Bug 1083359 - Part 2 - Allow C++ code to provide an async stack when calling a JS function. r=jimb

js/src/asmjs/AsmJSFrameIterator.cpp

@@ -115,8 +115,8 @@ AsmJSFrameIterator::computeLine(uint32_t *column) const
 static const unsigned PushedRetAddr = 0;
 static const unsigned PostStorePrePopFP = 0;
 # endif
-static const unsigned PushedFP = 10;
+static const unsigned PushedFP = 13;
-static const unsigned StoredFP = 14;
+static const unsigned StoredFP = 20;
 #elif defined(JS_CODEGEN_X86)
 # if defined(DEBUG)
 static const unsigned PushedRetAddr = 0;

js/src/builtin/TestingFunctions.cpp

@@ -1007,6 +1007,37 @@ SaveStack(JSContext *cx, unsigned argc, jsval *vp)
     return true;
 }
 
+static bool
+CallFunctionWithAsyncStack(JSContext *cx, unsigned argc, jsval *vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    if (args.length() != 3) {
+        JS_ReportError(cx, "The function takes exactly three arguments.");
+        return false;
+    }
+    if (!args[0].isObject() || !IsCallable(args[0])) {
+        JS_ReportError(cx, "The first argument should be a function.");
+        return false;
+    }
+    if (!args[1].isObject() || !args[1].toObject().is<SavedFrame>()) {
+        JS_ReportError(cx, "The second argument should be a SavedFrame.");
+        return false;
+    }
+    if (!args[2].isString() || args[2].toString()->empty()) {
+        JS_ReportError(cx, "The third argument should be a non-empty string.");
+        return false;
+    }
+
+    RootedObject function(cx, &args[0].toObject());
+    RootedObject stack(cx, &args[1].toObject());
+    RootedString asyncCause(cx, args[2].toString());
+
+    JS::AutoSetAsyncStackForNewCalls sas(cx, stack, asyncCause);
+    return Call(cx, UndefinedHandleValue, function,
+                JS::HandleValueArray::empty(), args.rval());
+}
+
 static bool
 EnableTrackAllocations(JSContext *cx, unsigned argc, jsval *vp)
 {
@@ -2438,6 +2469,13 @@ static const JSFunctionSpecWithHelp TestingFunctions[] = {
 "  of frames. If 'compartment' is given, allocate the js::SavedFrame instances\n"
 "  with the given object's compartment."),
 
+    JS_FN_HELP("callFunctionWithAsyncStack", CallFunctionWithAsyncStack, 0, 0,
+"callFunctionWithAsyncStack(function, stack, asyncCause)",
+"  Call 'function', using the provided stack as the async stack responsible\n"
+"  for the call, and propagate its return value or the exception it throws.\n"
+"  The function is called with no arguments, and 'this' is 'undefined'. The\n"
+"  specified |asyncCause| is attached to the provided stack frame."),
+
     JS_FN_HELP("enableTrackAllocations", EnableTrackAllocations, 0, 0,
 "enableTrackAllocations()",
 "  Start capturing the JS stack at every allocation. Note that this sets an\n"

js/src/gc/RootMarking.cpp

@@ -451,6 +451,14 @@ js::gc::GCRuntime::markRuntime(JSTracer *trc,
         MarkPersistentRootedChains(trc);
     }
 
+    if (rt->asyncStackForNewActivations)
+        MarkObjectRoot(trc, &rt->asyncStackForNewActivations,
+                       "asyncStackForNewActivations");
+
+    if (rt->asyncCauseForNewActivations)
+        MarkStringRoot(trc, &rt->asyncCauseForNewActivations,
+                       "asyncCauseForNewActivations");
+
     if (rt->scriptAndCountsVector) {
         ScriptAndCountsVector &vec = *rt->scriptAndCountsVector;
         for (size_t i = 0; i < vec.length(); i++)

