SERVER-38731 Implement ability to specify sync source read preference in initial sync

4 files changed, 84 insertions, 76 deletions

diff --git a/src/mongo/client/SConscript b/src/mongo/client/SConscript
index 8d216c960d5..51c3df426c9 100644
--- a/src/mongo/client/SConscript
+++ b/src/mongo/client/SConscript
@@ -33,6 +33,7 @@ env.Library(
     source=[
         'read_preference.cpp',
         env.Idlc('hedging_mode.idl')[0],
+        env.Idlc('read_preference.idl')[0],
     ],
     LIBDEPS=[
         '$BUILD_DIR/mongo/bson/util/bson_extract',
diff --git a/src/mongo/client/read_preference.cpp b/src/mongo/client/read_preference.cpp
index 5c686a839c6..3d52248f223 100644
--- a/src/mongo/client/read_preference.cpp
+++ b/src/mongo/client/read_preference.cpp
@@ -50,49 +50,6 @@ const char kTagsFieldName[] = "tags";
 const char kMaxStalenessSecondsFieldName[] = "maxStalenessSeconds";
 const char kHedgeFieldName[] = "hedge";
 
-const char kPrimaryOnly[] = "primary";
-const char kPrimaryPreferred[] = "primaryPreferred";
-const char kSecondaryOnly[] = "secondary";
-const char kSecondaryPreferred[] = "secondaryPreferred";
-const char kNearest[] = "nearest";
-
-StringData readPreferenceName(ReadPreference pref) {
-    switch (pref) {
-        case ReadPreference::PrimaryOnly:
-            return StringData(kPrimaryOnly);
-        case ReadPreference::PrimaryPreferred:
-            return StringData(kPrimaryPreferred);
-        case ReadPreference::SecondaryOnly:
-            return StringData(kSecondaryOnly);
-        case ReadPreference::SecondaryPreferred:
-            return StringData(kSecondaryPreferred);
-        case ReadPreference::Nearest:
-            return StringData(kNearest);
-        default:
-            MONGO_UNREACHABLE;
-    }
-}
-
-StatusWith<ReadPreference> parseReadPreferenceMode(StringData prefStr) {
-    if (prefStr == kPrimaryOnly) {
-        return ReadPreference::PrimaryOnly;
-    } else if (prefStr == kPrimaryPreferred) {
-        return ReadPreference::PrimaryPreferred;
-    } else if (prefStr == kSecondaryOnly) {
-        return ReadPreference::SecondaryOnly;
-    } else if (prefStr == kSecondaryPreferred) {
-        return ReadPreference::SecondaryPreferred;
-    } else if (prefStr == kNearest) {
-        return ReadPreference::Nearest;
-    }
-    return Status(ErrorCodes::FailedToParse,
-                  str::stream() << "Could not parse $readPreference mode '" << prefStr
-                                << "'. Only the modes '" << kPrimaryOnly << "', '"
-                                << kPrimaryPreferred << "', '" << kSecondaryOnly << "', '"
-                                << kSecondaryPreferred << "', and '" << kNearest
-                                << "' are supported.");
-}
-
 // Slight kludge here: if we weren't passed a TagSet, we default to the empty
 // TagSet if ReadPreference is Primary, or the default (wildcard) TagSet otherwise.
 // This maintains compatibility with existing code, while preserving the ability to round
@@ -108,6 +65,14 @@ TagSet defaultTagSetForMode(ReadPreference mode) {
 
 }  // namespace
 
+Status validateReadPreferenceMode(const std::string& prefStr) {
+    try {
+        ReadPreference_parse(IDLParserErrorContext(kModeFieldName), prefStr);
+    } catch (DBException& e) {
+        return e.toStatus();
+    }
+    return Status::OK();
+}
 
 /**
  * Replica set refresh period on the task executor.
@@ -157,11 +122,19 @@ StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromInnerBSON(const BSO
     }
 
