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|
| -# Service Manager User Guide
|
| -
|
| -## What is the Service Manager?
|
| -
|
| -The Service Manager is a tool that brokers connections and capabilities between
|
| -and manages instances of components, referred to henceforth as services.
|
| -
|
| -The Service Manager performs the following functions:
|
| -
|
| -* Brokering connections between services, including communicating policies such
|
| - as capabilities (which include access to interfaces), user identity, etc.
|
| -* Launching and managing the lifecycle services and processes (though services
|
| - may also create their own processes and tell the Service Manager about them).
|
| -* Tracks running services, and provides an API that allows services to
|
| - understand whats running.
|
| -
|
| -The Service Manager presents a series of Mojo interfaces to services, though in
|
| -practice interacting with the Service is made simpler with a client library.
|
| -Currently, there is only a client library written in C++, since that meets the
|
| -needs of most of the use cases in Chrome.
|
| -
|
| -## Details
|
| -
|
| -### Mojo Recap
|
| -
|
| -The Mojo system provides two key components of interest here - a lightweight
|
| -message pipe concept allowing two endpoints to communicate, and a bindings layer
|
| -that allows interfaces to be described to bind to those endpoints, with
|
| -ergonomic bindings for languages used in Chrome.
|
| -
|
| -Mojo message pipes are designed to be lightweight and may be read from/written
|
| -to and passed around from one process to the next. In most situations however
|
| -the developer wont interact with the pipes directly, rather with a generated
|
| -types encapsulating a bound interface.
|
| -
|
| -To use the bindings, a developer defines their interface in the Mojo IDL format,
|
| -**mojom**. With some build magic, the generated headers can then be included and
|
| -used from C++, JS and Java.
|
| -
|
| -It is important to note here that Mojo Interfaces have fully qualified
|
| -identifiers in string form, generated from the module path and interface name:
|
| -**`module.path.InterfaceName`**. This is how interfaces are referenced in
|
| -Service Manifests, and how they will be referenced throughout this document.
|
| -
|
| -This would be a good place for me to refer to this in-depth Mojo User Guide,
|
| -which spells all of this out in great detail.
|
| -
|
| -### Services
|
| -
|
| -A Service is any bit of code the Service Manager knows about. This could be a
|
| -unique process, or just a bit of code run in some existing process.
|
| -
|
| -The Service Manager disambiguates services by their **Identity**. Every service
|
| -has its own unique Identity. From the Service Managers perspective, a services
|
| -Identity is represented by the tuple of the its Name, UserId and Instance Name.
|
| -The Name is a formatted string that superficially represents a scheme:host pair,
|
| -but actually isnt a URL. More on the structure of these names later. The UserId
|
| -is a string GUID, representing the user the service is run as. The Instance Name
|
| -is a string, typically (but not necessarily) derived from the Name, which can be
|
| -used to allow multiple instances of a service to exist for the same Name,UserId
|
| -pair. In Chrome an example of this would be multiple instances of the renderer
|
| -or the same profile.
|
| -
|
| -A Service implements the Mojo interface shell.mojom.Service, which is the
|
| -primary means the Service Manager has of communicating with its service. Service
|
| -has two methods: OnStart(), called once at when the Service Manager first learns
|
| -about the service, and OnConnect(), which the Service Manager calls every time
|
| -some other service tries to connect to this one.
|
| -
|
| -Services have a link back to the Service Manager too, primarily in the form of
|
| -the shell.mojom.Connector interface. The Connector allows services to open
|
| -connections to other services.
|
| -
|
| -A unique connection from the Service Manager to a service is called an
|
| -instance, each with its own unique identifier, called an instance id. Every
|
| -instance has a unique Identity. It is possible to locate an existing instance
|
| -purely using its Identity.