js/src/jit-test/tests/saved-stacks/async-max-frame-count.js

@@ -0,0 +1,84 @@
+// Test that async stacks are limited on recursion.
+
+const defaultAsyncStackLimit = 60;
+
+function recur(n, limit) {
+  if (n > 0) {
+    return callFunctionWithAsyncStack(recur.bind(undefined, n - 1, limit),
+                                      saveStack(limit), "Recurse");
+  }
+  return saveStack(limit);
+}
+
+function checkRecursion(n, limit) {
+  print("checkRecursion(" + uneval(n) + ", " + uneval(limit) + ")");
+
+  var stack = recur(n, limit);
+
+  // Async stacks are limited even if we didn't ask for a limit. There is a
+  // default limit on frames attached on top of any synchronous frames. In this
+  // case the synchronous frame is the last call to `recur`.
+  if (limit == 0) {
+    limit = defaultAsyncStackLimit + 1;
+  }
+
+  // The first `n` or `limit` frames should have `recur` as their `asyncParent`.
+  for (var i = 0; i < Math.min(n, limit); i++) {
+    assertEq(stack.functionDisplayName, "recur");
+    assertEq(stack.parent, null);
+    stack = stack.asyncParent;
+  }
+
+  // This frame should be the first call to `recur`.
+  if (limit > n) {
+    assertEq(stack.functionDisplayName, "recur");
+    assertEq(stack.asyncParent, null);
+    stack = stack.parent;
+  } else {
+    assertEq(stack, null);
+  }
+
+  // This frame should be the call to `checkRecursion`.
+  if (limit > n + 1) {
+    assertEq(stack.functionDisplayName, "checkRecursion");
+    assertEq(stack.asyncParent, null);
+    stack = stack.parent;
+  } else {
+    assertEq(stack, null);
+  }
+
+  // We should be at the top frame, which is the test script itself.
+  if (limit > n + 2) {
+    assertEq(stack.functionDisplayName, null);
+    assertEq(stack.asyncParent, null);
+    assertEq(stack.parent, null);
+  } else {
+    assertEq(stack, null);
+  }
+}
+
+// Check capturing with no limit, which should still apply a default limit.
+checkRecursion(0, 0);
+checkRecursion(1, 0);
+checkRecursion(2, 0);
+checkRecursion(defaultAsyncStackLimit + 10, 0);
+
+// Limit of 1 frame.
+checkRecursion(0, 1);
+checkRecursion(1, 1);
+checkRecursion(2, 1);
+
+// Limit of 2 frames.
+checkRecursion(0, 2);
+checkRecursion(1, 2);
+checkRecursion(2, 2);
+
+// Limit of 3 frames.
+checkRecursion(0, 3);
+checkRecursion(1, 3);
+checkRecursion(2, 3);
+
+// Limit higher than the default limit.
+checkRecursion(defaultAsyncStackLimit + 10, defaultAsyncStackLimit + 10);
+checkRecursion(defaultAsyncStackLimit + 11, defaultAsyncStackLimit + 10);
+checkRecursion(defaultAsyncStackLimit + 12, defaultAsyncStackLimit + 10);

