Auto merge of #21825 - jdm:backtrace-helpers, r=Manishearth

Optional backtraces for JS errors and WebGL errors

This adds two new build-time features to enable useful debugging tools when investigating why JS and WebGL content isn't working. They're optional because they're quite heavyweight.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they're optional developer features.

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---
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16 files changed, 131 insertions, 16 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 85859bf7c72..6d839d69c0e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2981,6 +2981,7 @@ version = "0.0.1"
 dependencies = [
  "app_units 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "audio-video-metadata 0.1.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "backtrace 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "base64 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bitflags 1.0.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "bluetooth_traits 0.0.1",
diff --git a/components/canvas/Cargo.toml b/components/canvas/Cargo.toml
index d65c1b4b110..4006947acf6 100644
--- a/components/canvas/Cargo.toml
+++ b/components/canvas/Cargo.toml
@@ -9,6 +9,9 @@ publish = false
 name = "canvas"
 path = "lib.rs"
 
+[features]
+webgl_backtrace = ["canvas_traits/webgl_backtrace"]
+
 [dependencies]
 azure = {git = "https://github.com/servo/rust-azure"}
 canvas_traits = {path = "../canvas_traits"}
diff --git a/components/canvas/gl_context.rs b/components/canvas/gl_context.rs
index 00e3f3ba4a9..c90ecec9cca 100644
--- a/components/canvas/gl_context.rs
+++ b/components/canvas/gl_context.rs
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use canvas_traits::webgl::{WebGLCommand, WebGLVersion};
+use canvas_traits::webgl::{WebGLCommand, WebGLVersion, WebGLCommandBacktrace};
 use compositing::compositor_thread::{CompositorProxy, self};
 use euclid::Size2D;
 use gleam::gl;
@@ -144,13 +144,18 @@ impl GLContextWrapper {
         }
     }
 
-    pub fn apply_command(&self, cmd: WebGLCommand, state: &mut GLState) {
+    pub fn apply_command(
+        &self,
+        cmd: WebGLCommand,
+        backtrace: WebGLCommandBacktrace,
+        state: &mut GLState
+    ) {
         match *self {
             GLContextWrapper::Native(ref ctx) => {
-                WebGLImpl::apply(ctx, state, cmd);
+                WebGLImpl::apply(ctx, state, cmd, backtrace);
             }
             GLContextWrapper::OSMesa(ref ctx) => {
-                WebGLImpl::apply(ctx, state, cmd);
+                WebGLImpl::apply(ctx, state, cmd, backtrace);
             }
         }
     }
diff --git a/components/canvas/webgl_thread.rs b/components/canvas/webgl_thread.rs
index c6d9082b7e0..4d8a4c044a2 100644
--- a/components/canvas/webgl_thread.rs
+++ b/components/canvas/webgl_thread.rs
@@ -137,8 +137,8 @@ impl<VR: WebVRRenderHandler + 'static> WebGLThread<VR> {
             WebGLMsg::RemoveContext(ctx_id) => {
                 self.remove_webgl_context(ctx_id);
             },
-            WebGLMsg::WebGLCommand(ctx_id, command) => {
-                self.handle_webgl_command(ctx_id, command);
+            WebGLMsg::WebGLCommand(ctx_id, command, backtrace) => {
+                self.handle_webgl_command(ctx_id, command, backtrace);
             },
             WebGLMsg::WebVRCommand(ctx_id, command) => {
                 self.handle_webvr_command(ctx_id, command);
@@ -164,10 +164,15 @@ impl<VR: WebVRRenderHandler + 'static> WebGLThread<VR> {
     }
 
     /// Handles a WebGLCommand for a specific WebGLContext
-    fn handle_webgl_command(&mut self, context_id: WebGLContextId, command: WebGLCommand) {
+    fn handle_webgl_command(
+        &mut self,
+        context_id: WebGLContextId,
+        command: WebGLCommand,
+        backtrace: WebGLCommandBacktrace,
+    ) {
         let data = Self::make_current_if_needed_mut(context_id, &mut self.contexts, &mut self.bound_context_id);
         if let Some(data) = data {
-            data.ctx.apply_command(command, &mut data.state);
+            data.ctx.apply_command(command, backtrace, &mut data.state);
         }
     }
 
