dbus_singleton: use stack variable and extern

Currently, the |systemBus| connection is a static variable declared in
headers. This has a problem that every translation unit will keep its
own copy. It's not a problem today because there's only one translation
unit "webserver_main.cpp.o". This issue was brounght up in
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/54758

Actually, the |systemBus| doesn't need to be a singleton. It can just be
a stack variable, which is normally more efficient than heap variables.

To keep minimum changes treeside, this commits keeps the existing
|systemBus| variable as an external variable. It is defined in its own
translation unit. It is initialized in the main translation unit.

Reference:
1. Extern
https://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files

Tested:
1. Romulus QEMU robot Redfish test passed;
2. Start and restart service on real hardware, no issues;
3. No new validator failures
4. Code compies

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I03b387bd5f218a86c9d1765415a46e3c2ad83ff9

2 files changed, 17 insertions, 3 deletions

diff --git a/src/dbus_singleton.cpp b/src/dbus_singleton.cpp
new file mode 100644
index 0000000000..f78164fbe2
--- /dev/null
+++ b/src/dbus_singleton.cpp
@@ -0,0 +1,13 @@
+#include "dbus_singleton.hpp"
+
+#include <boost/asio/io_context.hpp>
+
+namespace crow
+{
+namespace connections
+{
+
+sdbusplus::asio::connection* systemBus = nullptr;
+
+} // namespace connections
+} // namespace crow
diff --git a/src/webserver_main.cpp b/src/webserver_main.cpp
index ff0bbd7420..c306dea958 100644
--- a/src/webserver_main.cpp
+++ b/src/webserver_main.cpp
@@ -65,8 +65,8 @@ static int run()
     auto io = std::make_shared<boost::asio::io_context>();
     App app(io);
 
-    crow::connections::systemBus =
-        std::make_shared<sdbusplus::asio::connection>(*io);
+    sdbusplus::asio::connection systemBus(*io);
+    crow::connections::systemBus = &systemBus;
 
     // Static assets need to be initialized before Authorization, because auth
     // needs to build the whitelist from the static routes
@@ -147,7 +147,8 @@ static int run()
     app.run();
     io->run();
 
-    crow::connections::systemBus.reset();
+    crow::connections::systemBus = nullptr;
+
     return 0;
 }