Documentation/Maemo 5 Developer Guide/DBus/Asynchronous GLib D-Bus
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Asynchronous GLib/D-Bus
Asynchronicity in D-Bus clients
So far all the RPC method calls that have been implemented here have been "fast", i.e. their execution has not depended on an access to slow services or external resources. In real life, however, it is quite likely that some services cannot be provided immediately, but will have to wait for some external service to complete, before completing the method call.
The GLib wrappers provide a version of making method calls, where the call will be launched (almost) immediately, and a callback will be executed when the method call returns (either with a return value, or an error).
Using the asynchronous wrappers is important, when the program needs to update some kind of status, or be reactive to the user (via a GUI or other interface). Otherwise, the program would block waiting for the RPC method to return, and would not be able to refresh the GUI or screen when required. An alternative solution would be to use separate threads that would run the synchronous methods, but synchronization between threads would become an issue, and debugging threaded programs is much harder than single-threaded ones. Also, implementation of threads might be suboptimal in some environments. These are the reasons why the thread scenario will not be covered here.
Slow running RPC methods will be simulated here by adding a delay into the server method implementations, so that it will become clear why asynchronous RPC mechanisms are important. As signals, by their nature, are asynchronous as well, they do not add anything to this example now. In order to simplify the code listings, the signal support from the asynchronous clients will be dropped here, even though the server still contains them and will emit the.
Slow Test Server
The only change on the server side is the addition of delays into each of the RPC methods (setvalue1, setvalue2, getvalue1 and getvalue2). This delay is added to the start of each function as follows: glib-dbus-async/server.c
/* How many microseconds to delay between each client operation. */ #define SERVER_DELAY_USEC (5*1000000UL) /*... Listing cut for brevity ...*/ gboolean value_object_setvalue1(ValueObject* obj, gint valueIn, GError** error) { dbg("Called (valueIn=%d)", valueIn); g_assert(obj != NULL); dbg("Delaying operation"); g_usleep(SERVER_DELAY_USEC); /* Compare the current value against old one. If they're the same, we don't need to do anything (except return success). */ if (obj->value1 != valueIn) {
Building the server is done as before, but we'll notice the delay when we call an RPC method:
[sbox-DIABLO_X86: ~/glib-dbus-async] > run-standalone.sh ./server & server:main Connecting to the Session D-Bus. server:main Registering the well-known name (org.maemo.Platdev_ex) server:main RequestName returned 1. server:main Creating one Value object. server:value_object_class_init: Called server:value_object_class_init: Creating signals server:value_object_class_init: Binding to GLib/D-Bus server:value_object_class_init: Done server:value_object_init: Called server:main Registering it on the D-Bus. server:main Ready to serve requests (daemonizing). server: Not daemonizing (built with NO_DAEMON-build define) [sbox-DIABLO_X86: ~/glib-dbus-async] > time run-standalone.sh dbus-send \ --type=method_call --print-reply --dest=org.maemo.Platdev_ex \ /GlobalValue org.maemo.Value.getvalue1 server:value_object_getvalue1: Called (internal value1 is 0) server:value_object_getvalue1: Delaying operation method return sender=:1.54 -> dest=:1.56 int32 0 real 0m5.066s user 0m0.004s sys 0m0.056s
In the example above, the time shell built-in command was used. It will run the given command while measuring the wall clock time (a.k.a. real time), and time used while executing the code and system calls. In this case, only the real time is of any interest. The method call will delay for about 5 seconds, as it should. The delay (even if given with microsecond resolution) is always approximate, and longer than the requested amount. Exact delay will depend on many factors, most of which cannot be directly influenced.
