SERVER-56307 Allow the donor to enter the critical section when the untransferred mods are within a convergence threshold.

(cherry picked from commit 44bdd2c38fd5c80881e15f40e2da6cacf5f35eb4)

5 files changed, 83 insertions, 7 deletions

diff --git a/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp b/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp
index e6815d05889..6b65ecd7a50 100644
--- a/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp
+++ b/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp
@@ -39,11 +39,13 @@
 #include "mongo/db/catalog_raii.h"
 #include "mongo/db/dbhelpers.h"
 #include "mongo/db/exec/working_set_common.h"
+#include "mongo/db/index/index_access_method.h"
 #include "mongo/db/index/index_descriptor.h"
 #include "mongo/db/repl/optime.h"
 #include "mongo/db/repl/replication_process.h"
 #include "mongo/db/s/collection_sharding_runtime.h"
 #include "mongo/db/s/migration_source_manager.h"
+#include "mongo/db/s/sharding_runtime_d_params_gen.h"
 #include "mongo/db/s/sharding_statistics.h"
 #include "mongo/db/s/start_chunk_clone_request.h"
 #include "mongo/db/service_context.h"
@@ -796,18 +798,18 @@ MigrationChunkClonerSourceLegacy::_getIndexScanExecutor(
     InternalPlanner::IndexScanOptions scanOption) {
     // Allow multiKey based on the invariant that shard keys must be single-valued. Therefore, any
     // multi-key index prefixed by shard key cannot be multikey over the shard key fields.
-    const IndexDescriptor* idx =
+    const IndexDescriptor* shardKeyIdx =
         collection->getIndexCatalog()->findShardKeyPrefixedIndex(opCtx,
                                                                  _shardKeyPattern.toBSON(),
                                                                  false);  // requireSingleKey
-    if (!idx) {
+    if (!shardKeyIdx) {
         return {ErrorCodes::IndexNotFound,
                 str::stream() << "can't find index with prefix " << _shardKeyPattern.toBSON()
                               << " in storeCurrentLocs for " << _args.getNss().ns()};
     }
 
     // Assume both min and max non-empty, append MinKey's to make them fit chosen index
-    const KeyPattern kp(idx->keyPattern());
+    const KeyPattern kp(shardKeyIdx->keyPattern());
 
     BSONObj min = Helpers::toKeyFormat(kp.extendRangeBound(_args.getMinKey(), false));
     BSONObj max = Helpers::toKeyFormat(kp.extendRangeBound(_args.getMaxKey(), false));
@@ -816,7 +818,7 @@ MigrationChunkClonerSourceLegacy::_getIndexScanExecutor(
     // being queued and will migrate in the 'transferMods' stage.
     return InternalPlanner::indexScan(opCtx,
                                       &collection,
-                                      idx,
+                                      shardKeyIdx,
                                       min,
                                       max,
                                       BoundInclusion::kIncludeStartKeyOnly,
@@ -896,6 +898,17 @@ Status MigrationChunkClonerSourceLegacy::_storeCurrentLocs(OperationContext* opC
 
     const uint64_t collectionAverageObjectSize = collection->averageObjectSize(opCtx);
 
+    uint64_t averageObjectIdSize = OID::kOIDSize;
+    if (totalRecs > 0) {
+        const auto idIdx = collection->getIndexCatalog()->findIdIndex(opCtx)->getEntry();
+        if (!idIdx) {
+            return {ErrorCodes::IndexNotFound,
+                    str::stream() << "can't find index '_id' in storeCurrentLocs for "
+                                  << _args.getNss().ns()};
+        }
+        averageObjectIdSize = idIdx->accessMethod()->getSpaceUsedBytes(opCtx) / totalRecs;
+    }
+
     if (isLargeChunk) {
         return {
             ErrorCodes::ChunkTooBig,
@@ -909,7 +922,7 @@ Status MigrationChunkClonerSourceLegacy::_storeCurrentLocs(OperationContext* opC
 
     stdx::lock_guard<Latch> lk(_mutex);
     _averageObjectSizeForCloneLocs = collectionAverageObjectSize + 12;
-
+    _averageObjectIdSize = averageObjectIdSize;
     return Status::OK();
 }
 
@@ -1023,6 +1036,31 @@ Status MigrationChunkClonerSourceLegacy::_checkRecipientCloningStatus(OperationC
             return Status::OK();
         }
 
+        bool supportsCriticalSectionDuringCatchUp = false;
+        if (auto featureSupportedField =
+                res[StartChunkCloneRequest::kSupportsCriticalSectionDuringCatchUp]) {
+            if (!featureSupportedField.booleanSafe()) {
+                return {ErrorCodes::Error(563070),
+                        str::stream()
+                            << "Illegal value for "
+                            << StartChunkCloneRequest::kSupportsCriticalSectionDuringCatchUp};
+            }
+            supportsCriticalSectionDuringCatchUp = true;
+        }
+
+        if (res["state"].String() == "catchup" && supportsCriticalSectionDuringCatchUp) {
+            int64_t estimatedUntransferredModsSize = _deleted.size() * _averageObjectIdSize +
+                _reload.size() * _averageObjectSizeForCloneLocs;
+            auto estimatedUntransferredChunkPercentage =
+                (std::min(_args.getMaxChunkSizeBytes(), estimatedUntransferredModsSize) * 100) /
+                _args.getMaxChunkSizeBytes();
+            if (estimatedUntransferredChunkPercentage < minCatchUpPercentageBeforeBlockingWrites) {
+                // The recipient is sufficiently caught-up with the writes on the donor.
+                // Block writes, so that it can drain everything.
+                return Status::OK();
+            }
+        }
+
         if (res["state"].String() == "fail") {
             return {ErrorCodes::OperationFailed,
                     str::stream() << "Data transfer error: " << res["errmsg"].str()};
diff --git a/src/mongo/db/s/migration_chunk_cloner_source_legacy.h b/src/mongo/db/s/migration_chunk_cloner_source_legacy.h
index 3f866595cda..f41127afad1 100644
--- a/src/mongo/db/s/migration_chunk_cloner_source_legacy.h
+++ b/src/mongo/db/s/migration_chunk_cloner_source_legacy.h
@@ -357,6 +357,9 @@ private:
     // pre-allocation (initial clone).
     uint64_t _averageObjectSizeForCloneLocs{0};
 
