From 4dd7387799a39078bba1c5ed649907b72328fbfe Mon Sep 17 00:00:00 2001
From: Josh Matthews <josh@joshmatthews.net>
Date: Tue, 14 Jul 2020 12:17:05 -0400
Subject: [PATCH] dom: Add Window.js_backtrace debugging extension method.

---
 components/script/Cargo.toml                |  6 +++---
 components/script/dom/webidls/Window.webidl |  2 ++
 components/script/dom/window.rs             | 17 ++++++++++++++++-
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/components/script/Cargo.toml b/components/script/Cargo.toml
index 225367a7090..d155919ba12 100644
--- a/components/script/Cargo.toml
+++ b/components/script/Cargo.toml
@@ -18,8 +18,8 @@ jitspew = ['js/jitspew']
 profilemozjs = ['js/profilemozjs']
 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"]
+webgl_backtrace = ["canvas_traits/webgl_backtrace"]
+js_backtrace = []
 refcell_backtrace = ["accountable-refcell"]
 uwp = ["js/uwp"]
 xr-profile = ["webxr-api/profile"]
@@ -33,7 +33,7 @@ serde_json = "1.0"
 accountable-refcell = { version = "0.2.0", optional = true }
 app_units = "0.7"
 arrayvec = "0.5.1"
-backtrace = { version = "0.3", optional = true }
+backtrace = "0.3"
 base64 = "0.10.1"
 bitflags = "1.0"
 bluetooth_traits = { path = "../bluetooth_traits" }
diff --git a/components/script/dom/webidls/Window.webidl b/components/script/dom/webidls/Window.webidl
index 80a8d31595a..fa3460e9889 100644
--- a/components/script/dom/webidls/Window.webidl
+++ b/components/script/dom/webidls/Window.webidl
@@ -142,6 +142,8 @@ partial interface Window {
   void gc();
   [Pref="dom.servo_helpers.enabled"]
   void trap();
+  [Pref="dom.servo_helpers.enabled"]
+  void js_backtrace();
 };
 
 // WebDriver extensions
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index d6a4ac22622..09497c0798a 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -72,6 +72,7 @@ use crate::task_source::{TaskSource, TaskSourceName};
 use crate::timers::{IsInterval, TimerCallback};
 use crate::webdriver_handlers::jsval_to_webdriver;
 use app_units::Au;
+use backtrace::Backtrace;
 use base64;
 use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLChan;
@@ -89,7 +90,7 @@ use js::jsapi::Heap;
 use js::jsapi::JSAutoRealm;
 use js::jsapi::JSObject;
 use js::jsapi::JSPROP_ENUMERATE;
-use js::jsapi::{GCReason, JS_GC};
+use js::jsapi::{GCReason, StackFormat, JS_GC};
 use js::jsval::UndefinedValue;
 use js::jsval::{JSVal, NullValue};
 use js::rust::wrappers::JS_DefineProperty;
@@ -1081,6 +1082,20 @@ impl WindowMethods for Window {
         unsafe { ::std::intrinsics::breakpoint() }
     }
 
+    #[allow(unsafe_code)]
+    fn Js_backtrace(&self) {
+        unsafe {
+            capture_stack!(in(*self.get_cx()) let stack);
+            let js_stack = stack.and_then(|s| s.as_string(None, StackFormat::SpiderMonkey));
+            let rust_stack = Backtrace::new();
+            println!(
+                "Current JS stack:\n{}\nCurrent Rust stack:\n{:?}",
+                js_stack.unwrap_or(String::new()),
+                rust_stack
+            );
+        }
+    }
+
     #[allow(unsafe_code)]
     fn WebdriverCallback(&self, cx: JSContext, val: HandleValue) {
         let rv = unsafe { jsval_to_webdriver(*cx, &self.globalscope, val) };