SERVER-12180 Clean up dependency tracking code

Behavior changes (none effect semantics of pipeline, just implementation):
* We now track what we know about fields and metadata separately. This allows
  us to prove that we won't need the text score if we see a $group or $out.
* If we don't need any fields from the source document we now use a projection
  that will return an empty document, or just the text score if it is needed.
  We used to use {_id: 0} which returned all other fields.

Code organization changes:
* Dependencies are now tracked using a dedicated struct rather than a
  set<string> with some magic strings.
* ParsedDeps is now a proper class rather than a typedef.
* Removed ExpressionFieldPath::_baseVar since its former role is fulfilled
  better by _variable.

1 files changed, 77 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/dependencies.h b/src/mongo/db/pipeline/dependencies.h
new file mode 100644
index 00000000000..47f8f46c432
--- /dev/null
+++ b/src/mongo/db/pipeline/dependencies.h
@@ -0,0 +1,77 @@
+/**
+ * Copyright (c) 2014 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 <boost/optional.hpp>
+#include <set>
+#include <string>
+
+#include "mongo/db/pipeline/document.h"
+
+namespace mongo {
+    class ParsedDeps;
+
+    /**
+     * This struct allows components in an agg pipeline to report what they need from their input.
+     */
+    struct DepsTracker {
+        DepsTracker()
+            : needWholeDocument(false)
+            , needTextScore(false)
+        {}
+
+        /**
+         * Returns a projection object covering the dependencies tracked by this class.
+         */
+        BSONObj toProjection() const;
+
+        boost::optional<ParsedDeps> toParsedDeps() const;
+
+        std::set<std::string> fields; // names of needed fields in dotted notation
+        bool needWholeDocument; // if true, ignore fields and assume the whole document is needed
+        bool needTextScore;
+    };
+
+    /**
+     * This class is designed to quickly extract the needed fields from a BSONObj into a Document.
+     * It should only be created by a call to DepsTracker::ParsedDeps
+     */
+    class ParsedDeps {
+    public:
+        Document extractFields(const BSONObj& input) const;
+
+    private:
+        friend struct DepsTracker; // so it can call constructor
+        explicit ParsedDeps(const Document& fields)
+            : _fields(fields)
+        {}
+
+        Document _fields;
+    };
+}