|
| -
|
| -Services define their own lifetimes. Services in processes started by other
|
| -services (rather than the Service Manager) may even outlive the connection with
|
| -the Service Manager. For processes launched by the Service Manager, when a
|
| -service wishes to terminate it closes the Service pipe with the Service Manager
|
| -and the Service Manager destroys its corresponding instance and asks the process
|
| -to exit.
|
| -
|
| -#### A simple Service example
|
| -
|
| -Consider this simple application that implements the Service interface:
|
| -
|
| -**app.cc:**
|
| -
|
| - #include "mojo/public/c/system/main.h"
|
| - #include "services/shell/public/cpp/application_runner.h"
|
| - #include "services/shell/public/cpp/connector.h"
|
| - #include "services/shell/public/cpp/connection.h"
|
| - #include "services/shell/public/cpp/identity.h"
|
| - #include "services/shell/public/cpp/service.h"
|
| -
|
| - class Service : public shell::Service {
|
| - public:
|
| - Service() {}
|
| - ~Service() override {}
|
| -
|
| - // Overridden from shell::Service:
|
| - void OnStart(const shell::Identity& identity) override {
|
| - }
|
| - bool OnConnect(shell::Connection* connection) override {
|
| - return true;
|
| - }
|
| - };
|
| -
|
| - MojoResult ServiceMain(MojoHandle service_request_handle) {
|
| - return shell::ServiceRunner(new Service).Run(service_request_handle);
|
| - }
|
| -
|
| - app_manifest.json:
|
| -
|
| - {
|
| - "manifest_version": 1,
|
| - "name": "mojo:app",
|
| - "display_name": "Example App",
|
| - "capabilities": {}
|
| - }
|
| -
|
| -**BUILD.gn:**
|
| -
|
| - import("//mojo/public/mojo_application.gni")
|
| -
|
| - service("app") {
|
| - sources = [ "app.cc" ]
|
| - deps = [ "//base", "//mojo/shell/public/cpp" ]
|
| - data_deps = [ ":manifest" ]
|
| - }
|
| -
|
| - service_manifest("manifest") {
|
| - name = "app"
|
| - source = "app_manifest.json"
|
| - }
|
| -
|
| -What does all this do? Building the app target produces two files in the output
|
| -directory: app/app.library and app/manifest.json. app.library is a DSO loaded by
|
| -the Service Manager in its own process when another service connects to the
|
| -mojo:app name. This is not the only way (nor even the most likely one) you can
|
| -implement a Service, but its the simplest and easiest to reason about.
|
| -
|
| -This service doesnt do much. Its implementation of OnStart() is empty, and its
|
| -implementation of OnConnect just returns true to allow the inbound connection to
|
| -complete. Lets study the parameters to these methods though, since theyll be
|
| -important as we begin to do more in our service.
|
| -
|
| -##### OnStart Parameters
|
| -
|
| -###### const shell::Identity& identity
|
| -This is the identity this service is known to the Service Manager as. It
|
| -includes the services Name, User ID and Instance Name.
|
| -
|
| -##### OnConnect Parameters
|
| -
|
| -###### shell::Connection* connection
|
| -This is a pointer to an object that encapsulates the connection with a remote
|
| -service. The service uses this object to learn about the service at the remote
|
| -end, to bind interfaces from it, and to expose interfaces to it. The
|
| -Connection concept is implemented under the hood by a pair of
|
| -shell.mojom.InterfaceProviders - this is the physical link between the service
|
| -that give the Connection its utility. The Connection object is owned by the
|
| -caller of OnConnect, and will outlive the underlying pipes.
|
| -
|
| -The service can decide to block the connection outright by returning false from
|
| -this method. In that scenario the underlying pipes will be closed and the remote
|
| -end will see an error and have the chance to recover.
|
| -
|
| -Before we add any functionality to our service, such as exposing an interface,
|
| -we should look at how we connect to another service and bind an interface from
|
| -it. This will lay the groundwork to understanding how to export an interface.
