SERVER-36528 Implement $planCacheStats agg source.

The $planCacheStats agg metadata source should typically be used
instead of the older 'planCacheListPlans' and
'planCacheListQueryShapes' commands (although these plan cache
commands are still fully supported).

Lookup by 'queryHash' can be achieved via a $planCacheStats-$match
pipeline.

1 files changed, 144 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/document_source_plan_cache_stats.h b/src/mongo/db/pipeline/document_source_plan_cache_stats.h
new file mode 100644
index 00000000000..f6d9f8dce0e
--- /dev/null
+++ b/src/mongo/db/pipeline/document_source_plan_cache_stats.h
@@ -0,0 +1,144 @@
+/**
+ * Copyright (C) 2018 MongoDB Inc.
+ *
+ * This program is free software: you can redistribute it and/or  modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * As a special exception, the copyright holders give permission to link the
+ * code of portions of this program with the OpenSSL library under certain
+ * conditions as described in each individual source file and distribute
+ * linked combinations including the program with the OpenSSL library. You
+ * must comply with the GNU Affero General Public License in all respects
+ * for all of the code used other than as permitted herein. If you modify
+ * file(s) with this exception, you may extend this exception to your
+ * version of the file(s), but you are not obligated to do so. If you do not
+ * wish to do so, delete this exception statement from your version. If you
+ * delete this exception statement from all source files in the program,
+ * then also delete it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/db/pipeline/document_source.h"
+#include "mongo/db/pipeline/document_source_match.h"
+
+namespace mongo {
+
+class DocumentSourcePlanCacheStats final : public DocumentSource {
+public:
+    static const char* kStageName;
+
+    class LiteParsed final : public LiteParsedDocumentSource {
+    public:
+        static std::unique_ptr<LiteParsed> parse(const AggregationRequest& request,
+                                                 const BSONElement& spec) {
+            return stdx::make_unique<LiteParsed>(request.getNamespaceString());
+        }
+
+        explicit LiteParsed(NamespaceString nss) : _nss(std::move(nss)) {}
+
+        stdx::unordered_set<NamespaceString> getInvolvedNamespaces() const override {
+            // There are no foreign collections.
+            return stdx::unordered_set<NamespaceString>();
+        }
+
+        PrivilegeVector requiredPrivileges(bool isMongos) const override {
+            return {Privilege(ResourcePattern::forExactNamespace(_nss), ActionType::planCacheRead)};
+        }
+
+        bool isInitialSource() const final {
+            return true;
+        }
+
+        bool allowedToForwardFromMongos() const override {
+            // $planCacheStats must be run locally on a mongod.
+            return false;
+        }
+
+        bool allowedToPassthroughFromMongos() const override {
+            // $planCacheStats must be run locally on a mongod.
+            return false;
+        }
+
+        void assertSupportsReadConcern(const repl::ReadConcernArgs& readConcern) const {
+            uassert(ErrorCodes::InvalidOptions,
+                    str::stream() << "Aggregation stage " << kStageName
+                                  << " requires read concern local but found "
+                                  << readConcern.toString(),
+                    readConcern.getLevel() == repl::ReadConcernLevel::kLocalReadConcern);
+        }
+
+    private:
+        const NamespaceString _nss;
+    };
+
+    static boost::intrusive_ptr<DocumentSource> createFromBson(
+        BSONElement elem, const boost::intrusive_ptr<ExpressionContext>& pExpCtx);
+
+    virtual ~DocumentSourcePlanCacheStats() = default;
+
+    GetNextResult getNext() override;
+
+    StageConstraints constraints(
+        Pipeline::SplitState = Pipeline::SplitState::kUnsplit) const override {
+        StageConstraints constraints{StreamType::kStreaming,
+                                     PositionRequirement::kFirst,
+                                     // This stage must run on a mongod, and will fail at parse time
+                                     // if an attempt is made to run it on mongos.
+                                     HostTypeRequirement::kAnyShard,
+                                     DiskUseRequirement::kNoDiskUse,
+                                     FacetRequirement::kNotAllowed,
+                                     TransactionRequirement::kNotAllowed};
+
+        constraints.requiresInputDocSource = false;
+        return constraints;
+    }
+
+    const char* getSourceName() const override {
+        return kStageName;
+    }
+
+    /**
+     * Absorbs a subsequent $match, in order to avoid copying the entire contents of the plan cache
+     * prior to filtering.
+     */
+    Pipeline::SourceContainer::iterator doOptimizeAt(Pipeline::SourceContainer::iterator itr,
+                                                     Pipeline::SourceContainer* container) override;
+
+    void serializeToArray(
+        std::vector<Value>& array,
+        boost::optional<ExplainOptions::Verbosity> explain = boost::none) const override;
+
+private:
+    DocumentSourcePlanCacheStats(const boost::intrusive_ptr<ExpressionContext>& expCtx);
+
+    Value serialize(
+        boost::optional<ExplainOptions::Verbosity> explain = boost::none) const override {
+        MONGO_UNREACHABLE;  // Should call serializeToArray instead.
+    }
+
+    // The result set for this change is produced through the mongo process interface on the first
+    // call to getNext(), and then held by this data member.
+    std::vector<BSONObj> _results;
+
+    // Whether '_results' has been populated yet.
+    bool _haveRetrievedStats = false;
+
+    // Used to spool out '_results' as calls to getNext() are made.
+    std::vector<BSONObj>::iterator _resultsIter;
+
+    // $planCacheStats can push a match down into the plan cache layer, in order to avoid copying
+    // the entire contents of the cache.
+    boost::intrusive_ptr<DocumentSourceMatch> _absorbedMatch;
+};
+
+}  // namespace mongo