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diff --git a/devtools/shared/commands/resource/legacy-listeners/source.js b/devtools/shared/commands/resource/legacy-listeners/source.js
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+/* 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/. */
+
+"use strict";
+
+const ResourceCommand = require("resource://devtools/shared/commands/resource/resource-command.js");
+
+/**
+ * Emit SOURCE resources, which represents a Javascript source and has the following attributes set on "available":
+ *
+ * - introductionType {null|String}: A string indicating how this source code was introduced into the system.
+ *   This will typically be set to "scriptElement", "eval", ...
+ *   But this may have many other values:
+ *     https://searchfox.org/mozilla-central/rev/ac142717cc067d875e83e4b1316f004f6e063a46/dom/script/ScriptLoader.cpp#2628-2639
+ *     https://searchfox.org/mozilla-central/search?q=symbol:_ZN2JS14CompileOptions19setIntroductionTypeEPKc&redirect=false
+ *     https://searchfox.org/mozilla-central/rev/ac142717cc067d875e83e4b1316f004f6e063a46/devtools/server/actors/source.js#160-169
+ * - sourceMapBaseURL {String}: Base URL where to look for a source map.
+ *   This isn't the source map URL.
+ * - sourceMapURL {null|String}: URL of the source map, if there is one.
+ * - url {null|String}: URL of the source, if it relates to a particular URL.
+ *   Evaled sources won't have any related URL.
+ * - isBlackBoxed {Boolean}: Specifying whether the source actor's 'black-boxed' flag is set.
+ * - extensionName {null|String}: If the source comes from an add-on, the add-on name.
+ */
+module.exports = async function ({ targetCommand, targetFront, onAvailable }) {
+  const isBrowserToolbox =
+    targetCommand.descriptorFront.isBrowserProcessDescriptor;
+  const isNonTopLevelFrameTarget =
+    !targetFront.isTopLevel &&
+    targetFront.targetType === targetCommand.TYPES.FRAME;
+
+  if (isBrowserToolbox && isNonTopLevelFrameTarget) {
+    // In the BrowserToolbox, non-top-level frame targets are already
+    // debugged via content-process targets.
+    return;
+  }
+
+  const threadFront = await targetFront.getFront("thread");
+
+  // Use a list of all notified SourceFront as we don't have a newSource event for all sources
+  // but we sometime get sources notified both via newSource event *and* sources() method...
+  // We store actor ID instead of SourceFront as it appears that multiple SourceFront for the same
+  // actor are created...
+  const sourcesActorIDCache = new Set();
+
+  // Forward new sources (but also existing ones, see next comment)
+  threadFront.on("newSource", ({ source }) => {
+    if (sourcesActorIDCache.has(source.actor)) {
+      return;
+    }
+    sourcesActorIDCache.add(source.actor);
+    // source is a SourceActor's form, add the resourceType attribute on it
+    source.resourceType = ResourceCommand.TYPES.SOURCE;
+    onAvailable([source]);
+  });
+
+  // Forward already existing sources
+  // Note that calling `sources()` will end up emitting `newSource` event for all existing sources.
+  // But not in some cases, for example, when the thread is already paused.
+  // (And yes, it means that already existing sources can be transfered twice over the wire)
+  //
+  // Also, browser_ext_devtools_inspectedWindow_targetSwitch.js creates many top level targets,
+  // for which the SourceMapURLService will fetch sources. But these targets are destroyed while
+  // the test is running and when they are, we purge all pending requests, including this one.
+  // So ignore any error if this request failed on destruction.
+  let sources;
+  try {
+    sources = await threadFront.sources();
+  } catch (e) {
+    if (threadFront.isDestroyed()) {
+      return;
+    }
+    throw e;
+  }
+
+  // Note that `sources()` doesn't encapsulate SourceFront into a `source` attribute
+  // while `newSource` event does.
+  sources = sources.filter(source => {
+    return !sourcesActorIDCache.has(source.actor);
+  });
+  for (const source of sources) {
+    sourcesActorIDCache.add(source.actor);
+    // source is a SourceActor's form, add the resourceType attribute on it
+    source.resourceType = ResourceCommand.TYPES.SOURCE;
+  }
+  onAvailable(sources);
+};