SERVER-31633 If the featureCompatibilityVersion is 3.6, upgrading, or downgrading, respond to isMaster with minWireVersion=maxWireVersion

5 files changed, 69 insertions, 19 deletions

diff --git a/jstests/multiVersion/2_test_launching_cluster.js b/jstests/multiVersion/2_test_launching_cluster.js
index 47368a27331..4e81fce3602 100644
--- a/jstests/multiVersion/2_test_launching_cluster.js
+++ b/jstests/multiVersion/2_test_launching_cluster.js
@@ -15,6 +15,7 @@ load('./jstests/multiVersion/libs/verify_versions.js');
     "use strict";
     // Check our latest versions
     var versionsToCheck = ["last-stable", "latest"];
+    var versionsToCheckConfig = ["latest"];
     var versionsToCheckMongos = ["last-stable"];
 
     jsTest.log("Testing mixed versions...");
@@ -25,7 +26,7 @@ load('./jstests/multiVersion/libs/verify_versions.js');
         mongos: 2,
         other: {
             mongosOptions: {binVersion: versionsToCheckMongos},
-            configOptions: {binVersion: versionsToCheck},
+            configOptions: {binVersion: versionsToCheckConfig},
             shardOptions: {binVersion: versionsToCheck},
             enableBalancer: true
         }
@@ -52,7 +53,7 @@ load('./jstests/multiVersion/libs/verify_versions.js');
     for (var j = 0; j < configs.length; j++)
         versionsFound.push(configs[j].getBinVersion());
 
-    assert.allBinVersions(versionsToCheck, versionsFound);
+    assert.allBinVersions(versionsToCheckConfig, versionsFound);
 
     st.stop();
 })();
diff --git a/jstests/multiVersion/set_feature_compatibility_version.js b/jstests/multiVersion/set_feature_compatibility_version.js
index d85e660cb83..2ee29b36404 100644
--- a/jstests/multiVersion/set_feature_compatibility_version.js
+++ b/jstests/multiVersion/set_feature_compatibility_version.js
@@ -367,18 +367,13 @@ TestData.skipCheckingUUIDsConsistentAcrossCluster = true;
     assert.writeOK(
         primaryAdminDB.getSiblingDB("test").coll.insert({awaitRepl: true}, {writeConcern: {w: 3}}));
 
-    // Test that a 3.4 secondary crashes when syncing from a 3.6 primary and the
+    // Test that a 3.4 secondary can no longer replicate from the primary after the
     // featureCompatibilityVersion is set to 3.6.
     assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: "3.6"}));
-    assert.soon(function() {
-        try {
-            secondaryAdminDB.runCommand({ping: 1});
-        } catch (e) {
-            return true;
-        }
-        return false;
-    }, "Expected 3.4 secondary to terminate due to replicating featureCompatibilityVersion=3.6");
-    rst.stop(secondary, undefined, {allowedExitCode: MongoRunner.EXIT_ABRUPT});
+    checkFCV34(secondaryAdminDB, "3.4");
+    assert.writeOK(primaryAdminDB.getSiblingDB("test").coll.insert({shouldReplicate: false}));
+    assert.eq(secondaryAdminDB.getSiblingDB("test").coll.find({shouldReplicate: false}).itcount(),
+              0);
     rst.stopSet();
 
