SERVER-57256: Add deprecation message for count command and direct reader to docs

2 files changed, 48 insertions, 0 deletions

diff --git a/jstests/noPassthrough/deprecated_count.js b/jstests/noPassthrough/deprecated_count.js
new file mode 100644
index 00000000000..aafee4ca749
--- /dev/null
+++ b/jstests/noPassthrough/deprecated_count.js
@@ -0,0 +1,37 @@
+// The count command is deprecated in 5.0.
+//
+// In this test, we run the count command several times.
+// We want to make sure that the deprecation warning message is only logged once despite
+// the multiple invocations in an effort to not clutter the dev's console.
+// More specifically, we expect to only log 1/127 of count() events.
+
+(function() {
+"use strict";
+load("jstests/libs/log.js");  // For findMatchingLogLine, findMatchingLogLines
+
+jsTest.log('Test standalone');
+const standalone = MongoRunner.runMongod({});
+const dbName = 'test';
+const collName = "test_count_command_deprecation_messaging";
+const db = standalone.getDB(dbName);
+const coll = db.getCollection(collName);
+
+coll.drop();
+var res = assert.commandWorked(db.runCommand({count: collName}));
+assert.eq(0, res.n);
+assert.commandWorked(coll.insert({i: 1}));
+res = assert.commandWorked(db.runCommand({count: collName}));
+assert.eq(1, res.n);
+assert.commandWorked(coll.insert({i: 1}));
+res = assert.commandWorked(db.runCommand({count: collName}));
+assert.eq(2, res.n);
+
+const globalLogs = db.adminCommand({getLog: 'global'});
+const fieldMatcher = {
+    msg:
+        "The count command is deprecated. For more information, see https://docs.mongodb.com/manual/reference/method/db.collection.count/"
+};
+const matchingLogLines = [...findMatchingLogLines(globalLogs.log, fieldMatcher)];
+assert.eq(matchingLogLines.length, 1, matchingLogLines);
+MongoRunner.stopMongod(standalone);
+})();
diff --git a/src/mongo/db/commands/count_cmd.cpp b/src/mongo/db/commands/count_cmd.cpp
index 2d4769c5f86..626a3669ae6 100644
--- a/src/mongo/db/commands/count_cmd.cpp
+++ b/src/mongo/db/commands/count_cmd.cpp
@@ -61,10 +61,14 @@ using std::unique_ptr;
 // Failpoint which causes to hang "count" cmd after acquiring the DB lock.
 MONGO_FAIL_POINT_DEFINE(hangBeforeCollectionCount);
 
+// The count command is deprecated in 5.0
+
 /**
  * Implements the MongoD side of the count command.
  */
 class CmdCount : public BasicCommand {
+    inline static Rarely _sampler;
+
 public:
     CmdCount() : BasicCommand("count") {}
 
@@ -214,6 +218,13 @@ public:
              const string& dbname,
              const BSONObj& cmdObj,
              BSONObjBuilder& result) override {
+
+        if (_sampler.tick()) {
+            LOGV2_WARNING(5725601,
+                          "The count command is deprecated. For more information, see "
+                          "https://docs.mongodb.com/manual/reference/method/db.collection.count/");
+        }
+
         CommandHelpers::handleMarkKillOnClientDisconnect(opCtx);
         // Acquire locks and resolve possible UUID. The RAII object is optional, because in the case
         // of a view, the locks need to be released.