SERVER-15917 fix explain of write commands against a non-existent database

9 files changed, 174 insertions, 42 deletions

diff --git a/jstests/core/explain_null_collection.js b/jstests/core/explain_null_collection.js
new file mode 100644
index 00000000000..47d576e6fbf
--- /dev/null
+++ b/jstests/core/explain_null_collection.js
@@ -0,0 +1,104 @@
+// Test explain of various operations against non-existent databases and collections.
+
+var explain;
+var explainColl;
+
+//
+// Part 1: Non-existent database.
+//
+
+var explainMissingDb = db.getSiblingDB("explainMissingDb");
+
+// .find()
+explainMissingDb.dropDatabase();
+explain = explainMissingDb.collection.explain("executionStats").find().finish();
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .count()
+explainMissingDb.dropDatabase();
+explain = explainMissingDb.collection.explain("executionStats").count();
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .group()
+explainMissingDb.dropDatabase();
+explainColl = explainMissingDb.collection.explain("executionStats");
+explain = explainColl.group({key: "a", initial: {}, reduce: function() { } });
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .remove()
+explainMissingDb.dropDatabase();
+explain = explainMissingDb.collection.explain("executionStats").remove({a: 1});
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .update() with upsert: false
+explainMissingDb.dropDatabase();
+explainColl = explainMissingDb.collection.explain("executionStats");
+explain = explainColl.update({a: 1}, {b: 1});
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .update() with upsert: true
+explainMissingDb.dropDatabase();
+explainColl = explainMissingDb.collection.explain("executionStats");
+explain = explainColl.update({a: 1}, {b: 1}, {upsert: true});
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .aggregate()
+explainMissingDb.dropDatabase();
+explain = explainMissingDb.collection.explain("executionStats").aggregate([{$match: {a: 1}}]);
+assert.commandWorked(explain);
+
+//
+// Part 2: Non-existent collection.
+//
+
+var missingColl = db.explain_null_collection;
+
+// .find()
+missingColl.drop();
+explain = missingColl.explain("executionStats").find().finish();
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .count()
+missingColl.drop();
+explain = missingColl.explain("executionStats").count();
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .group()
+missingColl.drop();
+explainColl = missingColl.explain("executionStats");
+explain = explainColl.group({key: "a", initial: {}, reduce: function() { } });
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .remove()
+missingColl.drop();
+explain = missingColl.explain("executionStats").remove({a: 1});
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .update() with upsert: false
+missingColl.drop();
+explainColl = missingColl.explain("executionStats");
+explain = explainColl.update({a: 1}, {b: 1});
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .update() with upsert: true
+missingColl.drop();
+explainColl = missingColl.explain("executionStats");
+explain = explainColl.update({a: 1}, {b: 1}, {upsert: true});
+assert.commandWorked(explain);
+assert("executionStats" in explain);
+
+// .aggregate()
+missingColl.drop();
+explain = missingColl.explain("executionStats").aggregate([{$match: {a: 1}}]);
+assert.commandWorked(explain);
diff --git a/src/mongo/db/commands/write_commands/write_commands.cpp b/src/mongo/db/commands/write_commands/write_commands.cpp
index f1f7c2a3919..e20ff8c883a 100644
--- a/src/mongo/db/commands/write_commands/write_commands.cpp
+++ b/src/mongo/db/commands/write_commands/write_commands.cpp
@@ -47,6 +47,7 @@
 #include "mongo/db/repl/repl_coordinator_global.h"
 #include "mongo/db/server_parameters.h"
 #include "mongo/db/stats/counters.h"
+#include "mongo/s/d_state.h"
 
 namespace mongo {
 
@@ -214,11 +215,16 @@ namespace mongo {
 
             // Explains of write commands are read-only, but we take write locks so
             // that timing info is more accurate.
-            Lock::DBLock dlk(txn->lockState(), nsString.db(), MODE_IX);
-            Lock::CollectionLock colLock(txn->lockState(), nsString.ns(), MODE_IX);
-            Client::Context ctx(txn, nsString);
+            AutoGetDb autoDb( txn, nsString.db(), MODE_IX );
+            Lock::CollectionLock colLock( txn->lockState(), nsString.ns(), MODE_IX );
+
+            // We check the shard version explicitly here rather than using Client::Context,
+            // as Context can do implicit database creation if the db does not exist. We want
+            // explain to be a no-op that reports a trivial EOF plan against non-existent dbs
+            // or collections.
+            ensureShardVersionOKOrThrow( nsString.ns() );
 