@@ -672,7 +677,8 @@ impl WebGLImpl {
     pub fn apply<Native: NativeGLContextMethods>(
         ctx: &GLContext<Native>,
         state: &mut GLState,
-        command: WebGLCommand
+        command: WebGLCommand,
+        _backtrace: WebGLCommandBacktrace,
     ) {
         match command {
             WebGLCommand::GetContextAttributes(ref sender) =>
@@ -1193,7 +1199,14 @@ impl WebGLImpl {
         // TODO: update test expectations in order to enable debug assertions
         let error = ctx.gl().get_error();
         if error != gl::NO_ERROR {
-            error!("Last GL operation failed: {:?}", command)
+            error!("Last GL operation failed: {:?}", command);
+            #[cfg(feature = "webgl_backtrace")]
+            {
+                error!("Backtrace from failed WebGL API:\n{}", _backtrace.backtrace);
+                if let Some(backtrace) = _backtrace.js_backtrace {
+                    error!("JS backtrace from failed WebGL API:\n{}", backtrace);
+                }
+            }
         }
         assert_eq!(error, gl::NO_ERROR, "Unexpected WebGL error: 0x{:x} ({})", error, error);
     }
diff --git a/components/canvas_traits/Cargo.toml b/components/canvas_traits/Cargo.toml
index b5599232b5c..187b46d617e 100644
--- a/components/canvas_traits/Cargo.toml
+++ b/components/canvas_traits/Cargo.toml
@@ -9,6 +9,9 @@ publish = false
 name = "canvas_traits"
 path = "lib.rs"
 
+[features]
+webgl_backtrace = []
+
 [dependencies]
 cssparser = "0.24.0"
 euclid = "0.19"
diff --git a/components/canvas_traits/webgl.rs b/components/canvas_traits/webgl.rs
index 8f9f4e70a31..ad10ed7edd0 100644
--- a/components/canvas_traits/webgl.rs
+++ b/components/canvas_traits/webgl.rs
@@ -24,6 +24,14 @@ pub use ::webgl_channel::WebGLPipeline;
 /// Entry point channel type used for sending WebGLMsg messages to the WebGL renderer.
 pub use ::webgl_channel::WebGLChan;
 
+#[derive(Clone, Deserialize, Serialize)]
+pub struct WebGLCommandBacktrace {
+    #[cfg(feature = "webgl_backtrace")]
+    pub backtrace: String,
+    #[cfg(feature = "webgl_backtrace")]
+    pub js_backtrace: Option<String>,
+}
+
 /// WebGL Message API
 #[derive(Deserialize, Serialize)]
 pub enum WebGLMsg {
@@ -35,7 +43,7 @@ pub enum WebGLMsg {
     /// Drops a WebGLContext.
     RemoveContext(WebGLContextId),
     /// Runs a WebGLCommand in a specific WebGLContext.
-    WebGLCommand(WebGLContextId, WebGLCommand),
+    WebGLCommand(WebGLContextId, WebGLCommand, WebGLCommandBacktrace),
     /// Runs a WebVRCommand in a specific WebGLContext.
     WebVRCommand(WebGLContextId, WebVRCommand),
     /// Locks a specific WebGLContext. Lock messages are used for a correct synchronization
@@ -121,8 +129,8 @@ impl WebGLMsgSender {
 
     /// Send a WebGLCommand message
     #[inline]
-    pub fn send(&self, command: WebGLCommand) -> WebGLSendResult {
-        self.sender.send(WebGLMsg::WebGLCommand(self.ctx_id, command))
+    pub fn send(&self, command: WebGLCommand, backtrace: WebGLCommandBacktrace) -> WebGLSendResult {
+        self.sender.send(WebGLMsg::WebGLCommand(self.ctx_id, command, backtrace))
     }
 
     /// Send a WebVRCommand message
diff --git a/components/script/Cargo.toml b/components/script/Cargo.toml
index 1c8feb0f17e..ca859bb472a 100644
--- a/components/script/Cargo.toml
+++ b/components/script/Cargo.toml
@@ -16,6 +16,8 @@ debugmozjs = ['mozjs/debugmozjs']
 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"
@@ -29,6 +31,7 @@ tinyfiledialogs = "3.0"
 [dependencies]
 app_units = "0.7"
 audio-video-metadata = "0.1.4"
+backtrace = {version = "0.3", optional = true}
 base64 = "0.6"
 bitflags = "1.0"
 bluetooth_traits = {path = "../bluetooth_traits"}
diff --git a/components/script/dom/bindings/error.rs b/components/script/dom/bindings/error.rs
index ce045b2c9d3..64b581f316d 100644
--- a/components/script/dom/bindings/error.rs
+++ b/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());
diff --git a/components/script/dom/webglrenderingcontext.rs b/components/script/dom/webglrenderingcontext.rs
index 7367a38fdcf..754637ad52b 100644
--- a/components/script/dom/webglrenderingcontext.rs
+++ b/components/script/dom/webglrenderingcontext.rs
@@ -2,9 +2,11 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#[cfg(feature = "webgl_backtrace")]
+use backtrace::Backtrace;
 use byteorder::{ByteOrder, NativeEndian, WriteBytesExt};
 use canvas_traits::canvas::{byte_swap, multiply_u8_pixel};
-use canvas_traits::webgl::{DOMToTextureCommand, Parameter};
+use canvas_traits::webgl::{DOMToTextureCommand, Parameter, WebGLCommandBacktrace};
 use canvas_traits::webgl::{TexParameter, WebGLCommand, WebGLContextShareMode, WebGLError};
 use canvas_traits::webgl::{WebGLFramebufferBindingRequest, WebGLMsg, WebGLMsgSender};
 use canvas_traits::webgl::{WebGLProgramId, WebGLResult, WebGLSLVersion, WebGLSender};
@@ -316,7 +318,7 @@ impl WebGLRenderingContext {
 
