
@@ -130,10 +139,18 @@ export const externalReferenceAttachmentSOAttributesWithoutRefs = omit(
   'externalReferenceId'
 );
 
-export const getPersistableStateAttachmentTypeRegistry =
+export const createPersistableStateAttachmentTypeRegistryMock =
   (): PersistableStateAttachmentTypeRegistry => {
     const persistableStateAttachmentTypeRegistry = new PersistableStateAttachmentTypeRegistry();
     persistableStateAttachmentTypeRegistry.register(getPersistableAttachment());
 
     return persistableStateAttachmentTypeRegistry;
   };
+
+export const createExternalReferenceAttachmentTypeRegistryMock =
+  (): ExternalReferenceAttachmentTypeRegistry => {
+    const externalReferenceAttachmentTypeRegistry = new ExternalReferenceAttachmentTypeRegistry();
+    externalReferenceAttachmentTypeRegistry.register(getExternalReferenceAttachment());
+
+    return externalReferenceAttachmentTypeRegistry;
+  };

x-pack/plugins/cases/server/attachment_framework/so_references.test.ts

@@ -7,7 +7,7 @@
 
 import { CommentType, SECURITY_SOLUTION_OWNER } from '../../common';
 import {
-  getPersistableStateAttachmentTypeRegistry,
+  createPersistableStateAttachmentTypeRegistryMock,
   persistableStateAttachment,
   persistableStateAttachmentAttributes,
 } from './mocks';
@@ -19,7 +19,7 @@ import {
 } from './so_references';
 
 describe('Persistable state SO references', () => {
-  const persistableStateAttachmentTypeRegistry = getPersistableStateAttachmentTypeRegistry();
+  const persistableStateAttachmentTypeRegistry = createPersistableStateAttachmentTypeRegistryMock();
   const references = [
     {
       id: 'testRef',

x-pack/plugins/cases/server/client/cases/create.test.ts

@@ -0,0 +1,80 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { SECURITY_SOLUTION_OWNER } from '../../../common';
+import { CaseSeverity, ConnectorTypes } from '../../../common/api';
+import { mockCases } from '../../mocks';
+import { createCasesClientMockArgs } from '../mocks';
+import { create } from './create';
+
+describe('create', () => {
+  const theCase = {
+    title: 'My Case',
+    tags: [],
+    description: 'testing sir',
+    connector: {
+      id: '.none',
+      name: 'None',
+      type: ConnectorTypes.none,
+      fields: null,
+    },
+    settings: { syncAlerts: true },
+    severity: CaseSeverity.LOW,
+    owner: SECURITY_SOLUTION_OWNER,
+    assignees: [{ uid: '1' }],
+  };
+
+  const caseSO = mockCases[0];
+
+  describe('Assignees', () => {
+    const clientArgs = createCasesClientMockArgs();
+    clientArgs.services.caseService.postNewCase.mockResolvedValue(caseSO);
+
+    beforeEach(() => {
+      jest.clearAllMocks();
+    });
+
+    it('notifies single assignees', async () => {
+      await create(theCase, clientArgs);
+
+      expect(clientArgs.services.notificationService.notifyAssignees).toHaveBeenCalledWith({
+        assignees: theCase.assignees,
+        theCase: caseSO,
+      });
+    });
+
+    it('notifies multiple assignees', async () => {
+      await create({ ...theCase, assignees: [{ uid: '1' }, { uid: '2' }] }, clientArgs);
+
+      expect(clientArgs.services.notificationService.notifyAssignees).toHaveBeenCalledWith({
+        assignees: [{ uid: '1' }, { uid: '2' }],
+        theCase: caseSO,
+      });
+    });
+
+    it('does not notify when there are no assignees', async () => {
+      await create({ ...theCase, assignees: [] }, clientArgs);
+
+      expect(clientArgs.services.notificationService.notifyAssignees).not.toHaveBeenCalled();
+    });
+
+    it('does not notify the current user', async () => {
+      await create(
+        {
+          ...theCase,
+          assignees: [{ uid: '1' }, { uid: 'u_J41Oh6L9ki-Vo2tOogS8WRTENzhHurGtRc87NgEAlkc_0' }],
+        },
+        clientArgs
+      );
+
+      expect(clientArgs.services.notificationService.notifyAssignees).toHaveBeenCalledWith({
+        assignees: [{ uid: '1' }],
+        theCase: caseSO,
+      });
+    });
+  });
+});

x-pack/plugins/cases/server/client/cases/create.ts

@@ -41,7 +41,7 @@ export const create = async (
 ): Promise<CaseResponse> => {
   const {
     unsecuredSavedObjectsClient,
-    services: { caseService, userActionService, licensingService },
+    services: { caseService, userActionService, licensingService, notificationService },
     user,
     logger,
     authorization: auth,
@@ -116,11 +116,22 @@ export const create = async (
       owner: newCase.attributes.owner,
     });
 
-    return CaseResponseRt.encode(
-      flattenCaseSavedObject({
-        savedObject: newCase,
-      })
-    );
+    const flattenedCase = flattenCaseSavedObject({
+      savedObject: newCase,
+    });
+
+    if (query.assignees && query.assignees.length !== 0) {
+      const assigneesWithoutCurrentUser = query.assignees.filter(
+        (assignee) => assignee.uid !== user.profile_uid
+      );
+
+      await notificationService.notifyAssignees({
+        assignees: assigneesWithoutCurrentUser,
+        theCase: newCase,
+      });
+    }
+
+    return CaseResponseRt.encode(flattenedCase);
   } catch (error) {
     throw createCaseError({ message: `Failed to create case: ${error}`, error, logger });
   }

x-pack/plugins/cases/server/client/cases/get.ts

@@ -9,7 +9,7 @@ import { pipe } from 'fp-ts/lib/pipeable';
 import { fold } from 'fp-ts/lib/Either';
 import { identity } from 'fp-ts/lib/function';
 
-import type { SavedObject, SavedObjectsResolveResponse } from '@kbn/core/server';
+import type { SavedObjectsResolveResponse } from '@kbn/core/server';
 import type {
   CaseResponse,
   CaseResolveResponse,
@@ -37,6 +37,7 @@ import type { CasesClientArgs } from '..';
 import { Operations } from '../../authorization';
 import { combineAuthorizedAndOwnerFilter } from '../utils';
 import { CasesService } from '../../services';
+import type { CaseSavedObject } from '../../common/types';
 
 /**
  * Parameters for finding cases IDs using an alert ID
@@ -182,7 +183,7 @@ export const get = async (
   } = clientArgs;
 
   try {
-    const theCase: SavedObject<CaseAttributes> = await caseService.getCase({
+    const theCase: CaseSavedObject = await caseService.getCase({
       id,
     });