SERVER-43821 Make PlanStage and PlanExecutor return errors by throwing

This eliminates the need for the FAILURE status codes in
PlanStage and PlanExecutor, and brings query execution's
error reporting more in line with that of the rest of the
server. It also makes it easier for future implementations
of PlanExecutor to comply with the interface.

1 files changed, 33 insertions, 44 deletions

diff --git a/src/mongo/db/pipeline/document_source_cursor.cpp b/src/mongo/db/pipeline/document_source_cursor.cpp
index 6125fc6746b..d4e75b1d2b3 100644
--- a/src/mongo/db/pipeline/document_source_cursor.cpp
+++ b/src/mongo/db/pipeline/document_source_cursor.cpp
@@ -134,58 +134,47 @@ void DocumentSourceCursor::loadBatch() {
 
     PlanExecutor::ExecState state;
     Document resultObj;
-    {
-        AutoGetCollectionForRead autoColl(pExpCtx->opCtx, _exec->nss());
-        uassertStatusOK(repl::ReplicationCoordinator::get(pExpCtx->opCtx)
-                            ->checkCanServeReadsFor(pExpCtx->opCtx, _exec->nss(), true));
-
-        _exec->restoreState();
-
-        {
-            ON_BLOCK_EXIT([this] { recordPlanSummaryStats(); });
-
-            while ((state = _exec->getNext(&resultObj, nullptr)) == PlanExecutor::ADVANCED) {
-                _currentBatch.enqueue(transformDoc(std::move(resultObj)));
-
-                // As long as we're waiting for inserts, we shouldn't do any batching at this level
-                // we need the whole pipeline to see each document to see if we should stop waiting.
-                if (awaitDataState(pExpCtx->opCtx).shouldWaitForInserts ||
-                    static_cast<long long>(_currentBatch.memUsageBytes()) >
-                        internalDocumentSourceCursorBatchSizeBytes.load()) {
-                    // End this batch and prepare PlanExecutor for yielding.
-                    _exec->saveState();
-                    return;
-                }
-            }
-            // Special case for tailable cursor -- EOF doesn't preclude more results, so keep
-            // the PlanExecutor alive.
-            if (state == PlanExecutor::IS_EOF && pExpCtx->isTailableAwaitData()) {
+
+    AutoGetCollectionForRead autoColl(pExpCtx->opCtx, _exec->nss());
+    uassertStatusOK(repl::ReplicationCoordinator::get(pExpCtx->opCtx)
+                        ->checkCanServeReadsFor(pExpCtx->opCtx, _exec->nss(), true));
+
+    _exec->restoreState();
+
+    try {
+        ON_BLOCK_EXIT([this] { recordPlanSummaryStats(); });
+
+        while ((state = _exec->getNext(&resultObj, nullptr)) == PlanExecutor::ADVANCED) {
+            _currentBatch.enqueue(transformDoc(std::move(resultObj)));
+
+            // As long as we're waiting for inserts, we shouldn't do any batching at this level we
+            // need the whole pipeline to see each document to see if we should stop waiting.
+            if (awaitDataState(pExpCtx->opCtx).shouldWaitForInserts ||
+                static_cast<long long>(_currentBatch.memUsageBytes()) >
+                    internalDocumentSourceCursorBatchSizeBytes.load()) {
+                // End this batch and prepare PlanExecutor for yielding.
                 _exec->saveState();
                 return;
             }
         }
 
-        // If we got here, there won't be any more documents, so destroy our PlanExecutor. Note we
-        // must hold a collection lock to destroy '_exec', but we can only assume that our locks are
-        // still held if '_exec' did not end in an error. If '_exec' encountered an error during a
-        // yield, the locks might be yielded.
-        if (state != PlanExecutor::FAILURE) {
-            cleanupExecutor();
-        }
-    }
+        invariant(state == PlanExecutor::IS_EOF);
 
-    switch (state) {
-        case PlanExecutor::ADVANCED:
-        case PlanExecutor::IS_EOF:
-            return;  // We've reached our limit or exhausted the cursor.
-        case PlanExecutor::FAILURE: {
-            _execStatus = WorkingSetCommon::getMemberObjectStatus(resultObj).withContext(
-                "Error in $cursor stage");
-            uassertStatusOK(_execStatus);
+        // Special case for tailable cursor -- EOF doesn't preclude more results, so keep the
+        // PlanExecutor alive.
+        if (pExpCtx->isTailableAwaitData()) {
+            _exec->saveState();
+            return;
         }
-        default:
-            MONGO_UNREACHABLE;
+    } catch (...) {
+        // Record error details before re-throwing the exception.
+        _execStatus = exceptionToStatus().withContext("Error in $cursor stage");
+        throw;
     }
+
+    // If we got here, there won't be any more documents, so destroy our PlanExecutor. Note we must
+    // hold a collection lock to destroy '_exec'.
+    cleanupExecutor();
 }
 
 void DocumentSourceCursor::_updateOplogTimestamp() {