1 files changed, 104 insertions, 0 deletions
diff --git a/xpcom/threads/nsMemoryPressure.cpp b/xpcom/threads/nsMemoryPressure.cpp
new file mode 100644
index 0000000000..dbd3a92f79
--- /dev/null
+++ b/xpcom/threads/nsMemoryPressure.cpp
@@ -0,0 +1,104 @@
+/* -*- 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/. */
+
+#include "nsMemoryPressure.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Services.h"
+
+#include "nsThreadUtils.h"
+#include "nsIObserverService.h"
+
+using namespace mozilla;
+
+const char* const kTopicMemoryPressure = "memory-pressure";
+const char* const kTopicMemoryPressureStop = "memory-pressure-stop";
+const char16_t* const kSubTopicLowMemoryNew = u"low-memory";
+const char16_t* const kSubTopicLowMemoryOngoing = u"low-memory-ongoing";
+
+// This is accessed from any thread through NS_NotifyOfEventualMemoryPressure
+static Atomic<MemoryPressureState, Relaxed> sMemoryPressurePending(
+    MemoryPressureState::NoPressure);
+
+void NS_NotifyOfEventualMemoryPressure(MemoryPressureState aState) {
+  MOZ_ASSERT(aState != MemoryPressureState::None);
+
+  /*
+   * A new memory pressure event erases an ongoing (or stop of) memory pressure,
+   * but an existing "new" memory pressure event takes precedence over a new
+   * "ongoing" or "stop" memory pressure event.
+   */
+  switch (aState) {
+    case MemoryPressureState::None:
+    case MemoryPressureState::LowMemory:
+      sMemoryPressurePending = aState;
+      break;
+    case MemoryPressureState::NoPressure:
+      sMemoryPressurePending.compareExchange(MemoryPressureState::None, aState);
+      break;
+  }
+}
+
+nsresult NS_NotifyOfMemoryPressure(MemoryPressureState aState) {
+  NS_NotifyOfEventualMemoryPressure(aState);
+  nsCOMPtr<nsIRunnable> event =
+      new Runnable("NS_DispatchEventualMemoryPressure");
+  return NS_DispatchToMainThread(event);
+}
+
+void NS_DispatchMemoryPressure() {
+  MOZ_ASSERT(NS_IsMainThread());
+  static MemoryPressureState sMemoryPressureStatus =
+      MemoryPressureState::NoPressure;
+
+  MemoryPressureState mpPending =
+      sMemoryPressurePending.exchange(MemoryPressureState::None);
+  if (mpPending == MemoryPressureState::None) {
+    return;
+  }
+
+  nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+  if (!os) {
+    NS_WARNING("Can't get observer service!");
+    return;
+  }
+
+  switch (mpPending) {
+    case MemoryPressureState::None:
+      MOZ_ASSERT_UNREACHABLE("Already handled this case above.");
+      break;
+    case MemoryPressureState::LowMemory:
+      switch (sMemoryPressureStatus) {
+        case MemoryPressureState::None:
+          MOZ_ASSERT_UNREACHABLE("The internal status should never be None.");
+          break;
+        case MemoryPressureState::NoPressure:
+          sMemoryPressureStatus = MemoryPressureState::LowMemory;
+          os->NotifyObservers(nullptr, kTopicMemoryPressure,
+                              kSubTopicLowMemoryNew);
+          break;
+        case MemoryPressureState::LowMemory:
+          os->NotifyObservers(nullptr, kTopicMemoryPressure,
+                              kSubTopicLowMemoryOngoing);
+          break;
+      }
+      break;
+    case MemoryPressureState::NoPressure:
+      switch (sMemoryPressureStatus) {
+        case MemoryPressureState::None:
+          MOZ_ASSERT_UNREACHABLE("The internal status should never be None.");
+          break;
+        case MemoryPressureState::NoPressure:
+          // Already no pressure.  Do nothing.
+          break;
+        case MemoryPressureState::LowMemory:
+          sMemoryPressureStatus = MemoryPressureState::NoPressure;
+          os->NotifyObservers(nullptr, kTopicMemoryPressureStop, nullptr);
+          break;
+      }
+      break;
+  }
+}