|
| -
|
| -### Connecting
|
| -
|
| -Once we have a Connector, we can connect to other services and bind interfaces
|
| -from them. In the trivial app above we can do this directly in OnStart:
|
| -
|
| - void OnStart(const shell::Identity& identity) override {
|
| - scoped_ptr<shell::Connection> connection =
|
| - connector()->Connect("mojo:service");
|
| - mojom::SomeInterfacePtr some_interface;
|
| - connection->GetInterface(&some_interface);
|
| - some_interface->Foo();
|
| - }
|
| -
|
| -This assumes an interface called mojo.SomeInterface with a method Foo()
|
| -exported by another Mojo client identified by the name mojo:service.
|
| -
|
| -What is happening here? Lets look line-by-line
|
| -
|
| -
|
| - scoped_ptr<shell::Connection> connection =
|
| - connector->Connect("mojo:service");
|
| -
|
| -This asks the Service Manager to open a connection to the service named
|
| -mojo:service. The Connect() method returns a Connection object similar to the
|
| -one received by OnConnect() - in fact this Connection object binds the other
|
| -ends of the pipes of the Connection object received by OnConnect in the remote
|
| -service. This time, the caller of Connect() takes ownership of the Connection,
|
| -and when it is destroyed the connection (and the underlying pipes) is closed. A
|
| -note on this later.
|
| -
|
| - mojom::SomeInterfacePtr some_interface;
|
| -
|
| -This is a shorthand from the mojom bindings generator, producing an
|
| -instantiation of a mojo::InterfacePtr<mojom::SomeInterface>. At this point the
|
| -InterfacePtr is unbound (has no pipe handle), and calling is_bound() on it will
|
| -return false. Before we can call any methods, we need to bind it to a Mojo
|
| -message pipe. This is accomplished on the following line:
|
| -
|
| - connection->GetInterface(&some_interface);
|
| -
|
| -Calling this method allocates a Mojo message pipe, binds the client handle to
|
| -the provided InterfacePtr, and sends the server handle to the remote service,
|
| -where it will eventually (asynchronously) be bound to an object implementing the
|
| -requested interface. Now that our InterfacePtr has been bound, we can start
|
| -calling methods on it:
|
| -
|
| - some_interface->Foo();
|
| -
|
| -Now an important note about lifetimes. At this point the Connection returned by
|
| -Connect() goes out of scope, and is destroyed. This closes the underlying
|
| -InterfaceProvider pipes with the remote client. But Mojo methods are
|
| -asynchronous. Does this mean that the call to Foo() above is lost? No. Before
|
| -closing, queued writes to the pipe are flushed.
|
| -
|
| -### Implementing an Interface
|
| -
|
| -Lets look at how to implement an interface now from a client and expose it to
|
| -inbound connections from other clients. To do this well need to implement
|
| -OnConnect() in our Service implementation, and implement a couple of other
|
| -interfaces. For the sake of this example, well imagine now were writing the
|
| -mojo:service client, implementing the interface defined in this mojom:
|
| -
|
| -**some_interface.mojom:**
|
| -
|
| - module mojom;
|
| -
|
| - interface SomeInterface {
|
| - Foo();
|
| - };
|
| -
|
| -To build this mojom we need to invoke the mojom gn template from
|
| -`//mojo/public/tools/bindings/mojom.gni`. Once we do that and look at the
|
| -output, we can see that the C++ class mojom::SomeInterface is generated and can
|
| -be #included from the same path as the .mojom file at some_interface.mojom.h.
|
| -In our implementation of the mojo:service client, well need to derive from this
|
| -class to implement the interface. But thats not enough. Well also have to find
|
| -a way to bind inbound requests to bind this interface to the object that
|
| -implements it. Lets look at a snippet of a class that does all of this:
|
| -
|
| -**service.cc:**
|
| -
|
| - class Service : public shell::Service,
|
| - public shell::InterfaceFactory<mojom::SomeInterface>,
|
| - public mojom::SomeInterface {
|
| - public:
|
| - ..