js/src/jit-test/tests/saved-stacks/async-principals.js

@@ -0,0 +1,244 @@
+// Test cases when a SavedFrame or one of its ancestors have a principal that is
+// not subsumed by the caller's principal, when async parents are present.
+
+function checkVisibleStack(stackFrame, expectedFrames) {
+  // We check toString separately from properties like asyncCause to check that
+  // it walks the physical SavedFrame chain consistently with the properties.
+  var stringFrames = stackFrame.toString().split("\n");
+
+  while (expectedFrames.length) {
+    let expectedFrame = expectedFrames.shift();
+    let stringFrame = stringFrames.shift();
+
+    // Check the frame properties.
+    assertEq(stackFrame.functionDisplayName, expectedFrame.name);
+    assertEq(stackFrame.asyncCause, expectedFrame.asyncCause);
+
+    // Check the stringified version.
+    let expectedStart =
+      (expectedFrame.asyncCause ? expectedFrame.asyncCause + "*" : "") +
+      expectedFrame.name;
+    assertEq(stringFrame.replace(/@.*$/, ""), expectedStart);
+
+    // If the next frame has an asyncCause, it should be an asyncParent.
+    if (expectedFrames.length && expectedFrames[0].asyncCause) {
+      assertEq(stackFrame.parent, null);
+      stackFrame = stackFrame.asyncParent;
+    } else {
+      assertEq(stackFrame.asyncParent, null);
+      stackFrame = stackFrame.parent;
+    }
+  }
+}
+
+var low = newGlobal({ principal: 0 });
+var high = newGlobal({ principal: 0xfffff });
+
+// Test with synchronous cross-compartment calls.
+//
+// With arrows representing child-to-parent links, and fat arrows representing
+// child-to-async-parent links, create a SavedFrame stack like this:
+//
+//     low.e -> high.d => high.c => high.b -> low.a
+//
+// This stack captured in function `e` would be seen in its complete version if
+// accessed by `high`'s compartment, while in `low`'s compartment it would look
+// like this:
+//
+//     low.e => low.a
+//
+// The asyncCause seen on `low.a` above should not leak information about the
+// real asyncCause on `high.c` and `high.d`.
+//
+// The stack captured in function `d` would be seen in its complete version if
+// accessed by `high`'s compartment, while in `low`'s compartment it would look
+// like this:
+//
+//     low.a
+
+// We'll move these functions into the right globals below before invoking them.
+function a() {
+  b();
+}
+function b() {
+  callFunctionWithAsyncStack(c, saveStack(), "BtoC");
+}
+function c() {
+  callFunctionWithAsyncStack(d, saveStack(), "CtoD");
+}
+function d() {
+  let stackD = saveStack();
+
+  print("high.checkVisibleStack(stackD)");
+  checkVisibleStack(stackD, [
+    { name: "d", asyncCause: null   },
+    { name: "c", asyncCause: "CtoD" },
+    { name: "b", asyncCause: "BtoC" },
+    { name: "a", asyncCause: null   },
+  ]);
+
+  let stackE = e(saveStack(0, low));
+
+  print("high.checkVisibleStack(stackE)");
+  checkVisibleStack(stackE, [
+    { name: "e", asyncCause: null   },
+    { name: "d", asyncCause: null   },
+    { name: "c", asyncCause: "CtoD" },
+    { name: "b", asyncCause: "BtoC" },
+    { name: "a", asyncCause: null   },
+  ]);
+}
+function e(stackD) {
+  print("low.checkVisibleStack(stackD)");
+  checkVisibleStack(stackD, [
+    { name: "a", asyncCause: "Async" },
+  ]);
+
+  let stackE = saveStack();
+
+  print("low.checkVisibleStack(stackE)");
+  checkVisibleStack(stackE, [
+    { name: "e", asyncCause: null    },
+    { name: "a", asyncCause: "Async" },
+  ]);
+
+  return saveStack(0, high);
+}
+