-            Status prepInLockStatus = updateExecutor.prepareInLock(ctx.db());
+            Status prepInLockStatus = updateExecutor.prepareInLock( autoDb.getDb() );
             if ( !prepInLockStatus.isOK() ) {
                 return prepInLockStatus;
             }
@@ -255,12 +261,15 @@ namespace mongo {
             // Explains of write commands are read-only, but we take write locks so that timing
             // info is more accurate.
             AutoGetDb autoDb(txn, nsString.db(), MODE_IX);
-            if (!autoDb.getDb()) return Status::OK();
-
             Lock::CollectionLock colLock(txn->lockState(), nsString.ns(), MODE_IX);
-            Client::Context ctx(txn, nsString);
 
-            Status prepInLockStatus = deleteExecutor.prepareInLock(ctx.db());
+            // We check the shard version explicitly here rather than using Client::Context,
+            // as Context can do implicit database creation if the db does not exist. We want
+            // explain to be a no-op that reports a trivial EOF plan against non-existent dbs
+            // or collections.
+            ensureShardVersionOKOrThrow( nsString.ns() );
+
+            Status prepInLockStatus = deleteExecutor.prepareInLock( autoDb.getDb() );
             if ( !prepInLockStatus.isOK()) {
                 return prepInLockStatus;
             }
diff --git a/src/mongo/db/exec/update.cpp b/src/mongo/db/exec/update.cpp
index 59ba5593ad6..1c5c8ed4011 100644
--- a/src/mongo/db/exec/update.cpp
+++ b/src/mongo/db/exec/update.cpp
@@ -395,20 +395,17 @@ namespace mongo {
 
     UpdateStage::UpdateStage(const UpdateStageParams& params,
                              WorkingSet* ws,
-                             Database* db,
+                             Collection* collection,
                              PlanStage* child)
         : _params(params),
           _ws(ws),
-          _db(db),
+          _collection(collection),
           _child(child),
           _commonStats(kStageType),
           _updatedLocs(params.request->isMulti() ? new DiskLocSet() : NULL),
           _doc(params.driver->getDocument()) {
         // We are an update until we fall into the insert case.
         params.driver->setContext(ModifierInterface::ExecInfo::UPDATE_CONTEXT);
-
-        _collection = db->getCollection(params.request->getOpCtx(),
-                                        params.request->getNamespaceString().ns());
     }
 
     void UpdateStage::transformAndUpdate(BSONObj& oldObj, DiskLoc& loc) {
diff --git a/src/mongo/db/exec/update.h b/src/mongo/db/exec/update.h
index b543ebb5fdf..8e097bac999 100644
--- a/src/mongo/db/exec/update.h
+++ b/src/mongo/db/exec/update.h
@@ -28,7 +28,7 @@
 
 #pragma once
 
-#include "mongo/db/catalog/database.h"
+#include "mongo/db/catalog/collection.h"
 #include "mongo/db/exec/plan_stage.h"
 #include "mongo/db/jsobj.h"
 #include "mongo/db/ops/update_driver.h"
@@ -78,7 +78,7 @@ namespace mongo {
     public:
         UpdateStage(const UpdateStageParams& params,
                     WorkingSet* ws,
-                    Database* db,
+                    Collection* collection,
                     PlanStage* child);
 
         virtual bool isEOF();
@@ -135,8 +135,7 @@ namespace mongo {
         // Not owned by us.
         WorkingSet* _ws;
 
-        // Not owned by us.
-        Database* _db;
+        // Not owned by us. May be NULL.
         Collection* _collection;
 
         // Owned by us.
diff --git a/src/mongo/db/ops/delete_executor.cpp b/src/mongo/db/ops/delete_executor.cpp
index b66f153dac1..655babffb1c 100644
--- a/src/mongo/db/ops/delete_executor.cpp
+++ b/src/mongo/db/ops/delete_executor.cpp
@@ -110,10 +110,14 @@ namespace mongo {
             }
         }
 
