SERVER-19758 add support for "executionStats" and "allPlansExecution" to agg explain

Like other explainable commands, aggregate can now be
explained using the explain command, e.g.
db.runCommand({explain: {aggregate: ...}, verbosity:
"executionStats"}). The existing explain:true flag
corresponds to "queryPlanner" mode and is still supported.
However, explain:true cannot be specified when explaining
aggregate via the explain command.

Additional execution information is provided only in the
$cursor section of the aggregation explain output. Having
aggregation stages themselves track and report execution
info is further work.

1 files changed, 35 insertions, 10 deletions

diff --git a/src/mongo/db/pipeline/parsed_add_fields_test.cpp b/src/mongo/db/pipeline/parsed_add_fields_test.cpp
index d7b68afca1d..c9adc2b1a36 100644
--- a/src/mongo/db/pipeline/parsed_add_fields_test.cpp
+++ b/src/mongo/db/pipeline/parsed_add_fields_test.cpp
@@ -176,8 +176,13 @@ TEST(ParsedAddFieldsSerialize, SerializesToCorrectForm) {
         fromjson("{a: {$add: [\"$a\", {$const: 2}]}, b: {d: {$const: 3}}, x: {y: {$const: 4}}}"));
 
     // Should be the same if we're serializing for explain or for internal use.
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(false));
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(true));
+    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(boost::none));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kQueryPlanner));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecStats));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecAllPlans));
 }
 
 // Verify that serialize treats the _id field as any other field: including when explicity included.
@@ -190,8 +195,13 @@ TEST(ParsedAddFieldsSerialize, AddsIdToSerializeWhenExplicitlyIncluded) {
     auto expectedSerialization = Document(fromjson("{_id: {$const: false}}"));
 
     // Should be the same if we're serializing for explain or for internal use.
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(false));
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(true));
+    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(boost::none));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kQueryPlanner));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecStats));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecAllPlans));
 }
 
 // Verify that serialize treats the _id field as any other field: excluded when not explicitly
@@ -207,8 +217,13 @@ TEST(ParsedAddFieldsSerialize, OmitsIdFromSerializeWhenNotIncluded) {
     auto expectedSerialization = Document(fromjson("{a: {$const: true}}"));
 
     // Should be the same if we're serializing for explain or for internal use.
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(false));
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(true));
+    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(boost::none));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kQueryPlanner));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecStats));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecAllPlans));
 }
 
 // Verify that the $addFields stage optimizes expressions into simpler forms when possible.
@@ -220,8 +235,13 @@ TEST(ParsedAddFieldsOptimize, OptimizesTopLevelExpressions) {
     auto expectedSerialization = Document{{"a", Document{{"$const", 3}}}};
 
     // Should be the same if we're serializing for explain or for internal use.
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(false));
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(true));
+    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(boost::none));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kQueryPlanner));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecStats));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecAllPlans));
 }
 
 // Verify that the $addFields stage optimizes expressions even when they are nested.
@@ -233,8 +253,13 @@ TEST(ParsedAddFieldsOptimize, ShouldOptimizeNestedExpressions) {
     auto expectedSerialization = Document{{"a", Document{{"b", Document{{"$const", 3}}}}}};
 
     // Should be the same if we're serializing for explain or for internal use.
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(false));
-    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(true));
+    ASSERT_DOCUMENT_EQ(expectedSerialization, addition.serialize(boost::none));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kQueryPlanner));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecStats));
+    ASSERT_DOCUMENT_EQ(expectedSerialization,
+                       addition.serialize(ExplainOptions::Verbosity::kExecAllPlans));
 }
 
 //