+// Test with asynchronous cross-compartment calls and shared frames.
+//
+// With arrows representing child-to-parent links, and fat arrows representing
+// child-to-async-parent links, create a SavedFrame stack like this:
+//
+//     low.x => high.v => low.u
+//     low.y -> high.v => low.u
+//     low.z => high.w -> low.u
+//
+// This stack captured in functions `x`, `y`, and `z` would be seen in its
+// complete version if accessed by `high`'s compartment, while in `low`'s
+// compartment it would look like this:
+//
+//     low.x => low.u
+//     low.y => low.u
+//     low.z => low.u
+//
+// The stack captured in function `v` would be seen in its complete version if
+// accessed by `high`'s compartment, while in `low`'s compartment it would look
+// like this:
+//
+//     low.u
+
+// We'll move these functions into the right globals below before invoking them.
+function u() {
+  callFunctionWithAsyncStack(v, saveStack(), "UtoV");
+  w();
+}
+function v() {
+  let stackV = saveStack();
+  print("high.checkVisibleStack(stackV)");
+  checkVisibleStack(stackV, [
+    { name: "v", asyncCause: null   },
+    { name: "u", asyncCause: "UtoV" },
+  ]);
+
+  let xToCall = x.bind(undefined, saveStack(0, low));
+
+  let stackX = callFunctionWithAsyncStack(xToCall, saveStack(), "VtoX");
+
+  print("high.checkVisibleStack(stackX)");
+  checkVisibleStack(stackX, [
+    { name: "x", asyncCause: null   },
+    { name: "v", asyncCause: "VtoX" },
+    { name: "u", asyncCause: "UtoV" },
+  ]);
+
+  let stackY = y();
+
+  print("high.checkVisibleStack(stackY)");
+  checkVisibleStack(stackY, [
+    { name: "y", asyncCause: null   },
+    { name: "v", asyncCause: null   },
+    { name: "u", asyncCause: "UtoV" },
+  ]);
+}
+function w() {
+  let stackZ = callFunctionWithAsyncStack(z, saveStack(), "WtoZ");
+
+  print("high.checkVisibleStack(stackZ)");
+  checkVisibleStack(stackZ, [
+    { name: "z", asyncCause: null   },
+    { name: "w", asyncCause: "WtoZ" },
+    { name: "u", asyncCause: null   },
+  ]);
+}
+function x(stackV) {
+  print("low.checkVisibleStack(stackV)");
+  checkVisibleStack(stackV, [
+    { name: "u", asyncCause: "UtoV" },
+  ]);
+
+  let stackX = saveStack();
+
+  print("low.checkVisibleStack(stackX)");
+  checkVisibleStack(stackX, [
+    { name: "x", asyncCause: null   },
+    { name: "u", asyncCause: "UtoV" },
+  ]);
+
+  return saveStack(0, high);
+}
+function y() {
+  let stackY = saveStack();
+
+  print("low.checkVisibleStack(stackY)");
+  checkVisibleStack(stackY, [
+    { name: "y", asyncCause: null   },
+    { name: "u", asyncCause: "UtoV" },
+  ]);
+
+  return saveStack(0, high);
+}
+function z() {
+  let stackZ = saveStack();
+
+  print("low.checkVisibleStack(stackZ)");
+  checkVisibleStack(stackZ, [
+    { name: "z", asyncCause: null    },
+    { name: "u", asyncCause: "Async" },
+  ]);
+
+  return saveStack(0, high);
+}
+
+// Split the functions in their respective globals.
+low .eval(a.toSource());
+high.eval(b.toSource());
+high.eval(c.toSource());
+high.eval(d.toSource());
+low .eval(e.toSource());
+
+low .b = high.b;
+high.e = low .e;
+
+low .eval(u.toSource());
+high.eval(v.toSource());
+high.eval(w.toSource());
+low .eval(x.toSource());
+low .eval(y.toSource());
+low .eval(z.toSource());
+
+low .v = high.v;
+low .w = high.w;
+high.x = low .x;
+high.y = low .y;
+high.z = low .z;
+
+low .high = high;
+high.low  = low;
+
+low .eval(checkVisibleStack.toSource());
+high.eval(checkVisibleStack.toSource());
+
+// Execute the tests.
+low.a();
+low.u();