     ReadPreference mode;
-    auto swReadPrefMode = parseReadPreferenceMode(modeStr);
-    if (!swReadPrefMode.isOK()) {
-        return swReadPrefMode.getStatus();
+    try {
+        mode = ReadPreference_parse(IDLParserErrorContext(kModeFieldName), modeStr);
+    } catch (DBException& e) {
+        return e.toStatus().withContext(
+            str::stream() << "Could not parse $readPreference mode '" << modeStr
+                          << "'. Only the modes '"
+                          << ReadPreference_serializer(ReadPreference::PrimaryOnly) << "', '"
+                          << ReadPreference_serializer(ReadPreference::PrimaryPreferred) << "', '"
+                          << ReadPreference_serializer(ReadPreference::SecondaryOnly) << "', '"
+                          << ReadPreference_serializer(ReadPreference::SecondaryPreferred)
+                          << "', and '" << ReadPreference_serializer(ReadPreference::Nearest)
+                          << "' are supported.");
     }
-    mode = std::move(swReadPrefMode.getValue());
 
     boost::optional<HedgingMode> hedgingMode;
     if (auto hedgingModeEl = readPrefObj[kHedgeFieldName]) {
@@ -261,7 +234,7 @@ StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromContainingBSON(
 }
 
 void ReadPreferenceSetting::toInnerBSON(BSONObjBuilder* bob) const {
-    bob->append(kModeFieldName, readPreferenceName(pref));
+    bob->append(kModeFieldName, ReadPreference_serializer(pref));
     if (tags != defaultTagSetForMode(pref)) {
         bob->append(kTagsFieldName, tags.getTagBSON());
     }
diff --git a/src/mongo/client/read_preference.h b/src/mongo/client/read_preference.h
index 5ab06c6f397..5d003ec276a 100644
--- a/src/mongo/client/read_preference.h
+++ b/src/mongo/client/read_preference.h
@@ -31,6 +31,7 @@
 
 #include "mongo/bson/simple_bsonobj_comparator.h"
 #include "mongo/client/hedging_mode_gen.h"
+#include "mongo/client/read_preference_gen.h"
 #include "mongo/db/jsobj.h"
 #include "mongo/db/operation_context.h"
 #include "mongo/db/repl/optime.h"
@@ -40,35 +41,12 @@ namespace mongo {
 template <typename T>
 class StatusWith;
 
-enum class ReadPreference {
-    /**
-     * Read from primary only. All operations produce an error (throw an exception where
-     * applicable) if primary is unavailable. Cannot be combined with tags.
-     */
-    PrimaryOnly = 0,
-
-    /**
-     * Read from primary if available, otherwise a secondary. Tags will only be applied in the
-     * event that the primary is unavailable and a secondary is read from. In this event only
-     * secondaries matching the tags provided would be read from.
-     */
-    PrimaryPreferred,
-
-    /**
-     * Read from secondary if available, otherwise error.
-     */
-    SecondaryOnly,
+using ReadPreference = ReadPreferenceEnum;
 
-    /**
-     * Read from a secondary if available, otherwise read from the primary.
-     */
-    SecondaryPreferred,
-
-    /**
-     * Read from any member.
-     */
-    Nearest,
-};
+/**
+ * Validate a ReadPreference string. This is intended for use as an IDL validator callback.
+ */
+Status validateReadPreferenceMode(const std::string& prefStr);
 
 /**
  * A simple object for representing the list of tags requested by a $readPreference.
diff --git a/src/mongo/client/read_preference.idl b/src/mongo/client/read_preference.idl
new file mode 100644
index 00000000000..d98376baef8
--- /dev/null
+++ b/src/mongo/client/read_preference.idl
@@ -0,0 +1,56 @@
+# Copyright (C) 2020-present MongoDB, Inc.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the Server Side Public License, version 1,
+# as published by MongoDB, Inc.
+#
+# 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
+# Server Side Public License for more details.
+#
+# You should have received a copy of the Server Side Public License
+# along with this program. If not, see
+# <http://www.mongodb.com/licensing/server-side-public-license>.
+#
+# 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 Server Side 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.
+#
+
+global:
+   cpp_namespace: "mongo"
+
+imports:
+    - "mongo/idl/basic_types.idl"
+
+enums:
+  ReadPreference:
+      description: Enumeration representing Read Preference Modes
+      type: string
+      values:
+          # Read from primary only. All operations produce an error (throw an exception where
+          # applicable) if primary is unavailable. Cannot be combined with tags.
+          PrimaryOnly: "primary"
+          #
+          # Read from primary if available, otherwise a secondary. Tags will only be applied in the
+          # event that the primary is unavailable and a secondary is read from. In this event only
+          # secondaries matching the tags provided would be read from.
+          PrimaryPreferred: "primaryPreferred"
+          #
+          # Read from secondary if available, otherwise error.
+          SecondaryOnly: "secondary"
+          #
+          # Read from a secondary if available, otherwise read from the primary.
+          SecondaryPreferred: "secondaryPreferred"
+          #
+          # Read from any member.
+          Nearest: "nearest"