feat(shell-api): make db.hello() fall back to legacy variant MONGOSH-558 (#827)

If `db.hello()` is not available, fall back to the legacy variant, and
always make `isWritablePrimary` available in the return value for both
variants.

Author: addaleax

packages/shell-api/src/database.ts

@@ -812,22 +812,33 @@ export default class Database extends ShellApiClass {
   @returnsPromise
   async isMaster(): Promise<Document> {
     this._emitDatabaseApiCall('isMaster', {});
-    return await this._runCommand(
+    const result = await this._runCommand(
       {
         isMaster: 1,
       }
     );
+    result.isWritablePrimary = result.ismaster;
+    return result;
   }
 
   @returnsPromise
   @serverVersions(['5.0.0', ServerVersions.latest])
   async hello(): Promise<Document> {
     this._emitDatabaseApiCall('hello', {});
-    return await this._runCommand(
-      {
-        hello: 1,
+    try {
+      return await this._runCommand(
+        {
+          hello: 1,
+        }
+      );
+    } catch (err) {
+      if (err.codeName === 'CommandNotFound') {
+        const result = await this.isMaster();
+        delete result.ismaster;
+        return result;
       }
-    );
+      throw err;
+    }
   }
 
   @returnsPromise

packages/shell-api/src/integration.spec.ts

@@ -1927,15 +1927,18 @@ describe('Shell API (integration)', function() {
   describe('database commands', () => {
     it('db.isMaster() works', async() => {
       expect((await database.isMaster()).ismaster).to.equal(true);
+      expect((await database.isMaster()).isWritablePrimary).to.equal(true);
+    });
+
+    it('db.hello() works', async() => {
+      const result = await database.hello();
+      expect(result.ismaster).to.equal(undefined);
+      expect(result.isWritablePrimary).to.equal(true);
     });
 
     context('with 5.0+ server', () => {
       skipIfServerVersion(testServer, '<= 4.4');
 
-      it('db.hello() works', async() => {
-        expect((await database.hello()).isWritablePrimary).to.equal(true);
-      });
-
       it('db.rotateCertificates() works', async() => {
         expect((await database.rotateCertificates()).ok).to.equal(1);
         expect((await database.rotateCertificates('message')).ok).to.equal(1);