js/src/jit-test/tests/saved-stacks/async.js

@@ -0,0 +1,24 @@
+// Test calling a function using a previously captured stack as an async stack.
+
+function getAsyncStack() {
+  return saveStack();
+}
+
+// asyncCause may contain non-ASCII characters.
+let testAsyncCause = "Tes" + String.fromCharCode(355) + "String";
+
+callFunctionWithAsyncStack(function asyncCallback() {
+  let stack = saveStack();
+
+  assertEq(stack.functionDisplayName, "asyncCallback");
+  assertEq(stack.parent, null);
+  assertEq(stack.asyncCause, null);
+
+  assertEq(stack.asyncParent.functionDisplayName, "getAsyncStack");
+  assertEq(stack.asyncParent.asyncCause, testAsyncCause);
+  assertEq(stack.asyncParent.asyncParent, null);
+
+  assertEq(stack.asyncParent.parent.asyncCause, null);
+  assertEq(stack.asyncParent.parent.asyncParent, null);
+  assertEq(stack.asyncParent.parent.parent, null);
+}, getAsyncStack(), testAsyncCause);

js/src/jsapi.cpp

@@ -4318,6 +4318,28 @@ JS_RestoreFrameChain(JSContext *cx)
     cx->restoreFrameChain();
 }
 
+JS::AutoSetAsyncStackForNewCalls::AutoSetAsyncStackForNewCalls(
+  JSContext *cx, HandleObject stack, HandleString asyncCause)
+  : cx(cx),
+    oldAsyncStack(cx, cx->runtime()->asyncStackForNewActivations),
+    oldAsyncCause(cx, cx->runtime()->asyncCauseForNewActivations)
+{
+    CHECK_REQUEST(cx);
+
+    SavedFrame *asyncStack = &stack->as<SavedFrame>();
+    MOZ_ASSERT(!asyncCause->empty());
+
+    cx->runtime()->asyncStackForNewActivations = asyncStack;
+    cx->runtime()->asyncCauseForNewActivations = asyncCause;
+}
+
+JS::AutoSetAsyncStackForNewCalls::~AutoSetAsyncStackForNewCalls()
+{
+    cx->runtime()->asyncCauseForNewActivations = oldAsyncCause;
+    cx->runtime()->asyncStackForNewActivations =
+      oldAsyncStack ? &oldAsyncStack->as<SavedFrame>() : nullptr;
+}
+
 /************************************************************************/
 JS_PUBLIC_API(JSString *)
 JS_NewStringCopyN(JSContext *cx, const char *s, size_t n)

js/src/jsapi.h

@@ -3811,6 +3811,40 @@ JS_SaveFrameChain(JSContext *cx);
 extern JS_PUBLIC_API(void)
 JS_RestoreFrameChain(JSContext *cx);
 
+namespace JS {
+
+/*
+ * This class can be used to store a pointer to the youngest frame of a saved
+ * stack in the specified JSContext. This reference will be picked up by any new
+ * calls performed until the class is destroyed, with the specified asyncCause,
+ * that must not be empty.
+ *
+ * Any stack capture initiated during these new calls will go through the async
+ * stack instead of the current stack.
+ *
+ * Capturing the stack before a new call is performed will not be affected.
+ *
+ * The provided chain of SavedFrame objects can live in any compartment,
+ * although it will be copied to the compartment where the stack is captured.
+ */
+class MOZ_STACK_CLASS JS_PUBLIC_API(AutoSetAsyncStackForNewCalls)
+{
+    JSContext *cx;
+    RootedObject oldAsyncStack;
+    RootedString oldAsyncCause;
+
+  public:
+    // The stack parameter cannot be null by design, because it would be
+    // ambiguous whether that would clear any scheduled async stack and make the
+    // normal stack reappear in the new call, or just keep the async stack
+    // already scheduled for the new call, if any.
+    AutoSetAsyncStackForNewCalls(JSContext *cx, HandleObject stack,
+                                 HandleString asyncCause);
+    ~AutoSetAsyncStackForNewCalls();
+};
+
+}
+
 /************************************************************************/
 