-        // Note that 'collection' may by NULL in the case that the collection we are trying to
-        // delete from does not exist. NULL 'collection' is handled by getExecutorDelete(); we
-        // expect to get back a plan executor whose plan is a DeleteStage on top of an EOFStage.
-        Collection* collection = db->getCollection(_request->getOpCtx(), ns.ns());
+        // Note that 'collection' may by NULL in the case that the collection or database we are
+        // trying to delete from does not exist. NULL 'collection' is handled by
+        // getExecutorDelete(); we expect to get back a plan executor whose plan is a DeleteStage
+        // on top of an EOFStage.
+        Collection* collection = NULL;
+        if (db) {
+            collection = db->getCollection(_request->getOpCtx(), ns.ns());
+        }
 
         if (collection && collection->isCapped()) {
             return Status(ErrorCodes::IllegalOperation,
diff --git a/src/mongo/db/ops/update_executor.cpp b/src/mongo/db/ops/update_executor.cpp
index d403ea9edeb..cd9ea94dd80 100644
--- a/src/mongo/db/ops/update_executor.cpp
+++ b/src/mongo/db/ops/update_executor.cpp
@@ -111,7 +111,17 @@ namespace mongo {
 
         validateUpdate(nsString.ns().c_str(), _request->getUpdates(), _request->getQuery());
 
-        Collection* collection = db->getCollection(_request->getOpCtx(), nsString.ns());
+        // The batch executor is responsible for creating a database if this update is being
+        // performed against a non-existent database. However, it is possible for either the
+        // database or collection to be NULL for an explain. In this case, the explain is
+        // a no-op which returns a trivial EOF plan.
+        Collection* collection = NULL;
+        if (db) {
+            collection = db->getCollection(_request->getOpCtx(), nsString.ns());
+        }
+        else {
+            invariant(_request->isExplain());
+        }
 
         // The update stage does not create its own collection.  As such, if the update is
         // an upsert, create the collection that the update stage inserts into beforehand.
@@ -177,14 +187,26 @@ namespace mongo {
         Status getExecStatus = Status::OK();
         if (_canonicalQuery.get()) {
             // This is the regular path for when we have a CanonicalQuery.
-            getExecStatus = getExecutorUpdate(_request->getOpCtx(), db, _canonicalQuery.release(),
-                                              _request, &_driver, _opDebug, policy, &rawExec);
+            getExecStatus = getExecutorUpdate(_request->getOpCtx(),
+                                              collection,
+                                              _canonicalQuery.release(),
+                                              _request,
+                                              &_driver,
+                                              _opDebug,
+                                              policy,
+                                              &rawExec);
         }
         else {
             // This is the idhack fast-path for getting a PlanExecutor without doing the work
             // to create a CanonicalQuery.
-            getExecStatus = getExecutorUpdate(_request->getOpCtx(), db, nsString.ns(), _request,
-                                              &_driver, _opDebug, policy, &rawExec);
+            getExecStatus = getExecutorUpdate(_request->getOpCtx(),
+                                              collection,
+                                              nsString.ns(),
+                                              _request,
+                                              &_driver,
+                                              _opDebug,
+                                              policy,
+                                              &rawExec);
         }
 
         if (!getExecStatus.isOK()) {
diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index 8c474c0f948..a26118061ec 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -545,7 +545,7 @@ namespace mongo {
     //
 
     Status getExecutorUpdate(OperationContext* txn,
-                             Database* db,
+                             Collection* collection,
                              CanonicalQuery* rawCanonicalQuery,
                              const UpdateRequest* request,
                              UpdateDriver* driver,
@@ -554,8 +554,6 @@ namespace mongo {
                              PlanExecutor** execOut) {
         auto_ptr<CanonicalQuery> canonicalQuery(rawCanonicalQuery);
         auto_ptr<WorkingSet> ws(new WorkingSet());
-        Collection* collection = db->getCollection(request->getOpCtx(),
-                                                   request->getNamespaceString().ns());
 
         PlanStage* root;
         QuerySolution* querySolution;
@@ -568,7 +566,7 @@ namespace mongo {
         invariant(root);
         UpdateStageParams updateStageParams(request, driver, opDebug);
         updateStageParams.canonicalQuery = rawCanonicalQuery;
-        root = new UpdateStage(updateStageParams, ws.get(), db, root);
+        root = new UpdateStage(updateStageParams, ws.get(), collection, root);
         // We must have a tree of stages in order to have a valid plan executor, but the query
         // solution may be null. Takes ownership of all args other than 'collection' and 'txn'
         return PlanExecutor::make(txn,
@@ -582,7 +580,7 @@ namespace mongo {
     }
 
     Status getExecutorUpdate(OperationContext* txn,
-                             Database* db,
+                             Collection* collection,
                              const std::string& ns,
                              const UpdateRequest* request,
                              UpdateDriver* driver,
@@ -590,8 +588,6 @@ namespace mongo {
                              PlanExecutor::YieldPolicy yieldPolicy,
                              PlanExecutor** execOut) {
         auto_ptr<WorkingSet> ws(new WorkingSet());
-        Collection* collection = db->getCollection(request->getOpCtx(),
-                                                   request->getNamespaceString().ns());
         const BSONObj& unparsedQuery = request->getQuery();
 
