script: Optionally store backtraces when throwing DOM exceptions.

components/script/Cargo.toml

@@ -17,6 +17,7 @@ unstable = []
 unrooted_must_root_lint = ["script_plugins/unrooted_must_root_lint"]
 default = ["unrooted_must_root_lint"]
 webgl_backtrace = ["backtrace", "canvas_traits/webgl_backtrace"]
+js_backtrace = ["backtrace"]
 
 [build-dependencies]
 cmake = "0.1"

components/script/dom/bindings/error.rs

@@ -4,6 +4,10 @@
 
 //! Utilities to throw exceptions from Rust bindings.
 
+#[cfg(feature = "js_backtrace")]
+use backtrace::Backtrace;
+#[cfg(feature = "js_backtrace")]
+use dom::bindings::cell::DomRefCell;
 use dom::bindings::codegen::Bindings::DOMExceptionBinding::DOMExceptionMethods;
 use dom::bindings::codegen::PrototypeList::proto_id_to_name;
 use dom::bindings::conversions::{ConversionResult, FromJSValConvertible, ToJSValConvertible};
@@ -24,6 +28,11 @@ use js::rust::wrappers::JS_SetPendingException;
 use libc::c_uint;
 use std::slice::from_raw_parts;
 
+/// An optional stringified JS backtrace and stringified native backtrace from the
+/// the last DOM exception that was reported.
+#[cfg(feature = "js_backtrace")]
+thread_local!(static LAST_EXCEPTION_BACKTRACE: DomRefCell<Option<(Option<String>, String)>> = DomRefCell::new(None));
+
 /// DOM exceptions that can be thrown by a native DOM method.
 #[derive(Clone, Debug, MallocSizeOf)]
 pub enum Error {
@@ -90,6 +99,16 @@ pub type ErrorResult = Fallible<()>;
 
 /// Set a pending exception for the given `result` on `cx`.
 pub unsafe fn throw_dom_exception(cx: *mut JSContext, global: &GlobalScope, result: Error) {
+    #[cfg(feature = "js_backtrace")]
+    {
+        capture_stack!(in(cx) let stack);
+        let js_stack = stack.and_then(|s| s.as_string(None));
+        let rust_stack = Backtrace::new();
+        LAST_EXCEPTION_BACKTRACE.with(|backtrace| {
+            *backtrace.borrow_mut() = Some((js_stack, format!("{:?}", rust_stack)));
+        });
+    }
+
     let code = match result {
         Error::IndexSize => DOMErrorName::IndexSizeError,
         Error::NotFound => DOMErrorName::NotFoundError,
@@ -244,6 +263,17 @@ pub unsafe fn report_pending_exception(cx: *mut JSContext, dispatch_event: bool)
         "Error at {}:{}:{} {}",
         error_info.filename, error_info.lineno, error_info.column, error_info.message
     );
+    #[cfg(feature = "js_backtrace")]
+    {
+        LAST_EXCEPTION_BACKTRACE.with(|backtrace| {
+            if let Some((js_backtrace, rust_backtrace)) = backtrace.borrow_mut().take() {
+                if let Some(stack) = js_backtrace {
+                    eprintln!("JS backtrace:\n{}", stack);
+                }
+                eprintln!("Rust backtrace:\n{}", rust_backtrace);
+            }
+        });
+    }
 
     if dispatch_event {
         GlobalScope::from_context(cx).report_an_error(error_info, value.handle());

components/script/lib.rs

@@ -19,7 +19,7 @@
 
 extern crate app_units;
 extern crate audio_video_metadata;
-#[cfg(feature = "webgl_backtrace")]
+#[cfg(any(feature = "webgl_backtrace", feature = "js_backtrace"))]
 extern crate backtrace;
 extern crate base64;
 #[macro_use]

components/servo/Cargo.toml

@@ -17,6 +17,7 @@ webdriver = ["webdriver_server"]
 energy-profiling = ["profile_traits/energy-profiling"]
 debugmozjs = ["script/debugmozjs"]
 googlevr = ["webvr/googlevr"]
+js_backtrace = ["script/js_backtrace"]
 webrender_debugger = ["webrender/debugger"]
 oculusvr = ["webvr/oculusvr"]
 unstable = [

ports/libsimpleservo/Cargo.toml

@@ -42,4 +42,5 @@ debugmozjs = ["libservo/debugmozjs"]
 unstable = ["libservo/unstable"]
 googlevr = ["libservo/googlevr"]
 oculusvr = ["libservo/oculusvr"]
-webgl_backtrace = ["libservo/webgl_backtrace"]
+webgl_backtrace = ["libservo/webgl_backtrace"]
+js_backtrace = ["libservo/js_backtrace"]

ports/servo/Cargo.toml

@@ -31,6 +31,7 @@ energy-profiling = ["libservo/energy-profiling"]
 debugmozjs = ["libservo/debugmozjs"]
 unstable = ["libservo/unstable"]
 webgl_backtrace = ["libservo/webgl_backtrace"]
+js_backtrace = ["libservo/js_backtrace"]
 
 [target.'cfg(not(target_os = "android"))'.dependencies]
 backtrace = "0.3"

python/servo/build_commands.py

@@ -257,6 +257,11 @@ def build(self, target=None, release=False, dev=False, jobs=None,
         if debug_mozjs:
             features += ["debugmozjs"]
 
+        if self.config["build"]["webgl-backtrace"]:
+            features += ["webgl-backtrace"]
+        if self.config["build"]["dom-backtrace"]:
+            features += ["dom-backtrace"]
+
         if features:
             opts += ["--features", "%s" % ' '.join(features)]
 

python/servo/command_base.py

@@ -288,6 +288,8 @@ def resolverelative(category, key):
         self.config["build"].setdefault("rustflags", "")
         self.config["build"].setdefault("incremental", None)
         self.config["build"].setdefault("thinlto", False)
+        self.config["build"].setdefault("webgl-backtrace", False)
+        self.config["build"].setdefault("dom-backtrace", False)
 
         self.config.setdefault("android", {})
         self.config["android"].setdefault("sdk", "")

servobuild.example

@@ -32,6 +32,14 @@ android = false
 # Set "debug-mozjs" or use `mach build --debug-mozjs` to build a debug spidermonkey.
 debug-mozjs = false
 
+# When a GL error occurs as a result of a WebGL operation, print the stack trace for the content
+# JS and native Rust code that triggered the failed operation. Warning: very slow.
+webgl-backtrace = false
+
+# When a DOM exception is reported, print the stack trace for the content JS and native Rust code
+# that triggered it.
+dom-backtrace = false
+
 # Set to the path to your ccache binary to enable caching of compiler outputs
 #ccache = "/usr/local/bin/ccache"