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diff --git a/js/src/builtin/streams/MiscellaneousOperations.cpp b/js/src/builtin/streams/MiscellaneousOperations.cpp
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+++ b/js/src/builtin/streams/MiscellaneousOperations.cpp
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+/* Miscellaneous operations. */
+
+#include "builtin/streams/MiscellaneousOperations.h"
+
+#include "mozilla/Assertions.h"     // MOZ_ASSERT
+#include "mozilla/Attributes.h"     // MOZ_MUST_USE
+#include "mozilla/FloatingPoint.h"  // mozilla::IsNaN
+
+#include "jsapi.h"  // JS_ReportErrorNumberASCII
+
+#include "js/Conversions.h"           // JS::ToNumber
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/RootingAPI.h"            // JS::{,Mutable}Handle, JS::Rooted
+#include "vm/Interpreter.h"           // js::{Call,GetAndClearException}
+#include "vm/JSContext.h"             // JSContext
+#include "vm/ObjectOperations.h"      // js::GetProperty
+#include "vm/PromiseObject.h"         // js::PromiseObject
+#include "vm/StringType.h"            // js::PropertyName
+
+#include "vm/JSContext-inl.h"  // JSContext::check
+#include "vm/JSObject-inl.h"   // js::IsCallable
+
+using JS::Handle;
+using JS::MutableHandle;
+using JS::ToNumber;
+using JS::Value;
+
+MOZ_MUST_USE js::PromiseObject* js::PromiseRejectedWithPendingError(
+    JSContext* cx) {
+  Rooted<Value> exn(cx);
+  if (!cx->isExceptionPending() || !GetAndClearException(cx, &exn)) {
+    // Uncatchable error. This happens when a slow script is killed or a
+    // worker is terminated. Propagate the uncatchable error. This will
+    // typically kill off the calling asynchronous process: the caller
+    // can't hook its continuation to the new rejected promise.
+    return nullptr;
+  }
+  return PromiseObject::unforgeableReject(cx, exn);
+}
+
+/*** 6.3. Miscellaneous operations ******************************************/
+
+/**
+ * Streams spec, 6.3.1.
+ *      CreateAlgorithmFromUnderlyingMethod ( underlyingObject, methodName,
+ *                                            algoArgCount, extraArgs )
+ *
+ * This function only partly implements the standard algorithm. We do not
+ * actually create a new JSFunction completely encapsulating the new algorithm.
+ * Instead, this just gets the specified method and checks for errors. It's the
+ * caller's responsibility to make sure that later, when the algorithm is
+ * "performed", the appropriate steps are carried out.
+ */
+MOZ_MUST_USE bool js::CreateAlgorithmFromUnderlyingMethod(
+    JSContext* cx, Handle<Value> underlyingObject,
+    const char* methodNameForErrorMessage, Handle<PropertyName*> methodName,
+    MutableHandle<Value> method) {
+  cx->check(underlyingObject);
+  cx->check(methodName);
+  cx->check(method);
+
+  // Step 1: Assert: underlyingObject is not undefined.
+  MOZ_ASSERT(!underlyingObject.isUndefined());
+
+  // Step 2: Assert: ! IsPropertyKey(methodName) is true (implicit).
+  // Step 3: Assert: algoArgCount is 0 or 1 (omitted).
+  // Step 4: Assert: extraArgs is a List (omitted).
+
+  // Step 5: Let method be ? GetV(underlyingObject, methodName).
+  if (!GetProperty(cx, underlyingObject, methodName, method)) {
+    return false;
+  }
+
+  // Step 6: If method is not undefined,
+  if (!method.isUndefined()) {
+    // Step a: If ! IsCallable(method) is false, throw a TypeError
+    //         exception.
+    if (!IsCallable(method)) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_NOT_FUNCTION, methodNameForErrorMessage);
+      return false;
+    }
+
+    // Step b: If algoArgCount is 0, return an algorithm that performs the
+    //         following steps:
+    //     Step i: Return ! PromiseCall(method, underlyingObject,
+    //             extraArgs).
