fix: add back read concern application logic

Author: baileympearson

etc/notes/CHANGES_5.0.0.md

@@ -25,6 +25,21 @@ to migrate code that uses `Collection.mapReduce` to use the aggregation pipeline
 If the `mapReduce` command must be used, the `Db.command()` helper can be used to run the raw
 `mapReduce` command.
 
+```typescript
+const command = {
+  mapReduce: 'my-collection',
+  map: 'function() { emit(this.user_id, 1); }',
+  reduce: 'function(k,vals) { return 1; }',
+  out: 'inline',
+  readConcern: 'majority'
+}
+
+await db.command(command);
+```
+
+**Note** When using the `Db.command()` helper, all `mapReduce` options should be specified
+on the raw command object and should not be passed through the options object.
+
 ### `AddUserOptions.digestPassword` removed
 
 The `digestPassword` option has been removed from the add user helper.

src/sessions.ts

@@ -1007,7 +1007,7 @@ export function applySession(
     if (
       session.supports.causalConsistency &&
       session.operationTime &&
-      commandSupportsReadConcern(command)
+      commandSupportsReadConcern(command, options)
     ) {
       command.readConcern = command.readConcern || {};
       Object.assign(command.readConcern, { afterClusterTime: session.operationTime });

src/utils.ts

@@ -1326,10 +1326,23 @@ export function shuffle<T>(sequence: Iterable<T>, limit = 0): Array<T> {
   return limit % items.length === 0 ? items : items.slice(lowerBound);
 }
 
-// TODO: this should be codified in command construction
+// TODO(NODE-4936): read concern eligibility for commands should be codified in command construction
 // @see https://github.com/mongodb/specifications/blob/master/source/read-write-concern/read-write-concern.rst#read-concern
-export function commandSupportsReadConcern(command: Document): boolean {
-  return command.aggregate || command.count || command.distinct || command.find || command.geoNear;
+export function commandSupportsReadConcern(command: Document, options?: Document): boolean {
+  if (command.aggregate || command.count || command.distinct || command.find || command.geoNear) {
+    return true;
+  }
+
+  if (
+    command.mapReduce &&
+    options &&
+    options.out &&
+    (options.out.inline === 1 || options.out === 'inline')
+  ) {
+    return true;
+  }
+
+  return false;
 }
 
 /** A utility function to get the instance of mongodb-client-encryption, if it exists. */