|
| -
|
| - // Overridden from shell::Service:
|
| - bool OnConnect(shell::Connection* connection) override {
|
| - connection->AddInterface<mojom::SomeInterface>(this);
|
| - return true;
|
| - }
|
| -
|
| - // Overridden from shell::InterfaceFactory<mojom::SomeInterface>:
|
| - void Create(shell::Connection* connection,
|
| - mojom::SomeInterfaceRequest request) override {
|
| - bindings_.AddBinding(this, std::move(request));
|
| - }
|
| -
|
| - // Overridden from mojom::SomeInterface:
|
| - void Foo() override { /* .. */ }
|
| -
|
| - mojo::BindingSet<mojom::SomeInterface> bindings_;
|
| - };
|
| -
|
| -Lets study whats going on, starting with the obvious - we derive from
|
| -`mojom::SomeInterface` and implement `Foo()`. How do we bind this implementation
|
| -to a pipe handle from a connected service? First we have to advertise the
|
| -interface to the client through the inbound connection. This is accomplished in
|
| -OnConnect():
|
| -
|
| - connection->AddInterface<mojom::SomeInterface>(this);
|
| -
|
| -This adds the `mojom.SomeInterface` interface name to the inbound Connection
|
| -objects InterfaceRegistry, and tells the InterfaceRegistry to consult this
|
| -object when it needs to construct an implementation to bind. Why this object?
|
| -Well in addition to Service and SomeInterface, we also implement an
|
| -instantiation of the generic interface InterfaceFactory. InterfaceFactory is the
|
| -missing piece - it binds a request for SomeInterface (in the form of a message
|
| -pipe server handle) to the object that implements the interface (this). This is
|
| -why we implement Create():
|
| -
|
| - bindings_.AddBinding(this, std::move(request));
|
| -
|
| -In this case, this single instance binds requests for this interface from all
|
| -connected clients, so we use a mojo::BindingSet to hold them all. Alternatively,
|
| -we could construct an object per request, and use mojo::Binding.
|
| -
|
| -### Capabilities
|
| -
|
| -While the code above looks like it should work, if we were to type it all in,
|
| -build it and run it it still wouldnt. In fact, if we ran it, wed see this
|
| -error in the console:
|
| -
|
| -`Capabilities prevented connection from: mojo:app to mojo:service`
|
| -
|
| -The answer lies in an omission in one of the files I didnt discuss earlier, the
|
| -manifest.json, specifically the empty capabilities dictionary.
|
| -
|
| -You can think of an interface (and its underlying client handle) as a
|
| -capability. If you have it, and its bound, you can call methods on it and
|
| -something will happen. If you dont have a bound InterfacePtr, you (effectively)
|
| -dont have that capability.
|
| -
|
| -At the top level, the Service Manager implements the delegation of capabilities
|
| -in accordance with rules spelled out in each services manifest.
|
| -
|
| -Each service produces a manifest file with some typical metadata about itself,
|
| -and a capability spec. A capability spec describes classes of
|
| -capabilities offered by the service, classes of capabilities and individual
|
| -capabilities consumed by the service. Lets study a fairly complete capability
|
| -spec from another services manifest:
|
| -
|
| - "capabilities": {
|
| - "provided": {
|
| - "web": ["if1", "if2"],
|
| - "uid": []
|
| - "god-mode": ["*"]
|
| - },
|
| - "required": {
|
| - "*": { "classes": ["c1", "c2"], "interfaces": ["if3", "if4"] },
|
| - "mojo:foo": { "classes": ["c3"], "interfaces": ["if5"] }
|
| - }
|
| - }
|
| -
|
| -At the top level of the capabilities dictionary are two sub-dictionaries.
