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Fix sporadic race condition between Variant and underlying bus

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The underlying bus was thread_local, but the design assumption that Variants built on top of that instance won't outlive the thread was incorrect. In stress tests, Variants were moved (and this is completely legal) to a different thread.
This commit is contained in:
sangelovic
2020-01-25 22:31:41 +01:00
parent 245db893b8
commit 75709e31f1
8 changed files with 43 additions and 60 deletions
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src/Message.cpp
+4 -55
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@@ -28,7 +28,8 @@
#include <sdbus-c++/Types.h>
#include <sdbus-c++/Error.h>
#include "MessageUtils.h"
#include "SdBus.h"
#include "ISdBus.h"
#include "IConnection.h"
#include "ScopeGuard.h"
#include <systemd/sd-bus.h>
#include <cassert>
@@ -712,60 +713,8 @@ void Signal::send() const
PlainMessage createPlainMessage()
{
int r;
// All references to the bus (like created messages) must not outlive this thread (because messages refer to sdbus
// which is thread-local, and because BusReferenceKeeper below destroys the bus at thread exit).
// A more flexible solution would be that the caller would already provide an ISdBus reference as a parameter.
// Variant is one of the callers. This means Variant could no more be created in a stand-alone way, but
// through a factory of some existing facility (Object, Proxy, Connection...).
// TODO: Consider this alternative of creating Variant, it may live next to the current one. This function would
// get IConnection* parameter and IConnection would provide createPlainMessage factory (just like it already
// provides e.g. createMethodCall). If this parameter were null, the current mechanism would be used.
thread_local internal::SdBus sdbus;
sd_bus* bus{};
SCOPE_EXIT{ sd_bus_unref(bus); };
r = sd_bus_default_system(&bus);
SDBUS_THROW_ERROR_IF(r < 0, "Failed to get default system bus", -r);
thread_local struct BusReferenceKeeper
{
explicit BusReferenceKeeper(sd_bus* bus) : bus_(sd_bus_ref(bus)) { sd_bus_flush(bus_); }
~BusReferenceKeeper() { sd_bus_flush_close_unref(bus_); }
sd_bus* bus_{};
} busReferenceKeeper{bus};
// thread_local struct BusKeeper
// {
// BusKeeper()
// {
// auto r = sd_bus_open_system(&bus);
// sd_bus_flush(bus);
// SDBUS_THROW_ERROR_IF(r < 0, "Failed to get default system bus", -r);
// }
// ~BusKeeper()
// {
// sd_bus_flush_close_unref(bus);
// }
// sd_bus* bus{};
// internal::SdBus intf;
// } busKeeper;
// Shelved here as handy thing for potential future tracing purposes:
//#include <unistd.h>
//#include <sys/syscall.h>
//#define gettid() syscall(SYS_gettid)
//printf("createPlainMessage: sd_bus*=[%p], n_ref=[%d], TID=[%d]\n", bus, *(unsigned*)bus, gettid());
sd_bus_message* sdbusMsg{};
r = sd_bus_message_new(bus, &sdbusMsg, _SD_BUS_MESSAGE_TYPE_INVALID);
SDBUS_THROW_ERROR_IF(r < 0, "Failed to create a new message", -r);
return Message::Factory::create<PlainMessage>(sdbusMsg, &sdbus, adopt_message);
static auto connection = internal::createConnection();
return connection->createPlainMessage();
}
}