The next experiment deals with a likely scenario, where another method call comes along while the first one is still being executed. This is best tested by just repeating the sending command twice, but running the first one on the background (so that the shell does not wait for it to complete first). The server is still running on the background from the previous test:
[sbox-DIABLO_X86: ~/glib-dbus-async] > time run-standalone.sh dbus-send \ --type=method_call --print-reply --dest=org.maemo.Platdev_ex \ /GlobalValue org.maemo.Value.getvalue1 & [2] 17010 server:value_object_getvalue1: Called (internal value1 is 0) server:value_object_getvalue1: Delaying operation [sbox-DIABLO_X86: ~/glib-dbus-async] > time run-standalone.sh dbus-send \ --type=method_call --print-reply --dest=org.maemo.Platdev_ex \ /GlobalValue org.maemo.Value.getvalue1 method return sender=:1.54 -> dest=:1.57 int32 0 real 0m5.176s user 0m0.008s sys 0m0.092s server:value_object_getvalue1: Called (internal value1 is 0) server:value_object_getvalue1: Delaying operation method return sender=:1.54 -> dest=:1.58 int32 0 real 0m9.852s user 0m0.004s sys 0m0.052s
What can be seen from the above output is that the first client is delayed for about 5 seconds, while the second client (which was launched shortly after the first) is already delayed by a much longer period. This is to be expected, as the server can only process one request at a time, and will delay each request by 5 seconds.
Some server concurrency issues will be covered later, but for now, it is necessary that the clients are able to continue their "normal work" while they wait for the response from the server. Since this is just example code, "normal work" for our clients would be just waiting for the response, while blocking on incoming events (converted into callbacks). However, if the example programs were graphical, the asynchronous approach would make it possible for them to react to user input. D-Bus by itself does not support cancellation of method calls, once processing has started on the server side, so adding cancellation support would require a separate method call to the server. Since the server only handles one operation at a time, the current server cannot support method call cancellations at all.
Asynchronous Method Calls Using Stubs
When the glib-bindings-tool is run, it will already generate the necessary wrapping stubs to support launching asynchronous method calls. What is then left to do is implementing the callback functions correctly, processing the return errors and launching the method call. glib-dbus-async/client-stubs.c
/* Pull in the client stubs that were generated with dbus-binding-tool */ #include "value-client-stub.h"
The client has been simplified, so that it now only operates on value1. The callback that is called from the stub code is presented next: glib-dbus-async/client-stubs.c
/** * This function will be called when the async setvalue1 will either * complete, timeout or fail (our server however does not signal * errors, but the client D-Bus library might). When this example * program is left running for a while, you will see all three cases. * * The prototype must match the one generated by the dbus-binding-tool * (org_maemo_Value_setvalue1_reply). * * Since there is no return value from the RPC, the only useful * parameter that we get is the error object, which we'll check. * If error is NULL, that means no error. Otherwise the RPC call * failed and we should check what the cause was. */ static void setValue1Completed(DBusGProxy* proxy, GError *error, gpointer userData) { g_print(PROGNAME ":%s:setValue1Completed\n", timestamp()); if (error != NULL) { g_printerr(PROGNAME " ERROR: %s\n", error->message); /* We need to release the error object since the stub code does not do it automatically. */ g_error_free(error); } else { g_print(PROGNAME " SUCCESS\n"); } }
Since the method call does not return any data, the parameters for the callback are at minimum (those three will always be received). Handling errors must be performed within the callback, since errors could be delayed from the server, and not visible immediately at launch time. N.B. The callback will not terminate the program on errors. This is done on purpose in order to demonstrate some common asynchronous problems below. The timestamp function is a small utility function to return a pointer to a string, representing the number of seconds since the program started (useful to visualize the order of the different asynchronous events below). glib-dbus-async/client-stubs.c
/** * This function will be called repeatedly from within the mainloop * timer launch code. * * It will launch asynchronous RPC method to set value1 with ever * increasing argument. */ static gboolean timerCallback(DBusGProxy* remoteobj) { /* Local value that we'll start updating to the remote object. */ static gint localValue1 = -80; /* Start the RPC. This is done by calling the stub function that will take the new value and the callback function to call on reply getting back. The stub returns a DBusGProxyCall object, but we don't need it so we'll ignore the return value. The return value could be used to cancel a pending request (from client side) with dbus_g_proxy_cancel_call. We could also pass a pointer to user-data (last parameter), but we don't need one in this example. It would normally be used to "carry around" the application state. */ g_print(PROGNAME ":%s:timerCallback launching setvalue1\n", timestamp()); org_maemo_Value_setvalue1_async(remoteobj, localValue1, setValue1Completed, NULL); g_print(PROGNAME ":%s:timerCallback setvalue1 launched\n", timestamp()); /* Step the local value forward. */ localValue1 += 10; /* Repeat timer later. */ return TRUE; }
Using the stub code is rather simple. For each generated synchronous version of a method wrapper, there will also be a _async version of the call. The main difference with the parameters is the removal of the GError pointer (since errors will be handled in the callback), and the addition of the callback function to use when the method completes, times out or encounters an error.