 /*

js/src/vm/Runtime.cpp

@@ -123,6 +123,8 @@ JSRuntime::JSRuntime(JSRuntime *parentRuntime)
     profilerSampleBufferGen_(0),
     profilerSampleBufferLapCount_(1),
     asmJSActivationStack_(nullptr),
+    asyncStackForNewActivations(nullptr),
+    asyncCauseForNewActivations(nullptr),
     parentRuntime(parentRuntime),
     interrupt_(false),
     telemetryCallback(nullptr),

js/src/vm/Runtime.h

@@ -660,6 +660,22 @@ struct JSRuntime : public JS::shadow::Runtime,
     js::AsmJSActivation * volatile asmJSActivationStack_;
 
   public:
+    /*
+     * Youngest frame of a saved stack that will be picked up as an async stack
+     * by any new Activation, and is nullptr when no async stack should be used.
+     *
+     * The JS::AutoSetAsyncStackForNewCalls class can be used to set this.
+     *
+     * New activations will reset this to nullptr on construction after getting
+     * the current value, and will restore the previous value on destruction.
+     */
+    js::SavedFrame *asyncStackForNewActivations;
+
+    /*
+     * Value of asyncCause to be attached to asyncStackForNewActivations.
+     */
+    JSString *asyncCauseForNewActivations;
+
     js::Activation *const *addressOfActivation() const {
         return &activation_;
     }

js/src/vm/SavedStacks.cpp

@@ -38,6 +38,11 @@ using mozilla::Maybe;
 
 namespace js {
 
+/**
+ * Maximum number of saved frames returned for an async stack.
+ */
+const unsigned ASYNC_STACK_MAX_FRAME_COUNT = 60;
+
 struct SavedFrame::Lookup {
     Lookup(JSAtom *source, uint32_t line, uint32_t column,
            JSAtom *functionDisplayName, JSAtom *asyncCause, SavedFrame *parent,
@@ -53,6 +58,18 @@ struct SavedFrame::Lookup {
         MOZ_ASSERT(source);
     }
 
+    explicit Lookup(SavedFrame &savedFrame)
+      : source(savedFrame.getSource()),
+        line(savedFrame.getLine()),
+        column(savedFrame.getColumn()),
+        functionDisplayName(savedFrame.getFunctionDisplayName()),
+        asyncCause(savedFrame.getAsyncCause()),
+        parent(savedFrame.getParent()),
+        principals(savedFrame.getPrincipals())
+    {
+        MOZ_ASSERT(source);
+    }
+
     JSAtom       *source;
     uint32_t     line;
     uint32_t     column;
@@ -797,13 +814,7 @@ SavedStacks::sweep(JSRuntime *rt)
                 }
 
                 if (obj != temp || parentMoved) {
-                    e.rekeyFront(SavedFrame::Lookup(frame->getSource(),
+                    e.rekeyFront(SavedFrame::Lookup(*frame),
-                                                    frame->getLine(),
-                                                    frame->getColumn(),
-                                                    frame->getFunctionDisplayName(),
-                                                    frame->getAsyncCause(),
-                                                    frame->getParent(),
-                                                    frame->getPrincipals()),
                                  ReadBarriered<SavedFrame *>(frame));
                 }
             }
@@ -864,9 +875,32 @@ SavedStacks::insertFrames(JSContext *cx, FrameIter &iter, MutableHandleSavedFram
     // pointers on the first pass, and then we fill in the parent pointers as we
     // return in the second pass.
 
+    Activation *asyncActivation = nullptr;
+    RootedSavedFrame asyncStack(cx, nullptr);
+    RootedString asyncCause(cx, nullptr);
+
     // Accumulate the vector of Lookup objects in |stackChain|.
     SavedFrame::AutoLookupVector stackChain(cx);
     while (!iter.done()) {
+        Activation &activation = *iter.activation();
+
+        if (!asyncActivation) {
+            asyncStack = activation.asyncStack();
+            if (asyncStack) {
+                // While walking from the youngest to the oldest frame, we found
+                // an activation that has an async stack set. We will use the
+                // youngest frame of the async stack as the parent of the oldest
+                // frame of this activation. We still need to iterate over other
+                // frames in this activation before reaching the oldest frame.
+                asyncCause = activation.asyncCause();
+                asyncActivation = &activation;
+            }
+        } else if (asyncActivation != &activation) {
+            // We found an async stack in the previous activation, and we
+            // walked past the oldest frame of that activation, we're done.
+            break;
+        }
+
         AutoLocationValueRooter location(cx);
 
