process: emit memoryPressure from the cgroup's memory.events on Linux

packages/bun-types/overrides.d.ts

@@ -100,10 +100,14 @@ declare global {
        * of polling.
        *
        * On macOS `level` distinguishes `"warning"` from `"critical"` based on
-       * the kernel's memorystatus thresholds. On Linux (PSI) and Windows the
-       * event is always emitted with `"critical"`. On Linux, the underlying
-       * PSI trigger requires `CAP_SYS_RESOURCE` on kernels before 6.6; when
-       * unavailable the event is never emitted.
+       * the kernel's memorystatus thresholds. On Linux, reclaim at one of the
+       * process's cgroup memory limits (the `low`, `high` and `max` counters
+       * of cgroup v2 `memory.events`) is emitted as `"warning"`, an OOM event
+       * in that cgroup and a system-wide PSI memory stall as `"critical"`.
+       * The PSI trigger requires `CAP_SYS_RESOURCE` on kernels before 6.4, and
+       * the cgroup counters only exist on cgroup v2. On Windows the event is
+       * always emitted with `"critical"`. When no source is available the
+       * event is never emitted.
        *
        * This listener does not keep the event loop alive.
        */

src/js/internal-for-testing.ts

@@ -673,19 +673,29 @@ export const isMemoryPressureWatcherInstalled: () => boolean = $newCppFunction(
   0,
 );
 
-// True when the installed watcher registered a real OS source (a PSI trigger
-// on Linux). The watcher installs silently without one when the kernel
-// refuses the trigger, so isMemoryPressureWatcherInstalled() cannot tell.
-export const memoryPressureWatcherHasOsBackend: () => boolean = $newRustFunction(
-  "memory_pressure.rs",
-  "jsWatcherHasOsBackend",
-  0,
-);
+// The OS sources the installed watcher holds: "psi" and/or "cgroup" on Linux,
+// "memorystatus" on macOS, "notification" on Windows. The watcher installs
+// silently with no source when the OS refuses every one of them, so
+// isMemoryPressureWatcherInstalled() cannot tell.
+export const memoryPressureArmedSources: () => ("psi" | "cgroup" | "memorystatus" | "notification")[] =
+  $newRustFunction("memory_pressure.rs", "jsArmedSources", 0);
 
 // The exact bytes Bun writes to /proc/pressure/memory to arm its trigger.
 // null where there is no PSI backend (everything except Linux).
 export const memoryPressurePsiTrigger: () => Buffer | null = $newRustFunction("memory_pressure.rs", "jsPsiTrigger", 0);
 
+// Linux. Feed a memory.events file body to the installed "cgroup" source as if
+// the kernel had signalled it. atMs is the holdoff clock reading for this
+// notification, on a clock that starts at the first call, so a test drives
+// the holdoff without waiting. The listener sees exactly what the real
+// notification would produce. Returns false when no "cgroup" source is
+// installed.
+export const memoryPressureInjectCgroupEvents: (text: string, atMs?: number) => boolean = $newRustFunction(
+  "memory_pressure.rs",
+  "jsInjectCgroupEvents",
+  2,
+);
+
 export const getEventLoopStats: () => {
   activeTasks: number;
   tasks: number;