     #[inline]
     pub fn send_command(&self, command: WebGLCommand) {
-        self.webgl_sender.send(command).unwrap();
+        self.webgl_sender.send(command, capture_webgl_backtrace(self)).unwrap();
     }
 
     #[inline]
@@ -1189,6 +1191,25 @@ impl WebGLRenderingContext {
     }
 }
 
+#[cfg(not(feature = "webgl_backtrace"))]
+#[inline]
+pub fn capture_webgl_backtrace<T: DomObject>(_: &T) -> WebGLCommandBacktrace {
+    WebGLCommandBacktrace {}
+}
+
+#[cfg(feature = "webgl_backtrace")]
+#[cfg_attr(feature = "webgl_backtrace", allow(unsafe_code))]
+pub fn capture_webgl_backtrace<T: DomObject>(obj: &T) -> WebGLCommandBacktrace {
+    let bt = Backtrace::new();
+    unsafe {
+        capture_stack!(in(obj.global().get_cx()) let stack);
+        WebGLCommandBacktrace {
+            backtrace: format!("{:?}", bt),
+            js_backtrace: stack.and_then(|s| s.as_string(None)),
+        }
+    }
+}
+
 impl Drop for WebGLRenderingContext {
     fn drop(&mut self) {
         let _ = self.webgl_sender.send_remove();
@@ -1521,9 +1542,10 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
         let (sender, receiver) = webgl_channel().unwrap();
 
         // If the send does not succeed, assume context lost
+        let backtrace = capture_webgl_backtrace(self);
         if self
             .webgl_sender
-            .send(WebGLCommand::GetContextAttributes(sender))
+            .send(WebGLCommand::GetContextAttributes(sender), backtrace)
             .is_err()
         {
             return None;
diff --git a/components/script/lib.rs b/components/script/lib.rs
index f8341323466..097f69d7808 100644
--- a/components/script/lib.rs
+++ b/components/script/lib.rs
@@ -19,6 +19,8 @@
 
 extern crate app_units;
 extern crate audio_video_metadata;
+#[cfg(any(feature = "webgl_backtrace", feature = "js_backtrace"))]
+extern crate backtrace;
 extern crate base64;
 #[macro_use]
 extern crate bitflags;
diff --git a/components/servo/Cargo.toml b/components/servo/Cargo.toml
index 423d6814810..7d2a7895215 100644
--- a/components/servo/Cargo.toml
+++ b/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 = [
@@ -24,6 +25,11 @@ unstable = [
     "profile/unstable",
     "script/unstable",
 ]
+webgl_backtrace = [
+    "script/webgl_backtrace",
+    "canvas/webgl_backtrace",
+    "canvas_traits/webgl_backtrace",
+]
 
 [dependencies]
 bluetooth_traits = {path = "../bluetooth_traits"}
diff --git a/ports/libsimpleservo/Cargo.toml b/ports/libsimpleservo/Cargo.toml
index 6cb4d4f6c3c..5af57fe8e69 100644
--- a/ports/libsimpleservo/Cargo.toml
+++ b/ports/libsimpleservo/Cargo.toml
@@ -42,3 +42,5 @@ debugmozjs = ["libservo/debugmozjs"]
 unstable = ["libservo/unstable"]
 googlevr = ["libservo/googlevr"]
 oculusvr = ["libservo/oculusvr"]
+webgl_backtrace = ["libservo/webgl_backtrace"]
+js_backtrace = ["libservo/js_backtrace"]
diff --git a/ports/servo/Cargo.toml b/ports/servo/Cargo.toml
index a31f6306514..298dae298f9 100644
--- a/ports/servo/Cargo.toml
+++ b/ports/servo/Cargo.toml
@@ -30,6 +30,8 @@ webdriver = ["libservo/webdriver"]
 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"
diff --git a/python/servo/build_commands.py b/python/servo/build_commands.py
index da74a201aa4..5dc21d53e7d 100644
--- a/python/servo/build_commands.py
+++ b/python/servo/build_commands.py
@@ -264,6 +264,11 @@ class MachCommands(CommandBase):
         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)]
 
diff --git a/python/servo/command_base.py b/python/servo/command_base.py
index b6faa53a5e1..3770de6c9cd 100644
--- a/python/servo/command_base.py
+++ b/python/servo/command_base.py
@@ -288,6 +288,8 @@ class CommandBase(object):
         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", "")
diff --git a/servobuild.example b/servobuild.example
index 2df2dc256e1..2070da1a59c 100644
--- a/servobuild.example
+++ b/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"