introduce a pipeline namespace installer

components/msg/Cargo.toml

@@ -13,8 +13,11 @@ test = false
 doctest = false
 
 [dependencies]
+lazy_static = "1"
+ipc-channel = "0.11"
 malloc_size_of = { path = "../malloc_size_of" }
 malloc_size_of_derive = "0.1"
+parking_lot = "0.8"
 serde = "1.0.60"
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}
 

components/msg/constellation_msg.rs

@@ -5,10 +5,13 @@
 //! The high-level interface from script to constellation. Using this abstract interface helps
 //! reduce coupling between these two components.
 
+use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
+use parking_lot::Mutex;
 use std::cell::Cell;
 use std::fmt;
 use std::mem;
 use std::num::NonZeroU32;
+use std::sync::Arc;
 use std::time::Duration;
 
 #[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
@@ -17,12 +20,71 @@ pub enum TraversalDirection {
     Back(usize),
 }
 
+#[derive(Debug, Deserialize, Serialize)]
+/// Request a pipeline-namespace id from the constellation.
+pub struct PipelineNamespaceRequest(pub IpcSender<PipelineNamespaceId>);
+
+/// A per-process installer of pipeline-namespaces.
+pub struct PipelineNamespaceInstaller {
+    request_sender: Option<IpcSender<PipelineNamespaceRequest>>,
+    namespace_sender: IpcSender<PipelineNamespaceId>,
+    namespace_receiver: IpcReceiver<PipelineNamespaceId>,
+}
+
+impl PipelineNamespaceInstaller {
+    pub fn new() -> Self {
+        let (namespace_sender, namespace_receiver) =
+            ipc::channel().expect("PipelineNamespaceInstaller ipc channel failure");
+        PipelineNamespaceInstaller {
+            request_sender: None,
+            namespace_sender: namespace_sender,
+            namespace_receiver: namespace_receiver,
+        }
+    }
+
+    /// Provide a request sender to send requests to the constellation.
+    pub fn set_sender(&mut self, sender: IpcSender<PipelineNamespaceRequest>) {
+        self.request_sender = Some(sender);
+    }
+
+    /// Install a namespace, requesting a new Id from the constellation.
+    pub fn install_namespace(&self) {
+        match self.request_sender.as_ref() {
+            Some(sender) => {
+                let _ = sender.send(PipelineNamespaceRequest(self.namespace_sender.clone()));
+                let namespace_id = self
+                    .namespace_receiver
+                    .recv()
+                    .expect("The constellation to make a pipeline namespace id available");
+                PipelineNamespace::install(namespace_id);
+            },
+            None => unreachable!("PipelineNamespaceInstaller should have a request_sender setup"),
+        }
+    }
+}
+
+lazy_static! {
+    /// A per-process unique pipeline-namespace-installer.
+    /// Accessible via PipelineNamespace.
+    ///
+    /// Use PipelineNamespace::set_installer_sender to initiate with a sender to the constellation,
+    /// when a new process has been created.
+    ///
+    /// Use PipelineNamespace::fetch_install to install a unique pipeline-namespace from the calling thread.
+    static ref PIPELINE_NAMESPACE_INSTALLER: Arc<Mutex<PipelineNamespaceInstaller>> =
+        Arc::new(Mutex::new(PipelineNamespaceInstaller::new()));
+}
+
 /// Each pipeline ID needs to be unique. However, it also needs to be possible to
 /// generate the pipeline ID from an iframe element (this simplifies a lot of other
 /// code that makes use of pipeline IDs).
 ///
 /// To achieve this, each pipeline index belongs to a particular namespace. There is
 /// a namespace for the constellation thread, and also one for every script thread.
+///
+/// A namespace can be installed for any other thread in a process
+/// where an pipeline-installer has been initialized.
+///
 /// This allows pipeline IDs to be generated by any of those threads without conflicting
 /// with pipeline IDs created by other script threads or the constellation. The
 /// constellation is the only code that is responsible for creating new *namespaces*.
@@ -39,6 +101,7 @@ pub struct PipelineNamespace {
 }
 
 impl PipelineNamespace {
+    /// Install a namespace for a given Id.
     pub fn install(namespace_id: PipelineNamespaceId) {
         PIPELINE_NAMESPACE.with(|tls| {
             assert!(tls.get().is_none());
@@ -49,6 +112,25 @@ impl PipelineNamespace {
         });
     }
 
+    /// Setup the pipeline-namespace-installer, by providing it with a sender to the constellation.
+    /// Idempotent in single-process mode.
+    pub fn set_installer_sender(sender: IpcSender<PipelineNamespaceRequest>) {
+        PIPELINE_NAMESPACE_INSTALLER.lock().set_sender(sender);
+    }
+
+    /// Install a namespace in the current thread, without requiring having a namespace Id ready.
+    /// Panics if called more than once per thread.
+    pub fn auto_install() {
+        // Note that holding the lock for the duration of the call is irrelevant to performance,
+        // since a thread would have to block on the ipc-response from the constellation,
+        // with the constellation already acting as a global lock on namespace ids,
+        // and only being able to handle one request at a time.
+        //
+        // Hence, any other thread attempting to concurrently install a namespace
+        // would have to wait for the current call to finish, regardless of the lock held here.
+        PIPELINE_NAMESPACE_INSTALLER.lock().install_namespace();
+    }
+
     fn next_index(&mut self) -> NonZeroU32 {
         self.index += 1;
         NonZeroU32::new(self.index).expect("pipeline id index wrapped!")

components/msg/lib.rs

@@ -4,6 +4,8 @@
 
 #![deny(unsafe_code)]
 
+#[macro_use]
+extern crate lazy_static;
 #[macro_use]
 extern crate malloc_size_of;
 #[macro_use]