|
| -
|
| -#### Provided Capability Classes
|
| -
|
| -The provided dictionary enumerates the capability classes provided by the
|
| -service. A capability class is an alias, either to some special behavior exposed
|
| -by the service to remote services that request that class, or to a set of
|
| -interfaces exposed by the service to remote services. In the former case, in the
|
| -dictionary we provide an empty array as the value of the class name key, in the
|
| -latter case we provide an array with a list of the fully qualified Mojo
|
| -interface names (module.path.InterfaceName). A special case of array is one that
|
| -contains the single entry *, which means all interfaces. In the example
|
| -above, when another service connects to this one and requests the god-mode
|
| -class in its manifest, it can connect to all interfaces exposed by this service.
|
| -
|
| -#### Required Capabilities
|
| -
|
| -The required dictionary enumerates the capability classes and interfaces
|
| -required by the service. The keys into this dictionary are the names of the
|
| -services it intends to connect to, and the values for each key are capability
|
| -specs that describe the capability classes and individual interfaces that this
|
| -class needs to operate correctly. Here again, an array value for the
|
| -interfaces key in the capability spec consisting of a single * means the
|
| -service needs to bind all interfaces exposed by that service. Additionally, a
|
| -* key in the required dictionary allows the service to provide a capability
|
| -spec that must be adhered to by all applications it connects to.
|
| -
|
| -Note that a service need not enumerate every interface it provides in the
|
| -provided dictionary. This is done effectively at runtime when the service calls
|
| -AddInterface() on inbound connections. The service merely describes groups of
|
| -interfaces in capability classes as an ergonomic measure. Without capability
|
| -classes, services would have to explicitly state every interface they intended
|
| -to bind, which would make the manifests very cumbersome to author.
|
| -
|
| -Armed with this knowledge, we can return to app_manifest.json from the first
|
| -example and fill out the capability spec:
|
| -
|
| - {
|
| - "manifest_version": 1,
|
| - "name": "mojo:app",
|
| - "display_name": "Example App",
|
| - "capabilities": {
|
| - "required": {
|
| - "mojo:service": [],
|
| - }
|
| - }
|
| - }
|
| -
|
| -If we just run now, it still wont work, and well see this error:
|
| -
|
| - Connection CapabilitySpec prevented binding to interface mojom.SomeInterface
|
| - connection_name: mojo:service remote_name: mojo:app
|
| -
|
| -The connection was allowed to complete, but the attempt to bind
|
| -`mojom.SomeInterface` was blocked. We need to add that interface to the array in
|
| -the manifest:
|
| -
|
| - "required": {
|
| - "mojo:service": [ "mojom::SomeInterface" ],
|
| - }
|
| -
|
| -Now everything should work.
|
| -
|
| -(Note that we didnt write a manifest for mojo:service. Wed need to do that
|
| -too, though for this example we wouldnt have to describe mojom.SomeInterface in
|
| -the provided section of its capability spec, since it wasnt part of a class.
|
| -Connecting services like mojo:app just need to state that interface.)
|
| -
|
| -### Testing
|
| -
|
| -Now that weve built a simple application and service, its time to write a test
|
| -for them. The Shell client library provides a gtest base class
|
| -**shell::test::ServiceTest** that makes writing integration tests of services
|
| -straightforward. Lets look at a simple test of our service:
|
| -
|
| - #include "base/bind.h"
|
| - #include "base/run_loop.h"
|
| - #include "mojo/shell/public/cpp/service_test.h"
|
| - #include "path/to/some_interface.mojom.h"
|
| -
|
| - void QuitLoop(base::RunLoop* loop) {
|
| - loop->Quit();
|
| - }
|
| -
|
| - class Test : public shell::test::ServiceTest {
|
| - public:
|
| - Test() : shell::test::ServiceTest(exe:service_unittest) {}
|
| - ~Test() override {}
|
| - }
|
| -
|
| - TEST_F(Test, Basic) {
|
| - mojom::SomeInterface some_interface;
|
| - connector()->ConnectToInterface("mojo:service", &some_interface);
|
| - base::RunLoop loop;
|
| - some_interface->Foo(base::Bind(&QuitLoop, &loop));
|
| - loop.Run();
|
| - }
|
| -
|
| -The BUILD.gn for this test file looks like any other using the test() template.