+    // Step c: Otherwise, return an algorithm that performs the following
+    //         steps, taking an arg argument:
+    //     Step i: Let fullArgs be a List consisting of arg followed by the
+    //             elements of extraArgs in order.
+    //     Step ii: Return ! PromiseCall(method, underlyingObject,
+    //                                   fullArgs).
+    // (These steps are deferred to the code that performs the algorithm.
+    // See Perform{Write,Close}Algorithm, ReadableStreamControllerCancelSteps,
+    // and ReadableStreamControllerCallPullIfNeeded.)
+    return true;
+  }
+
+  // Step 7: Return an algorithm which returns a promise resolved with
+  //         undefined (implicit).
+  return true;
+}
+
+/**
+ * Streams spec, 6.3.2. InvokeOrNoop ( O, P, args )
+ * As it happens, all callers pass exactly one argument.
+ */
+MOZ_MUST_USE bool js::InvokeOrNoop(JSContext* cx, Handle<Value> O,
+                                   Handle<PropertyName*> P, Handle<Value> arg,
+                                   MutableHandle<Value> rval) {
+  cx->check(O, P, arg);
+
+  // Step 1: Assert: O is not undefined.
+  MOZ_ASSERT(!O.isUndefined());
+
+  // Step 2: Assert: ! IsPropertyKey(P) is true (implicit).
+  // Step 3: Assert: args is a List (implicit).
+  // Step 4: Let method be ? GetV(O, P).
+  Rooted<Value> method(cx);
+  if (!GetProperty(cx, O, P, &method)) {
+    return false;
+  }
+
+  // Step 5: If method is undefined, return.
+  if (method.isUndefined()) {
+    return true;
+  }
+
+  // Step 6: Return ? Call(method, O, args).
+  return Call(cx, method, O, arg, rval);
+}
+
+/**
+ * Streams spec, 6.3.7. ValidateAndNormalizeHighWaterMark ( highWaterMark )
+ */
+MOZ_MUST_USE bool js::ValidateAndNormalizeHighWaterMark(
+    JSContext* cx, Handle<Value> highWaterMarkVal, double* highWaterMark) {
+  // Step 1: Set highWaterMark to ? ToNumber(highWaterMark).
+  if (!ToNumber(cx, highWaterMarkVal, highWaterMark)) {
+    return false;
+  }
+
+  // Step 2: If highWaterMark is NaN or highWaterMark < 0, throw a RangeError
+  // exception.
+  if (mozilla::IsNaN(*highWaterMark) || *highWaterMark < 0) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_STREAM_INVALID_HIGHWATERMARK);
+    return false;
+  }
+
+  // Step 3: Return highWaterMark.
+  return true;
+}
+
+/**
+ * Streams spec, 6.3.8. MakeSizeAlgorithmFromSizeFunction ( size )
+ *
+ * The standard makes a big deal of turning JavaScript functions (grubby,
+ * touched by users, covered with germs) into algorithms (pristine,
+ * respectable, purposeful). We don't bother. Here we only check for errors and
+ * leave `size` unchanged. Then, in ReadableStreamDefaultControllerEnqueue and
+ * WritableStreamDefaultControllerGetChunkSize where this value is used, we
+ * check for undefined and behave as if we had "made" an "algorithm" for it.
+ */
+MOZ_MUST_USE bool js::MakeSizeAlgorithmFromSizeFunction(JSContext* cx,
+                                                        Handle<Value> size) {
+  cx->check(size);
+
+  // Step 1: If size is undefined, return an algorithm that returns 1.
+  if (size.isUndefined()) {
+    // Deferred. Size algorithm users must check for undefined.
+    return true;
+  }
+
+  // Step 2: If ! IsCallable(size) is false, throw a TypeError exception.
+  if (!IsCallable(size)) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_FUNCTION,
+                              "ReadableStream argument options.size");
+    return false;
+  }
+
+  // Step 3: Return an algorithm that performs the following steps, taking a
+  //         chunk argument:
+  //     a. Return ? Call(size, undefined, « chunk »).
+  // Deferred. Size algorithm users must know how to call the size function.
+  return true;
+}