The main function remains the same from the previous client examples (a once-per-second timer will be created and run from the mainloop, until the program is terminated).
Problems with Asynchronicity
When the simple test program is built and run, it can be seen that everything starts off quite well. But at some point, problems will start to appear:
[sbox-DIABLO_X86: /glib-dbus-async] > make client-stubs
dbus-binding-tool --prefix=value_object --mode=glib-client \
value-dbus-interface.xml > value-client-stub.h
cc -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include \
-I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -g -Wall \
-DG_DISABLE_DEPRECATED -DNO_DAEMON -DPROGNAME=\"client-stubs\" \
-c client-stubs.c -o client-stubs.o
cc client-stubs.o -o client-stubs -ldbus-glib-1 -ldbus-1 -lgobject-2.0 -lglib-2.0
[sbox-DIABLO_X86: /glib-dbus-async] > run-standalone.sh ./client-stubs
client-stubs:main Connecting to Session D-Bus.
client-stubs:main Creating a GLib proxy object for Value.
client-stubs: 0.00:main Starting main loop (first timer in 1s).
client-stubs: 1.00:timerCallback launching setvalue1
client-stubs: 1.00:timerCallback setvalue1 launched
server:value_object_setvalue1: Called (valueIn=-80)
server:value_object_setvalue1: Delaying operation
client-stubs: 2.00:timerCallback launching setvalue1
client-stubs: 2.00:timerCallback setvalue1 launched
client-stubs: 3.01:timerCallback launching setvalue1
client-stubs: 3.01:timerCallback setvalue1 launched
client-stubs: 4.01:timerCallback launching setvalue1
client-stubs: 4.01:timerCallback setvalue1 launched
client-stubs: 5.02:timerCallback launching setvalue1
client-stubs: 5.02:timerCallback setvalue1 launched
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=-70)
server:value_object_setvalue1: Delaying operation
client-stubs: 6.01:setValue1Completed
client-stubs SUCCESS
client-stubs: 6.02:timerCallback launching setvalue1
client-stubs: 6.02:timerCallback setvalue1 launched
client-stubs: 7.02:timerCallback launching setvalue1
client-stubs: 7.02:timerCallback setvalue1 launched
...
client-stubs:25.04:timerCallback launching setvalue1
client-stubs:25.04:timerCallback setvalue1 launched
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=-30)
server:value_object_setvalue1: Delaying operation
client-stubs:26.03:setValue1Completed
client-stubs SUCCESS
client-stubs:26.05:timerCallback launching setvalue1
client-stubs:26.05:timerCallback setvalue1 launched
client-stubs:27.05:timerCallback launching setvalue1
client-stubs:27.05:timerCallback setvalue1 launched
client-stubs:28.05:timerCallback launching setvalue1
client-stubs:28.05:timerCallback setvalue1 launched
client-stubs:29.05:timerCallback launching setvalue1
client-stubs:29.05:timerCallback setvalue1 launched
client-stubs:30.05:timerCallback launching setvalue1
client-stubs:30.05:timerCallback setvalue1 launched
client-stubs:31.02:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=-20)
server:value_object_setvalue1: Delaying operation
client-stubs:31.05:timerCallback launching setvalue1
client-stubs:31.05:timerCallback setvalue1 launched
client-stubs:32.03:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
client-stubs:32.05:timerCallback launching setvalue1
client-stubs:32.05:timerCallback setvalue1 launched
client-stubs:33.03:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
client-stubs:33.05:timerCallback launching setvalue1
client-stubs:33.05:timerCallback setvalue1 launched
client-stubs:34.03:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
client-stubs:34.06:timerCallback launching setvalue1
client-stubs:34.06:timerCallback setvalue1 launched
client-stubs:35.03:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
client-stubs:35.05:timerCallback launching setvalue1
client-stubs:35.05:timerCallback setvalue1 launched
client-stubs:36.04:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=-10)
server:value_object_setvalue1: Delaying operation
client-stubs:36.06:timerCallback launching setvalue1
client-stubs:36.06:timerCallback setvalue1 launched
client-stubs:37.04:setValue1Completed
client-stubs ERROR: Did not receive a reply. Possible causes include:
the remote application did not send a reply, the message bus security policy
blocked the reply, the reply timeout expired, or the network connection was
broken.