|
| -It must also depend on //services/shell/public/cpp:shell_test_support.
|
| -
|
| -ServiceTest does a few things, but most importantly it register the test itself
|
| -as a Service, with the name you pass it via its constructor. In the example
|
| -above, we supplied the name exe:service_unittest. This name is has no special
|
| -meaning other than that henceforth it will be used to identify the test service.
|
| -
|
| -Behind the scenes, ServiceTest spins up the Service Manager on a background
|
| -thread, and asks it to create an instance for the test service on the main
|
| -thread, with the name supplied. ServiceTest blocks the main thread while the
|
| -Service Manager thread does this initialization. Once the Service Manager has
|
| -created the instance, it calls OnStart() (as for any other service), and the
|
| -main thread continues, running the test. At this point accessors defined in
|
| -service_test.h like connector() can be used to connect to other services.
|
| -
|
| -Youll note in the example above I made Foo() take a callback, this is to give
|
| -the test something interesting to do. In the mojom for SomeInterface wed have
|
| -the Foo() method return an empty response. In mojo:service, wed have Foo() take
|
| -the callback as a parameter, and run it. In the test, we spin a RunLoop until we
|
| -get that response. In real world cases we can pass back state & validate
|
| -expectations. You can see real examples of this test framework in use in the
|
| -Service Managers own suite of tests, under //services/shell/tests.
|
| -
|
| -### Packaging
|
| -
|
| -By default a .library statically links its dependencies, so having many of them
|
| -will yield an installed product many times larger than Chrome today. For this
|
| -reason its desirable to package several Services together in a single binary.
|
| -The Service Manager provides an interface **shell.mojom.ServiceFactory**:
|
| -
|
| - interface ServiceFactory {
|
| - CreateService(Service& service, string name);
|
| - };
|
| -
|
| -When implemented by a service, the service becomes a package of other
|
| -services, which are instantiated by this interface. Imagine we have two services
|
| -mojo:service1 and mojo:service2, and we wish to package them together in a
|
| -single package mojo:services. We write the Service implementations for
|
| -mojo:service1 and mojo:service2, and then a Service implementation for
|
| -mojo:services - the latter implements ServiceFactory and instantiates the other
|
| -two:
|
| -
|
| - using shell::mojom::ServiceFactory;
|
| - using shell::mojom::ServiceRequest;
|
| -
|
| - class Services : public shell::Service,
|
| - public shell::InterfaceFactory<ServiceFactory>,
|
| - public ServiceFactory {
|
| -
|
| - // Expose ServiceFactory to inbound connections and implement
|
| - // InterfaceFactory to bind requests for it to this object.
|
| - void CreateService(ServiceRequest request,
|
| - const std::string& name) {
|
| - if (name == mojo:service1)
|
| - new Service1(std::move(request));
|
| - else if (name == mojo:service2)
|
| - new Service2(std::move(request));
|
| - }
|
| - }
|
| -
|
| -This is only half the story though. While this does mean that mojo:service1 and
|
| -mojo:service2 are now packaged (statically linked) with mojo:services, as it
|
| -stands to connect to either packaged service youd have to connect to
|
| -mojo:services first, and call CreateService yourself. This is undesirable for a
|
| -couple of reasons, firstly in that it complicates the connect flow, secondly in
|
| -that it forces details of the packaging, which are a distribution-level
|
| -implementation detail on clients wishing to use a service.
|
| -
|
| -To solve this, the Service Manager actually automates resolving packaged service
|
| -names to the package service. The Service Manager considers the name of a
|
| -service provided by some other package service to be an alias to that package
|
| -service. The Service Manager resolves these aliases based on information found,
|
| -you guessed it, in the manifests for the package client.