         {
@@ -891,17 +925,73 @@ SavedStacks::insertFrames(JSContext *cx, FrameIter &iter, MutableHandleSavedFram
 
         ++iter;
 
-        if (maxFrameCount == 0) {
+        // If maxFrameCount is zero there's no limit on the number of frames.
-            // If maxFrameCount is zero, then there's no limit on the number of
+        if (maxFrameCount == 0)
-            // frames.
             continue;
-        } else if (maxFrameCount == 1) {
+
-            // Since we were only asked to save one frame, do not continue
+        if (maxFrameCount == 1) {
-            // walking the stack and saving frame state.
+            // The frame we just saved was the last one we were asked to save.
+            // If we had an async stack, ensure we don't use any of its frames.
+            asyncStack.set(nullptr);
             break;
-        } else {
-            maxFrameCount--;
         }
+
+        maxFrameCount--;
+    }
+
+    // Limit the depth of the async stack, if any, and ensure that the
+    // SavedFrame instances we use are stored in the same compartment as the
+    // rest of the synchronous stack chain.
+    RootedSavedFrame parentFrame(cx, nullptr);
+    if (asyncStack && !adoptAsyncStack(cx, asyncStack, asyncCause, &parentFrame, maxFrameCount))
+        return false;
+
+    // Iterate through |stackChain| in reverse order and get or create the
+    // actual SavedFrame instances.
+    for (size_t i = stackChain->length(); i != 0; i--) {
+        SavedFrame::HandleLookup lookup = stackChain[i-1];
+        lookup->parent = parentFrame;
+        parentFrame.set(getOrCreateSavedFrame(cx, lookup));
+        if (!parentFrame)
+            return false;
+    }
+
+    frame.set(parentFrame);
+    return true;
+}
+
+bool
+SavedStacks::adoptAsyncStack(JSContext *cx, HandleSavedFrame asyncStack,
+                             HandleString asyncCause,
+                             MutableHandleSavedFrame adoptedStack,
+                             unsigned maxFrameCount)
+{
+    RootedAtom asyncCauseAtom(cx, AtomizeString(cx, asyncCause));
+    if (!asyncCauseAtom)
+        return false;
+
+    // If maxFrameCount is zero, the caller asked for an unlimited number of
+    // stack frames, but async stacks are not limited by the available stack
+    // memory, so we need to set an arbitrary limit when collecting them. We
+    // still don't enforce an upper limit if the caller requested more frames.
+    if (maxFrameCount == 0)
+        maxFrameCount = ASYNC_STACK_MAX_FRAME_COUNT;
+
+    // Accumulate the vector of Lookup objects in |stackChain|.
+    SavedFrame::AutoLookupVector stackChain(cx);
+    SavedFrame *currentSavedFrame = asyncStack;
+    for (unsigned i = 0; i < maxFrameCount && currentSavedFrame; i++) {
+        // Use growByUninitialized and placement-new instead of just append.
+        // We'd ideally like to use an emplace method once Vector supports it.
+        if (!stackChain->growByUninitialized(1))
+            return false;
+        new (&stackChain->back()) SavedFrame::Lookup(*currentSavedFrame);
+
+        // Attach the asyncCause to the youngest frame.
+        if (i == 0)
+            stackChain->back().asyncCause = asyncCauseAtom;
+
+        currentSavedFrame = currentSavedFrame->getParent();
     }
 
