SERVER-27710 Move SNMP init hook out of db.cpp.

The `src/mongo/db.cpp` file is compiled into the main server program,
and causes broken-dependency issues in linking the enterprise modules.
By moving this symbol to another file, we can better express this
dependency.  Further, this change lays the groundwork to switch from
from a pointer to function to a `std::function< void () >` object.

6 files changed, 105 insertions, 9 deletions

diff --git a/src/mongo/SConscript b/src/mongo/SConscript
index ccd0bbf341c..401ff9d684b 100644
--- a/src/mongo/SConscript
+++ b/src/mongo/SConscript
@@ -280,6 +280,7 @@ mongodLibDeps = [
     'db/dbdirectclient',
     'db/ftdc/ftdc_mongod',
     'db/index_d',
+    'db/initialize_snmp',
     'db/mongod_options',
     'db/mongodandmongos',
     'db/op_observer_d',
diff --git a/src/mongo/db/SConscript b/src/mongo/db/SConscript
index 4e739fcf51b..969eacd6d0b 100644
--- a/src/mongo/db/SConscript
+++ b/src/mongo/db/SConscript
@@ -61,6 +61,14 @@ env.Library(
     ]
 )
 
+env.Library(
+    target='initialize_snmp',
+	source= [
+		'initialize_snmp.cpp',
+	],
+	LIBDEPS=[],
+)
+
 env.CppUnitTest(
     target= 'field_ref_test',
     source= 'field_ref_test.cpp',
diff --git a/src/mongo/db/db.cpp b/src/mongo/db/db.cpp
index d57bd570700..cbcf906326e 100644
--- a/src/mongo/db/db.cpp
+++ b/src/mongo/db/db.cpp
@@ -158,8 +158,6 @@ using std::vector;
 
 using logger::LogComponent;
 
-void (*snmpInit)() = NULL;
-
 extern int diagLogging;
 
 namespace {
@@ -558,11 +556,7 @@ ExitCode _initAndListen(int listenPort) {
         uassert(12590, ss.str().c_str(), boost::filesystem::exists(storageGlobalParams.repairpath));
     }
 
-    // TODO:  This should go into a MONGO_INITIALIZER once we have figured out the correct
-    // dependencies.
-    if (snmpInit) {
-        snmpInit();
-    }
+    initializeSNMP();
 
     if (!storageGlobalParams.readOnly) {
         boost::filesystem::remove_all(storageGlobalParams.dbpath + "/_tmp/");
diff --git a/src/mongo/db/db.h b/src/mongo/db/db.h
index f5ca27cd748..5b0c4e33764 100644
--- a/src/mongo/db/db.h
+++ b/src/mongo/db/db.h
@@ -33,6 +33,7 @@
 #include "mongo/db/catalog/database_holder.h"
 #include "mongo/db/client.h"
 #include "mongo/db/curop.h"
+#include "mongo/db/initialize_snmp.h"
 #include "mongo/util/net/message.h"
 
 namespace mongo {
@@ -41,6 +42,4 @@ namespace repl {
 class ReplSettings;
 }  // namespace repl
 
-extern void (*snmpInit)();
-
 }  // namespace mongo
diff --git a/src/mongo/db/initialize_snmp.cpp b/src/mongo/db/initialize_snmp.cpp
new file mode 100644
index 00000000000..8610d863280
--- /dev/null
+++ b/src/mongo/db/initialize_snmp.cpp
@@ -0,0 +1,51 @@
+/**
+ *    Copyright (C) 2017 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.
+ */
+
+#include "mongo/db/initialize_snmp.h"
+
+namespace mongo {
+// Exposed in `src/mongo/db/db.h`, for legacy reasons.
+void (*snmpInit)() = nullptr;
+
+namespace {
+std::function<void()> snmpInitializer = [] {
+    if (snmpInit) {
+        snmpInit();
+    }
+};
+}
+}
+
+void mongo::registerSNMPInitializer(stdx::function<void()> init) {
+    // TODO: Check for reinitialization
+    snmpInitializer = std::move(init);
+}
+
+void mongo::initializeSNMP() {
+    return snmpInitializer();
+}
diff --git a/src/mongo/db/initialize_snmp.h b/src/mongo/db/initialize_snmp.h
new file mode 100644
index 00000000000..9626cd2c56c
--- /dev/null
+++ b/src/mongo/db/initialize_snmp.h
@@ -0,0 +1,43 @@
+/**
+ *    Copyright (C) 2017 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/stdx/functional.h"
+
+namespace mongo {
+
+void registerSNMPInitializer(stdx::function<void()> init);
+
+void initializeSNMP();
+
+// Deprecated -- will be removed.
+extern void (*snmpInit)();
+
+}  // namespace mongo