|
| -
|
| -Lets imagine mojo:service1 and mojo:service2 have typical manifests of the form
|
| -we covered earlier. Now imagine mojo:services, the package service that combines
|
| -the two. In the application install directory rather than the following
|
| -structure:
|
| -
|
| - service1/service1.library,manifest.json
|
| - service2/service2.library,manifest.json
|
| -
|
| -Instead well have:
|
| -
|
| - package/services.library,manifest.json
|
| -
|
| -The manifest for the package service describes not only itself, but includes the
|
| -manifests of all the services it provides. Fortunately there is some GN build
|
| -magic that automates generating this meta-manifest, so you dont need to write
|
| -it by hand. In the service_manifest() template instantiation for services, we
|
| -add the following lines:
|
| -
|
| - deps = [ ":service1_manifest", ":service2_manifest" ]
|
| - packaged_services = [ "service1", "service2" ]
|
| -
|
| -The deps line lists the service_manifest targets for the packaged services to be
|
| -consumed, and the packaged_services line provides the service names, without the
|
| -mojo: prefix. The presence of these two lines will cause the Manifest Collator
|
| -script to run, merging the dependent manifests into the package manifest. You
|
| -can study the resulting manifest to see what gets generated.
|
| -
|
| -At startup, the Service Manager will scan the package directory and consume the
|
| -manifests it finds, so it can learn about how to resolve aliases that it might
|
| -encounter subsequently.
|
| -
|
| -### Executables
|
| -
|
| -Thus far, the examples weve covered have packaged Services in .library files.
|
| -Its also possible to have a conventional executable provide a Service. There
|
| -are two different ways to use executables with the Service Manager, the first is
|
| -to have the Service Manager start the executable itself, the second is to have
|
| -some other executable start the process and then tell the Service Manager about
|
| -it. In both cases, the target executable has to perform a handshake with the
|
| -Service Manager early on so it can bind the Service request the Service Manager
|
| -sends it.
|
| -
|
| -Assuming you have an executable that properly initializes the Mojo EDK, you add
|
| -the following lines at some point early in application startup to establish the
|
| -connection with the Service Manager:
|
| -
|
| - #include "services/shell/public/cpp/service.h"
|
| - #include "services/shell/public/cpp/service_context.h"
|
| - #include "services/shell/runner/child/runner_connection.h"
|
| -
|
| - class MyClient : public shell::Service {
|
| - ..
|
| - };
|
| -
|
| - shell::mojom::ServiceRequest request;
|
| - scoped_ptr<shell::RunnerConnection> connection(
|
| - shell::RunnerConnection::ConnectToRunner(
|
| - &request, ScopedMessagePipeHandle()));
|
| - MyService service;
|
| - shell::ServiceContext context(&service, std::move(request));
|
| -
|
| -Whats happening here? The Service/ServiceContext usage should be familiar from
|
| -our earlier examples. The interesting part here happens in
|
| -`RunnerConnection::ConnectToRunner()`. Before we look at what ConnectToRunner
|
| -does, its important to cover how this process is launched. In this example,
|
| -this process is launched by the Service Manager. This is achieved through the
|
| -use of the exe Service Name type. The Service Names weve covered thus far
|
| -have looked like mojo:foo. The mojo prefix means that the Shell should look
|
| -for a .library file at foo/foo.library alongside the Service Manager
|
| -executable. If the code above was linked into an executable app.exe alongside
|
| -the Service Manager executable in the output directory, it can be launched by
|
| -connecting to the name exe:app. When the Service Manager launches an
|
| -executable, it passes a pipe to it on the command line, which the executable is
|
| -expected to bind to receive a ServiceRequest on. Now back to ConnectToRunner.
|
| -It spins up a background control thread with the Service Manager, binds the
|
| -pipe from the command line parameter, and blocks the main thread until the
|
| -ServiceRequest arrives and can be bound.