+    // The estimated average object _id size during the clone phase.
+    uint64_t _averageObjectIdSize{0};
+
     // Represents all of the requested but not yet fulfilled operations to be tracked, with regards
     // to the chunk being cloned.
     uint64_t _outstandingOperationTrackRequests{0};
diff --git a/src/mongo/db/s/migration_destination_manager.cpp b/src/mongo/db/s/migration_destination_manager.cpp
index a6ce98edcb7..877cc863702 100644
--- a/src/mongo/db/s/migration_destination_manager.cpp
+++ b/src/mongo/db/s/migration_destination_manager.cpp
@@ -360,6 +360,7 @@ void MigrationDestinationManager::report(BSONObjBuilder& b,
     b.append("min", _min);
     b.append("max", _max);
     b.append("shardKeyPattern", _shardKeyPattern);
+    b.append(StartChunkCloneRequest::kSupportsCriticalSectionDuringCatchUp, true);
 
     b.append("state", stateToString(_state));
 
@@ -547,6 +548,24 @@ Status MigrationDestinationManager::startCommit(const MigrationSessionId& sessio
 
     stdx::unique_lock<Latch> lock(_mutex);
 
+    const auto convergenceTimeout =
+        Shard::kDefaultConfigCommandTimeout + Shard::kDefaultConfigCommandTimeout / 4;
+
+    // The donor may have started the commit while the recipient is still busy processing
+    // the last batch of mods sent in the catch up phase. Allow some time for synching up.
+    auto deadline = Date_t::now() + convergenceTimeout;
+
+    while (_state == CATCHUP) {
+        if (stdx::cv_status::timeout ==
+            _stateChangedCV.wait_until(lock, deadline.toSystemTimePoint())) {
+            return {ErrorCodes::CommandFailed,
+                    str::stream() << "startCommit timed out waiting for the catch up completion. "
+                                  << "Sender's session is " << sessionId.toString()
+                                  << ". Current session is "
+                                  << (_sessionId ? _sessionId->toString() : "none.")};
+        }
+    }
+
     if (_state != STEADY) {
         return {ErrorCodes::CommandFailed,
                 str::stream() << "Migration startCommit attempted when not in STEADY state."
@@ -574,8 +593,7 @@ Status MigrationDestinationManager::startCommit(const MigrationSessionId& sessio
 
     // Assigning a timeout slightly higher than the one used for network requests to the config
     // server. Enough time to retry at least once in case of network failures (SERVER-51397).
-    auto const deadline = Date_t::now() + Shard::kDefaultConfigCommandTimeout +
-        Shard::kDefaultConfigCommandTimeout / 4;
+    deadline = Date_t::now() + convergenceTimeout;
     while (_sessionId) {
         if (stdx::cv_status::timeout ==
             _isActiveCV.wait_until(lock, deadline.toSystemTimePoint())) {
@@ -1174,6 +1192,7 @@ void MigrationDestinationManager::_migrateDriver(OperationContext* outerOpCtx) {
             const auto& mods = res.response;
 
             if (mods["size"].number() == 0) {
+                // There are no more pending modifications to be applied. End the catchup phase
                 break;
             }
 
diff --git a/src/mongo/db/s/sharding_runtime_d_params.idl b/src/mongo/db/s/sharding_runtime_d_params.idl
index 58a7f95de2f..ef964aac54d 100644
--- a/src/mongo/db/s/sharding_runtime_d_params.idl
+++ b/src/mongo/db/s/sharding_runtime_d_params.idl
@@ -84,6 +84,19 @@ server_parameters:
         cpp_varname: migrationLockAcquisitionMaxWaitMS
         default: 500
 
+    minCatchUpPercentageBeforeBlockingWrites:
+        description: >-
+          The maximum percentage of untrasferred chunk mods at the end of a catch up iteration
+          that may be deferred to the next phase of the migration protocol
+          (where new writes get blocked).
+        set_at: [startup]
+        cpp_vartype: int
+        cpp_varname: minCatchUpPercentageBeforeBlockingWrites
+        validator:
+          gte: 0
+          lte: 100
+        default: 10
+
     orphanCleanupDelaySecs:
         description: 'How long to wait before starting cleanup of an emigrated chunk range.'
         set_at: [startup, runtime]
diff --git a/src/mongo/db/s/start_chunk_clone_request.h b/src/mongo/db/s/start_chunk_clone_request.h
index c88986887fb..c6ecba1f839 100644
--- a/src/mongo/db/s/start_chunk_clone_request.h
+++ b/src/mongo/db/s/start_chunk_clone_request.h
@@ -49,6 +49,9 @@ class StatusWith;
  */
 class StartChunkCloneRequest {
 public:
+    static constexpr auto kSupportsCriticalSectionDuringCatchUp =
+        "supportsCriticalSectionDuringCatchUp"_sd;
+
     /**
      * Parses the input command and produces a request corresponding to its arguments.
      */