[Ctrl+c]
[sbox-DIABLO_X86: /glib-dbus-async] >
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=30)
server:value_object_setvalue1: Delaying operation
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=40)
server:value_object_setvalue1: Delaying operation
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
server:value_object_setvalue1: Called (valueIn=50)
server:value_object_setvalue1: Delaying operation
server:value_object_emitSignal: Emitting signal id 0, with message 'value1'
...
What happens above is rather subtle. The timer callback in the client launches once per second and performs the RPC method launch. The server, however, still has the 5 second delay for each method call in it. It can be seen that the successive launches go on without any responses for a while. The first response comes back at about 6 seconds from the starting of the client. At this point, the server already has four other outstanding method calls that it has not handled. Slowly the method calls are accumulating at the server end, and it does not deal with them quickly enough to satisfy the client.
After about 30 seconds, it can be seen how the setValue1Completed callback is invoked, but the method call fails. This has managed to trigger the method call timeout mechanism. After this point, all the method calls that have accumulated into the server (into a message queue) will fail in the client, since they all will now return late, even if the server actually does handle them.
Once the client is terminated, it can be seen that the server is still happily continuing serving the requests, oblivious to the fact that there is no client to process the responses.
The above test demonstrates quite brutally that the services need to be designed properly, so that there is a clearly defined protocol what to do in case a method call is delayed. It is also advisable to design a notification protocol to tell clients that something has completed, instead of forcing them to time out. Using D-Bus signals is one way, but it is necessary to take care not to generate signals, when tere is nothing listening to them. This can be done by only sending signals when an long operation finishes (assuming this has been documented as part of the service description).
One partial fix would be for the client to track and make sure that only one method call to one service is outstanding at any given time. So, instead of just blindly launching the RPC methods, it should defer from launching, if it has not yet received a response from the server (and the call has not timed out).
However, this fix is not complete, since the same problem will manifest itself once there are multiple clients running in parallel and requesting the same methods. The proper fix is to make the server capable of serving multiple requests in parallel. Some hints on how to do this are presented later on.
Asynchronous Method Calls Using GLib Wrappers
Sometimes the interface XML will be missing, so the dbus-bindings-tool cannot be run to generate the stub code. The GLib wrappers are generic enough to enable building own method calls, when necessary.
It is often easiest to start with some known generated stub code to see, which parts can possibly be reused with some modifications. This is what is shown in the last step of this example, in order to make a version of the asynchronous client that will work without the stub generator.
The first step is to take a peek at the stub-generated code for the setvalue1 call (when used asynchronously): glib-dbus-async/value-client-stub.h
typedef void (*org_maemo_Value_setvalue1_reply) (DBusGProxy *proxy, GError *error, gpointer userdata); static void org_maemo_Value_setvalue1_async_callback (DBusGProxy *proxy, DBusGProxyCall *call, void *user_data) { DBusGAsyncData *data = user_data; GError *error = NULL; dbus_g_proxy_end_call (proxy, call, &error, G_TYPE_INVALID); (*(org_maemo_Value_setvalue1_reply)data->cb) (proxy, error, data->userdata); return; } static #ifdef G_HAVE_INLINE inline #endif DBusGProxyCall* org_maemo_Value_setvalue1_async (DBusGProxy *proxy, const gint IN_new_value, org_maemo_Value_setvalue1_reply callback, gpointer userdata) { DBusGAsyncData *stuff; stuff = g_new (DBusGAsyncData, 1); stuff->cb = G_CALLBACK (callback); stuff->userdata = userdata; return dbus_g_proxy_begin_call ( proxy, "setvalue1", org_maemo_Value_setvalue1_async_callback, stuff, g_free, G_TYPE_INT, IN_new_value, G_TYPE_INVALID); }
What is notable in the code snippet above is that the _async method will create a temporary small structure that will hold the pointer to the callback function, and a copy of the userdata pointer. This small structure will then be passed to dbus_g_proxy_begin_call, along with the address of the generated callback wrapper function (org_maemo_Value_setvalue1_async_callback). The GLib async launcher will also take a function pointer to a function to use when the supplied "user-data" (in this case, the small structure) will need to be disposed of after the call. Since it uses g_new to allocate the small structure, it passes g_free as the freeing function. Next comes the argument specification for the method call, which obeys the same rules as the LibOSSO ones before.