         UpdateStageParams updateStageParams(request, driver, opDebug);
@@ -601,7 +597,7 @@ namespace mongo {
             // UpdateStage, so in this case we put an UpdateStage on top of an EOFStage.
             LOG(2) << "Collection " << ns << " does not exist."
                    << " Using EOF stage: " << unparsedQuery.toString();
-            UpdateStage* updateStage = new UpdateStage(updateStageParams, ws.get(), db,
+            UpdateStage* updateStage = new UpdateStage(updateStageParams, ws.get(), collection,
                                                        new EOFStage());
             return PlanExecutor::make(txn, ws.release(), updateStage, ns, yieldPolicy, execOut);
         }
@@ -612,7 +608,8 @@ namespace mongo {
 
             PlanStage* idHackStage = new IDHackStage(txn, collection, unparsedQuery["_id"].wrap(),
                                                      ws.get());
-            UpdateStage* root = new UpdateStage(updateStageParams, ws.get(), db, idHackStage);
+            UpdateStage* root = new UpdateStage(updateStageParams, ws.get(), collection,
+                                                idHackStage);
             return PlanExecutor::make(txn, ws.release(), root, collection, yieldPolicy, execOut);
         }
 
@@ -627,7 +624,8 @@ namespace mongo {
             return status;
 
         // Takes ownership of 'cq'.
-        return getExecutorUpdate(txn, db, cq, request, driver, opDebug, yieldPolicy, execOut);
+        return getExecutorUpdate(txn, collection, cq, request, driver, opDebug, yieldPolicy,
+                                 execOut);
     }
 
     //
diff --git a/src/mongo/db/query/get_executor.h b/src/mongo/db/query/get_executor.h
index 86e8156dfd6..6828dbe4b13 100644
--- a/src/mongo/db/query/get_executor.h
+++ b/src/mongo/db/query/get_executor.h
@@ -37,7 +37,6 @@
 namespace mongo {
 
     class Collection;
-    class Database;
     struct CountRequest;
 
     struct GroupRequest;
@@ -192,7 +191,7 @@ namespace mongo {
      * If the query cannot be executed, returns a Status indicating why.
      */
     Status getExecutorUpdate(OperationContext* txn,
-                             Database* db,
+                             Collection* collection,
                              CanonicalQuery* rawCanonicalQuery,
                              const UpdateRequest* request,
                              UpdateDriver* driver,
@@ -212,7 +211,7 @@ namespace mongo {
      * If the query cannot be executed, returns a Status indicating why.
      */
     Status getExecutorUpdate(OperationContext* txn,
-                             Database* db,
+                             Collection* collection,
                              const std::string& ns,
                              const UpdateRequest* request,
                              UpdateDriver* driver,
diff --git a/src/mongo/dbtests/query_stage_update.cpp b/src/mongo/dbtests/query_stage_update.cpp
index b0e18879361..54357ebc954 100644
--- a/src/mongo/dbtests/query_stage_update.cpp
+++ b/src/mongo/dbtests/query_stage_update.cpp
@@ -185,7 +185,7 @@ namespace QueryStageUpdate {
                 CurOp& curOp = *c.curop();
                 OpDebug* opDebug = &curOp.debug();
                 UpdateDriver driver( (UpdateDriver::Options()) );
-                Database* db = ctx.ctx().db();
+                Collection* collection = ctx.getCollection();
 
                 // Collection should be empty.
                 ASSERT_EQUALS(0U, count(BSONObj()));
@@ -213,7 +213,7 @@ namespace QueryStageUpdate {
                 auto_ptr<EOFStage> eofStage(new EOFStage());
 
                 scoped_ptr<UpdateStage> updateStage(
-                    new UpdateStage(params, ws.get(), db, eofStage.release()));
+                    new UpdateStage(params, ws.get(), collection, eofStage.release()));
 
                 runUpdate(updateStage.get());
             }
@@ -292,7 +292,7 @@ namespace QueryStageUpdate {
                     new CollectionScan(&_txn, collScanParams, ws.get(), cq->root()));
 
                 scoped_ptr<UpdateStage> updateStage(
-                    new UpdateStage(updateParams, ws.get(), db, cs.release()));
+                    new UpdateStage(updateParams, ws.get(), coll, cs.release()));
 
                 const UpdateStats* stats =
                     static_cast<const UpdateStats*>(updateStage->getSpecificStats());