|
| -
|
| -Like services provided from .library files, we have to provide a manifest for
|
| -services provided from executables. The format is identical, but in the
|
| -service_manifest template we need to set the type property to exe to cause the
|
| -generation step to put the manifest in the right place (it gets placed alongside
|
| -the executable, with the name <exe_name>_manifest.json.)
|
| -
|
| -### Service-Launched Processes
|
| -
|
| -There are some scenarios where a service will need to launch its own process,
|
| -rather than relying on the Service Manager to do it. The Connector API provides
|
| -the ability to tell the Shell about a process that the service has or will
|
| -create. The executable that the service launches (henceforth referred to as the
|
| -target) should be written using RunnerConnection as discussed in the previous
|
| -section. The connect flow in the service that launches the target (henceforth
|
| -referred to as the driver) works like this:
|
| -
|
| - base::FilePath target_path;
|
| - base::PathService::Get(base::DIR_EXE, &target_path);
|
| - target_path = target_path.Append(FILE_PATH_LITERAL("target.exe"));
|
| - base::CommandLine target_command_line(target_path);
|
| -
|
| - mojo::edk::PlatformChannelPair pair;
|
| - mojo::edk::HandlePassingInformation info;
|
| - pair.PrepareToPassClientHandleToChildProcess(&target_command_line, &info);
|
| -
|
| - std::string token = mojo::edk::GenerateRandomToken();
|
| - target_command_line.AppendSwitchASCII(switches::kPrimordialPipeToken,
|
| - token);
|
| -
|
| - mojo::ScopedMessagePipeHandle pipe =
|
| - mojo::edk::CreateParentMessagePipe(token);
|
| -
|
| - shell::mojom::ServiceFactoryPtr factory;
|
| - factory.Bind(
|
| - mojo::InterfacePtrInfo<shell::mojom::ServiceFactory>(
|
| - std::move(pipe), 0u));
|
| - shell::mojom::PIDReceiverPtr receiver;
|
| -
|
| - shell::Identity target("exe:target",shell::mojom::kInheritUserID);
|
| - shell::Connector::ConnectParams params(target);
|
| - params.set_client_process_connection(std::move(factory),
|
| - GetProxy(&receiver));
|
| - scoped_ptr<shell::Connection> connection = connector->Connect(¶ms);
|
| -
|
| - base::LaunchOptions options;
|
| - options.handles_to_inherit = &info;
|
| - base::Process process = base::LaunchProcess(target_command_line, options);
|
| - mojo::edk::ChildProcessLaunched(process.Handle(), pair.PassServerHandle());
|
| -
|
| -Thats a lot. But it boils down to these steps:
|
| -1. Creating the message pipe to connect the target process and the Service
|
| -Manager.
|
| -2. Putting the server end of the pipe onto the command line to the target
|
| -process.
|
| -3. Binding the client end to a ServiceFactoryPtr, constructing an Identity for
|
| -the target process and passing both through Connector::Connect().
|
| -4. Starting the process with the configured command line.
|
| -
|
| -In this example the target executable could be the same as the previous example.
|
| -
|
| -A word about process lifetimes. Processes created by the shell are managed by
|
| -the Service Manager. While a service-launched process may quit itself at any
|
| -point, when the Service Manager shuts down it will also shut down any process it
|
| -started. Processes created by services themselves are left to those services to
|
| -manage.
|
| -
|
| -***
|
| -
|
| -TBD:
|
| -
|
| -Instances & Processes
|
| -
|
| -Client lifetime strategies
|
| -
|
| -Process lifetimes.
|
| -
|
| -Writing tests (ShellTest)
|
| -Under the Hood
|
| -Four major components: Shell API (Mojom), Shell, Catalog, Shell Client Lib.
|
| -The connect flow, catalog, etc.
|
| -Capability brokering in the shell
|
| -Userids
|
| -
|
| -Finer points:
|
| -
|
| -Mojo Names: mojo, exe
|
| -Exposing services on outbound connections
|
|
|