SERVER-61754 dbCheck does not hold strong locks during batches

The dbCheck command accepts an optional (default true) parameter,
'snapshotRead', that uses point-in-time reads to check each batch. When
true, dbCheck does not block writes. When set to false, dbCheck reverts to
blocking writes.

2 files changed, 15 insertions, 13 deletions

diff --git a/src/mongo/db/concurrency/deferred_writer.cpp b/src/mongo/db/concurrency/deferred_writer.cpp
index be9facf84ec..9c6abe5ff47 100644
--- a/src/mongo/db/concurrency/deferred_writer.cpp
+++ b/src/mongo/db/concurrency/deferred_writer.cpp
@@ -48,7 +48,6 @@ auto kLogInterval = stdx::chrono::minutes(1);
 void DeferredWriter::_logFailure(const Status& status) {
     if (TimePoint::clock::now() - _lastLogged > kLogInterval) {
         LOGV2(20516,
-              "Unable to write to collection {namespace}: {error}",
               "Unable to write to collection",
               "namespace"_attr = _nss.toString(),
               "error"_attr = status);
@@ -100,14 +99,13 @@ StatusWith<std::unique_ptr<AutoGetCollection>> DeferredWriter::_getCollection(
     return std::move(agc);
 }
 
-void DeferredWriter::_worker(InsertStatement stmt) try {
+Status DeferredWriter::_worker(InsertStatement stmt) noexcept try {
     auto uniqueOpCtx = Client::getCurrent()->makeOperationContext();
     OperationContext* opCtx = uniqueOpCtx.get();
     auto result = _getCollection(opCtx);
 
     if (!result.isOK()) {
-        _logFailure(result.getStatus());
-        return;
+        return result.getStatus();
     }
 
     auto agc = std::move(result.getValue());
@@ -128,13 +126,9 @@ void DeferredWriter::_worker(InsertStatement stmt) try {
     stdx::lock_guard<Latch> lock(_mutex);
 
     _numBytes -= stmt.doc.objsize();
-
-    // If a write to a deferred collection fails, periodically tell the log.
-    if (!status.isOK()) {
-        _logFailure(status);
-    }
+    return status;
 } catch (const DBException& e) {
-    _logFailure(e.toStatus());
+    return e.toStatus();
 }
 
 DeferredWriter::DeferredWriter(NamespaceString nss, CollectionOptions opts, int64_t maxSize)
@@ -155,7 +149,12 @@ void DeferredWriter::startup(std::string workerName) {
     options.threadNamePrefix = workerName;
     options.minThreads = 0;
     options.maxThreads = 1;
-    options.onCreateThread = [](const std::string& name) { Client::initThread(name); };
+    options.onCreateThread = [](const std::string& name) {
+        Client::initThread(name);
+
+        stdx::lock_guard<Client> lk(cc());
+        cc().setSystemOperationKillableByStepdown(lk);
+    };
     _pool = std::make_unique<ThreadPool>(options);
     _pool->startup();
 }
@@ -190,7 +189,10 @@ bool DeferredWriter::insertDocument(BSONObj obj) {
     _pool->schedule([this, obj](auto status) {
         fassert(40588, status);
 
-        _worker(InsertStatement(obj.getOwned()));
+        auto workerStatus = _worker(InsertStatement(obj.getOwned()));
+        if (!workerStatus.isOK()) {
+            _logFailure(workerStatus);
+        }
     });
     return true;
 }
diff --git a/src/mongo/db/concurrency/deferred_writer.h b/src/mongo/db/concurrency/deferred_writer.h
index 0ac8238fa8d..1f45fbb8d15 100644
--- a/src/mongo/db/concurrency/deferred_writer.h
+++ b/src/mongo/db/concurrency/deferred_writer.h
@@ -136,7 +136,7 @@ private:
     /**
      * The method that the worker thread will run.
      */
-    void _worker(InsertStatement stmt);
+    Status _worker(InsertStatement stmt) noexcept;
 
     /**
      * The options for the collection, in case we need to create it.