Resolves #4183 - Implemementing context-based MIME type sniffing

The version of the standard is not finalized at the time of this writing.
Specifications may be found here: https://mimesniff.spec.whatwg.org/#context-specific-sniffing .

components/script/document_loader.rs

@@ -7,7 +7,7 @@
 
 use msg::constellation_msg::PipelineId;
 use net_traits::AsyncResponseTarget;
-use net_traits::{PendingAsyncLoad, ResourceTask};
+use net_traits::{PendingAsyncLoad, ResourceTask, LoadContext};
 use std::sync::Arc;
 use url::Url;
 
@@ -30,6 +30,15 @@ impl LoadType {
             LoadType::PageSource(ref url) => url,
         }
     }
+
+    fn to_load_context(&self) -> LoadContext {
+        match *self {
+            LoadType::Image(_) => LoadContext::Image,
+            LoadType::Script(_) => LoadContext::Script,
+            LoadType::Subframe(_) | LoadType::PageSource(_) => LoadContext::Browsing,
+            LoadType::Stylesheet(_) => LoadContext::Style
+        }
+    }
 }
 
 #[derive(JSTraceable, HeapSizeOf)]
@@ -67,9 +76,10 @@ impl DocumentLoader {
     /// Create a new pending network request, which can be initiated at some point in
     /// the future.
     pub fn prepare_async_load(&mut self, load: LoadType) -> PendingAsyncLoad {
+        let context = load.to_load_context();
         let url = load.url().clone();
         self.blocking_loads.push(load);
-        PendingAsyncLoad::new((*self.resource_task).clone(), url, self.pipeline)
+        PendingAsyncLoad::new(context, (*self.resource_task).clone(), url, self.pipeline)
     }
 
     /// Create and initiate a new network request.
@@ -78,7 +88,6 @@ impl DocumentLoader {
         pending.load_async(listener)
     }
 
-
     /// Mark an in-progress network request complete.
     pub fn finish_load(&mut self, load: LoadType) {
         let idx = self.blocking_loads.iter().position(|unfinished| *unfinished == load);

components/script/dom/dedicatedworkerglobalscope.rs

@@ -26,7 +26,7 @@ use js::jsapi::{JSAutoCompartment, JSAutoRequest};
 use js::jsval::UndefinedValue;
 use js::rust::Runtime;
 use msg::constellation_msg::PipelineId;
-use net_traits::load_whole_resource;
+use net_traits::{LoadContext, load_whole_resource};
 use rand::random;
 use script_task::ScriptTaskEventCategory::WorkerEvent;
 use script_task::{ScriptTask, ScriptChan, ScriptPort, StackRootTLS, CommonScriptMsg};
@@ -220,7 +220,7 @@ impl DedicatedWorkerGlobalScope {
             let roots = RootCollection::new();
             let _stack_roots_tls = StackRootTLS::new(&roots);
 
-            let (url, source) = match load_whole_resource(&init.resource_task, worker_url, None) {
+            let (url, source) = match load_whole_resource(LoadContext::Script, &init.resource_task, worker_url, None) {
                 Err(_) => {
                     println!("error loading script {}", serialized_worker_url);
                     parent_sender.send(CommonScriptMsg::RunnableMsg(WorkerEvent,

components/script/dom/workerglobalscope.rs

@@ -21,7 +21,7 @@ use ipc_channel::ipc::IpcSender;
 use js::jsapi::{HandleValue, JSAutoRequest, JSContext};
 use js::rust::Runtime;
 use msg::constellation_msg::{ConstellationChan, PipelineId};
-use net_traits::{ResourceTask, load_whole_resource};
+use net_traits::{LoadContext, ResourceTask, load_whole_resource};
 use profile_traits::mem;
 use script_task::{CommonScriptMsg, ScriptChan, ScriptPort};
 use script_traits::ScriptMsg as ConstellationMsg;
@@ -203,7 +203,7 @@ impl WorkerGlobalScopeMethods for WorkerGlobalScope {
         }
 
         for url in urls {
-            let (url, source) = match load_whole_resource(&self.resource_task, url, None) {
+            let (url, source) = match load_whole_resource(LoadContext::Script, &self.resource_task, url, None) {
                 Err(_) => return Err(Error::Network),
                 Ok((metadata, bytes)) => {
                     (metadata.final_url, String::from_utf8(bytes).unwrap())

components/script/dom/xmlhttprequest.rs

@@ -46,7 +46,7 @@ use js::jsapi::{JSContext, JS_ParseJSON, RootedValue};
 use js::jsval::{JSVal, NullValue, UndefinedValue};
 use net_traits::ControlMsg::Load;
 use net_traits::{AsyncResponseListener, AsyncResponseTarget, Metadata};
-use net_traits::{LoadConsumer, LoadData, ResourceCORSData, ResourceTask};
+use net_traits::{LoadConsumer, LoadContext, LoadData, ResourceCORSData, ResourceTask};
 use network_listener::{NetworkListener, PreInvoke};
 use parse::html::{ParseContext, parse_html};
 use parse::xml::{self, parse_xml};
@@ -521,7 +521,10 @@ impl XMLHttpRequestMethods for XMLHttpRequest {
 
         let global = self.global.root();
         let pipeline_id = global.r().pipeline();
-        let mut load_data = LoadData::new(self.request_url.borrow().clone().unwrap(), Some(pipeline_id));
+        let mut load_data =
+            LoadData::new(LoadContext::Browsing,
+                          self.request_url.borrow().clone().unwrap(),
+                          Some(pipeline_id));
         if load_data.url.origin().ne(&global.r().get_url().origin()) {
             load_data.credentials_flag = self.WithCredentials();
         }

components/script/script_task.rs

@@ -70,7 +70,7 @@ use msg::webdriver_msg::WebDriverScriptCommand;
 use net_traits::LoadData as NetLoadData;
 use net_traits::image_cache_task::{ImageCacheChan, ImageCacheResult, ImageCacheTask};
 use net_traits::storage_task::StorageTask;
-use net_traits::{AsyncResponseTarget, ControlMsg, LoadConsumer, Metadata, ResourceTask};
+use net_traits::{AsyncResponseTarget, ControlMsg, LoadConsumer, LoadContext, Metadata, ResourceTask};
 use network_listener::NetworkListener;
 use page::{Frame, IterablePage, Page};
 use parse::html::{ParseContext, parse_html};
@@ -2003,6 +2003,7 @@ impl ScriptTask {
         }
 
         resource_task.send(ControlMsg::Load(NetLoadData {
+            context: LoadContext::Browsing,
             url: load_data.url,
             method: load_data.method,
             headers: Headers::new(),