     // Iterate through |stackChain| in reverse order and get or create the
@@ -915,7 +1005,7 @@ SavedStacks::insertFrames(JSContext *cx, FrameIter &iter, MutableHandleSavedFram
             return false;
     }
 
-    frame.set(parentFrame);
+    adoptedStack.set(parentFrame);
     return true;
 }
 

js/src/vm/SavedStacks.h

@@ -63,8 +63,6 @@ class SavedFrame : public NativeObject {
                     HashPolicy,
                     SystemAllocPolicy> Set;
 
-    typedef RootedGeneric<Lookup*> AutoLookupRooter;
-
     class AutoLookupVector;
 
     class MOZ_STACK_CLASS HandleLookup {
@@ -160,6 +158,10 @@ class SavedStacks {
 
     bool       insertFrames(JSContext *cx, FrameIter &iter, MutableHandleSavedFrame frame,
                             unsigned maxFrameCount = 0);
+    bool       adoptAsyncStack(JSContext *cx, HandleSavedFrame asyncStack,
+                               HandleString asyncCause,
+                               MutableHandleSavedFrame adoptedStack,
+                               unsigned maxFrameCount);
     SavedFrame *getOrCreateSavedFrame(JSContext *cx, SavedFrame::HandleLookup lookup);
     SavedFrame *createFrameFromLookup(JSContext *cx, SavedFrame::HandleLookup lookup);
     void       chooseSamplingProbability(JSContext* cx);

js/src/vm/Stack-inl.h

@@ -826,8 +826,12 @@ Activation::Activation(JSContext *cx, Kind kind)
     prevProfiling_(prev_ ? prev_->mostRecentProfiling() : nullptr),
     savedFrameChain_(0),
     hideScriptedCallerCount_(0),
+    asyncStack_(cx, cx->runtime_->asyncStackForNewActivations),
+    asyncCause_(cx, cx->runtime_->asyncCauseForNewActivations),
     kind_(kind)
 {
+    cx->runtime_->asyncStackForNewActivations = nullptr;
+    cx->runtime_->asyncCauseForNewActivations = nullptr;
     cx->runtime_->activation_ = this;
 }
 
@@ -837,6 +841,8 @@ Activation::~Activation()
     MOZ_ASSERT(cx_->runtime_->activation_ == this);
     MOZ_ASSERT(hideScriptedCallerCount_ == 0);
     cx_->runtime_->activation_ = prev_;
+    cx_->runtime_->asyncCauseForNewActivations = asyncCause_;
+    cx_->runtime_->asyncStackForNewActivations = asyncStack_;
 }
 
 bool

js/src/vm/Stack.h

@@ -35,6 +35,8 @@ class StaticBlockObject;
 
 class ScopeCoordinate;
 
+class SavedFrame;
+
 // VM stack layout
 //
 // A JSRuntime's stack consists of a linked list of activations. Every activation
@@ -1064,6 +1066,17 @@ class Activation
     // data structures instead.
     size_t hideScriptedCallerCount_;
 
+    // Youngest saved frame of an async stack that will be iterated during stack
+    // capture in place of the actual stack of previous activations. Note that
+    // the stack of this activation is captured entirely before this is used.
+    //
+    // Usually this is nullptr, meaning that normal stack capture will occur.
+    // When this is set, the stack of any previous activation is ignored.
+    Rooted<SavedFrame *> asyncStack_;
+
+    // Value of asyncCause to be attached to asyncStack_.
+    RootedString asyncCause_;
+
     enum Kind { Interpreter, Jit, AsmJS };
     Kind kind_;
 
@@ -1136,6 +1149,14 @@ class Activation
         return offsetof(Activation, prevProfiling_);
     }
 
+    SavedFrame *asyncStack() {
+        return asyncStack_;
+    }
+
+    JSString *asyncCause() {
+        return asyncCause_;
+    }
+
   private:
     Activation(const Activation &other) = delete;
     void operator=(const Activation &other) = delete;