     // A mixed 3.4/3.6 replica set without a featureCompatibilityVersion document unfortunately
diff --git a/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js b/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js
new file mode 100644
index 00000000000..53ef53bff5b
--- /dev/null
+++ b/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js
@@ -0,0 +1,55 @@
+// Tests that when the featureCompatibilityVersion is not equal to the downgrade version, running
+// isMaster with internalClient returns a response with minWireVersion=maxWireVersion. This ensures
+// that an older version mongod/mongos will fail to connect to the node when it is upgraded,
+// upgrading, or downgrading.
+(function() {
+    "use strict";
+
+    const adminDB = db.getSiblingDB("admin");
+    const isMasterCommand = {
+        isMaster: 1,
+        internalClient: {minWireVersion: NumberInt(0), maxWireVersion: NumberInt(6)}
+    };
+    const upgradeVersion = "3.6";
+    const downgradeVersion = "3.4";
+
+    // When the featureCompatibilityVersion is equal to the upgrade version, running isMaster with
+    // internalClient returns minWireVersion=maxWireVersion.
+    assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: upgradeVersion}));
+    let res = adminDB.runCommand(isMasterCommand);
+    assert.commandWorked(res);
+    assert.eq(res.minWireVersion, res.maxWireVersion, tojson(res));
+
+    // When the featureCompatibilityVersion is upgrading, running isMaster with internalClient
+    // returns minWireVersion=maxWireVersion.
+    assert.writeOK(adminDB.system.version.update(
+        {_id: "featureCompatibilityVersion"},
+        {$set: {version: downgradeVersion, targetVersion: upgradeVersion}}));
+    res = adminDB.runCommand(isMasterCommand);
+    assert.commandWorked(res);
+    assert.eq(res.minWireVersion, res.maxWireVersion, tojson(res));
+
+    // When the featureCompatibilityVersion is downgrading, running isMaster with internalClient
+    // returns minWireVersion=maxWireVersion.
+    assert.writeOK(adminDB.system.version.update(
+        {_id: "featureCompatibilityVersion"},
+        {$set: {version: downgradeVersion, targetVersion: downgradeVersion}}));
+    res = adminDB.runCommand(isMasterCommand);
+    assert.commandWorked(res);
+    assert.eq(res.minWireVersion, res.maxWireVersion, tojson(res));
+
+    // When the featureCompatibilityVersion is equal to the downgrade version, running isMaster with
+    // internalClient returns minWireVersion<maxWireVersion.
+    assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: downgradeVersion}));
+    res = adminDB.runCommand(isMasterCommand);
+    assert.commandWorked(res);
+    assert.lt(res.minWireVersion, res.maxWireVersion, tojson(res));
+
+    // When the internalClient field is missing from the isMaster command, the response has
+    // minWireVersion<maxWireVersion, even if the featureCompatibilityVersion is equal to the
+    // upgrade version.
+    assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: upgradeVersion}));
+    res = adminDB.runCommand({isMaster: 1});
+    assert.commandWorked(res);
+    assert.lt(res.minWireVersion, res.maxWireVersion, tojson(res));
+})();
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index a96335aca1a..45f430a5a24 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -337,8 +337,7 @@ void FeatureCompatibilityVersion::onInsertOrUpdate(OperationContext* opCtx, cons
         serverGlobalParams.featureCompatibility.setVersion(newVersion);
 
         // Close all connections from internal clients with binary versions lower than 3.6.
-        if (newVersion == ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36 ||
-            newVersion == ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36) {
+        if (newVersion != ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
             opCtx->getServiceContext()->getServiceEntryPoint()->endAllSessions(
                 transport::Session::kLatestVersionInternalClientKeepOpen |
                 transport::Session::kExternalClientKeepOpen);
diff --git a/src/mongo/db/repl/replication_info.cpp b/src/mongo/db/repl/replication_info.cpp
index 3391dc3aa19..5d116d1fe2e 100644
--- a/src/mongo/db/repl/replication_info.cpp
+++ b/src/mongo/db/repl/replication_info.cpp
@@ -359,12 +359,12 @@ public:
         result.append("maxWireVersion", WireSpec::instance().incoming.maxWireVersion);
         result.append("logicalSessionTimeoutMinutes", localLogicalSessionTimeoutMinutes);
 
-        // If the featureCompatibilityVersion is 3.6, respond with minWireVersion=maxWireVersion.
-        // Then if the connection is from a mongod/mongos of an earlier version, it will fail to
-        // connect.
+        // If the featureCompatibilityVersion is 3.6 (or upgrading or downgrading), respond with
+        // minWireVersion=maxWireVersion. Then if the connection is from a mongod/mongos of an
+        // earlier version, it will fail to connect.
         if (internalClientElement &&
-            serverGlobalParams.featureCompatibility.getVersion() ==
-                ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36) {
+            serverGlobalParams.featureCompatibility.getVersion() !=
+                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
             result.append("minWireVersion", WireSpec::instance().incoming.maxWireVersion);
         } else {
             result.append("minWireVersion", WireSpec::instance().incoming.minWireVersion);