1 files changed, 79 insertions, 12 deletions

diff --git a/src/mongo/db/pipeline/parsed_aggregation_projection.h b/src/mongo/db/pipeline/parsed_aggregation_projection.h
index a3420fdb954..6e8d9100845 100644
--- a/src/mongo/db/pipeline/parsed_aggregation_projection.h
+++ b/src/mongo/db/pipeline/parsed_aggregation_projection.h
@@ -28,26 +28,89 @@
 
 #pragma once
 
+#include "mongo/platform/basic.h"
+
 #include <boost/intrusive_ptr.hpp>
 #include <memory>
 
+#include "mongo/bson/bsonelement.h"
+#include "mongo/db/pipeline/document_source.h"
+#include "mongo/db/pipeline/field_path.h"
+
 namespace mongo {
 
 class BSONObj;
 class Document;
-struct DepsTracker;
 struct ExpressionContext;
 
 namespace parsed_aggregation_projection {
 
-enum class ProjectionType { kExclusion, kInclusion };
+enum class ProjectionType { kExclusion, kInclusion, kComputed };
+
+/**
+ * This class ensures that the specification was valid: that none of the paths specified conflict
+ * with one another, that there is at least one field, etc. Here "projection" includes both
+ * $project specifications and $addFields specifications.
+ */
+class ProjectionSpecValidator {
+public:
+    /**
+     * Returns a Status: either a Status::OK() if the specification is valid for a projection, or a
+     * non-OK Status, error number, and message with why not.
+     */
+    static Status validate(const BSONObj& spec);
+
+private:
+    ProjectionSpecValidator(const BSONObj& spec) : _rawObj(spec) {}
+
+    /**
+     * Uses '_seenPaths' to see if 'path' conflicts with any paths that have already been specified.
+     *
+     * For example, a user is not allowed to specify {'a': 1, 'a.b': 1}, or some similar conflicting
+     * paths.
+     */
+    Status ensurePathDoesNotConflictOrThrow(StringData path);
+
+    /**
+     * Returns the relevant error if an invalid projection specification is detected.
+     */
+    Status validate();
+
+    /**
+     * Parses a single BSONElement. 'pathToElem' should include the field name of 'elem'.
+     *
+     * Delegates to parseSubObject() if 'elem' is an object. Otherwise adds the full path to 'elem'
+     * to '_seenPaths'.
+     *
+     * Calls ensurePathDoesNotConflictOrThrow with the path to this element, which sets the _status
+     * appropriately for conflicting path specifications.
+     */
+    Status parseElement(const BSONElement& elem, const FieldPath& pathToElem);
+
+    /**
+     * Traverses 'thisLevelSpec', parsing each element in turn.
+     *
+     * Sets _status appropriately if any paths conflict with each other or existing paths,
+     * 'thisLevelSpec' contains a dotted path, or if 'thisLevelSpec' represents an invalid
+     * expression.
+     */
+    Status parseNestedObject(const BSONObj& thisLevelSpec, const FieldPath& prefix);
+
+    // The original object. Used to generate more helpful error messages.
+    const BSONObj& _rawObj;
+
+    // Tracks which paths we've seen to ensure no two paths conflict with each other.
+    // Can be a vector since we iterate through it.
+    std::vector<std::string> _seenPaths;
+};
 
 /**
- * A ParsedAggregationProjection is responsible for parsing and executing a projection. It
+ * A ParsedAggregationProjection is responsible for parsing and executing a $project. It
  * represents either an inclusion or exclusion projection. This is the common interface between the
  * two types of projections.
  */
-class ParsedAggregationProjection {
+class ParsedAggregationProjection
+    : public DocumentSourceSingleDocumentTransformation::TransformerInterface {
 public:
     /**
      * Main entry point for a ParsedAggregationProjection.
@@ -72,11 +135,6 @@ public:
     virtual void parse(const BSONObj& spec) = 0;
 
     /**
-     * Serialize this projection.
-     */
-    virtual Document serialize(bool explain = false) const = 0;
-
-    /**
      * Optimize any expressions contained within this projection.
      */
     virtual void optimize() {}
@@ -89,15 +147,24 @@ public:
     /**
      * Add any dependencies needed by this projection or any sub-expressions to 'deps'.
      */
-    virtual void addDependencies(DepsTracker* deps) const {}
+    virtual DocumentSource::GetDepsReturn addDependencies(DepsTracker* deps) const {
+        return DocumentSource::NOT_SUPPORTED;
+    }
 
     /**
-     * Apply the projection to 'input'.
+     * Apply the projection transformation.
      */
-    virtual Document applyProjection(Document input) const = 0;
+    Document applyTransformation(Document input) {
+        return applyProjection(input);
+    }
 
 protected:
     ParsedAggregationProjection() = default;
+
+    /**
+     * Apply the projection to 'input'.
+     */
+    virtual Document applyProjection(Document input) const = 0;
 };
 }  // namespace parsed_aggregation_projection
 }  // namespace mongo