On RPC completion, the generated callback will be invoked, and it will get the real callback function pointer and the userdata as its "user-data" parameter. It will first collect the exit code for the call with dbus_g_proxy_end_call, unpack the data and invoke the real callback. After returning, the GLib wrappers (which called the generated callback) will call g_free to release the small structure, and the whole RPC launch will end.
The next step is to re-implement pretty much the same logic, but also dispose of the small structure, since the callback will be implemented directly, not as a wrapper-callback (it also omits the need for one memory allocation and one free).
The first step for that is to implement the RPC asynchronous launch code: glib-dbus-async/client-glib.c
/** * This function will be called repeatedly from within the mainloop * timer launch code. * * It will launch asynchronous RPC method to set value1 with ever * increasing argument. */ static gboolean timerCallback(DBusGProxy* remoteobj) { /* Local value that we'll start updating to the remote object. */ static gint localValue1 = -80; /* Start the first RPC. The call using GLib/D-Bus is only slightly more complex than the stubs. The overall operation is the same. */ g_print(PROGNAME ":timerCallback launching setvalue1\n"); dbus_g_proxy_begin_call(remoteobj, /* Method name. */ "setvalue1", /* Callback to call on "completion". */ setValue1Completed, /* User-data to pass to callback. */ NULL, /* Function to call to free userData after callback returns. */ NULL, /* First argument GType. */ G_TYPE_INT, /* First argument value (passed by value) */ localValue1, /* Terminate argument list. */ G_TYPE_INVALID); g_print(PROGNAME ":timerCallback setvalue1 launched\n"); /* Step the local value forward. */ localValue1 += 10; /* Repeat timer later. */ return TRUE; }
And the callback that will be invoked on method call completion, timeouts or errors: glib-dbus-async/client-glib.c
/** * This function will be called when the async setvalue1 will either * complete, timeout or fail (same as before). The main difference in * using GLib/D-Bus wrappers is that we need to "collect" the return * value (or error). This is done with the _end_call function. * * Note that all callbacks that are to be registered for RPC async * notifications using dbus_g_proxy_begin_call must follow the * following prototype: DBusGProxyCallNotify . */ static void setValue1Completed(DBusGProxy* proxy, DBusGProxyCall* call, gpointer userData) { /* This will hold the GError object (if any). */ GError* error = NULL; g_print(PROGNAME ":setValue1Completed\n"); /* We next need to collect the results from the RPC call. The function returns FALSE on errors (which we check), although we could also check whether error-ptr is still NULL. */ if (!dbus_g_proxy_end_call(proxy, /* The call that we're collecting. */ call, /* Where to store the error (if any). */ &error, /* Next we list the GType codes for all the arguments we expect back. In our case there are none, so set to invalid. */ G_TYPE_INVALID)) { /* Some error occurred while collecting the result. */ g_printerr(PROGNAME " ERROR: %s\n", error->message); g_error_free(error); } else { g_print(PROGNAME " SUCCESS\n"); } }
The generated stub code is no longer needed, so the dependency rules for the stubless GLib version will also be somewhat different: glib-dbus-async/Makefile
client-glib: client-glib.o $(CC) $^ -o $@ $(LDFLAGS) # Note that the GLib client doesn't need the stub code. client-glib.o: client-glib.c common-defs.h $(CC) $(CFLAGS) -DPROGNAME=\"$(basename $@)\" -c $< -o $@
Since the example program logic has not changed from the previous version, testing client-glib is not presented here (it can of course be tested if so desired, since the source code contains the fully working program). This version of the client will also launch the method calls without waiting for previous method calls to complete.