test(crash-reporting): pin the two halves of the cgroup-capped label

`cgroupCeilingBelowHostRam` and the cgroup-over-host stall preference each had
one direction under test, so `lowest < total`, the `memory.high` term and the
`??` could all be deleted with the suite green. Each new test was verified to
fail against its own mutation.

src/main/crash-reporting/linux-cgroup-memory-limit.test.ts

@@ -200,6 +200,31 @@ describe('cgroup-capped linux crash memory details', () => {
     expect(details.systemMemoryPressureSignal).toBe('mem-available')
   })
 
+  it('keeps the plain label when the ceiling is not below host RAM', () => {
+    setSystemMemoryInfoReaderForTest(() => NO_HOST_PRESSURE)
+    // A container whose memory.max was set to the whole machine: a ceiling, but
+    // not one that made the 20 GB beside it unreachable, so it explains nothing.
+    setLinuxCgroupMemoryLimitReaderForTest(() => ({ maxBytes: 32_005 * 1024 * 1024 }))
+
+    const details = getSystemMemoryDetails('linux')
+
+    expect(details.systemMemoryCgroupMaxMB).toBe(32_005)
+    expect(details.systemMemoryPressureSignal).toBe('mem-available')
+  })
+
+  it('caps on memory.high alone, which throttles us long before memory.max would', () => {
+    setSystemMemoryInfoReaderForTest(() => NO_HOST_PRESSURE)
+    setLinuxCgroupMemoryLimitReaderForTest(() => ({
+      maxBytes: undefined,
+      highBytes: 2_147_483_648
+    }))
+
+    const details = getSystemMemoryDetails('linux')
+
+    expect(details.systemMemoryCgroupHighMB).toBe(2_048)
+    expect(details.systemMemoryPressureSignal).toBe('mem-available-cgroup-capped')
+  })
+
   it('adds nothing on a host with no v2 memory controller', () => {
     setSystemMemoryInfoReaderForTest(() => NO_HOST_PRESSURE)
     setLinuxCgroupMemoryLimitReaderForTest(() => undefined)

src/main/crash-reporting/linux-memory-pressure-stall.test.ts

@@ -162,6 +162,21 @@ describe('stall in linux crash memory details', () => {
     expect(getSystemMemoryDetails('linux').systemMemoryPressureSignal).toBe('mem-available-stalled')
   })
 
+  it('does not let a thrashing host label our own calm cgroup as stalled', () => {
+    setSystemMemoryInfoReaderForTest(() => NO_HOST_PRESSURE)
+    // A sibling cgroup is the hog: the host stalls, we do not, and whatever
+    // killed us was not this. Taking the higher of the two would misname it.
+    setLinuxMemoryPressureStallReaderForTest(() => ({
+      host: { fullAvg10: 90 },
+      cgroup: { fullAvg10: 1.2 }
+    }))
+
+    const details = getSystemMemoryDetails('linux')
+
+    expect(details.systemMemoryStallFullAvg10Pct).toBe(90)
+    expect(details.systemMemoryPressureSignal).toBe('mem-available')
+  })
+
   it('lets a cgroup ceiling outrank stall, since it explains the stall as well', () => {
     setSystemMemoryInfoReaderForTest(() => ({ ...NO_HOST_PRESSURE, total: 32_000 * 1024 }))
     setLinuxCgroupMemoryLimitReaderForTest(() => ({ maxBytes: 2_147_483_648 }))

src/main/crash-reporting/pre-gone-host-memory.ts

@@ -62,8 +62,8 @@ function withCarriedSwapVolume(sample: PreGoneSystemMemorySample): PreGoneSystem
 function commitHostMemorySample(nowMs: number): boolean {
   try {
     const details = getSystemMemoryDetails()
-    // Why not `length === 0`: the signal label is appended unconditionally, so a
-    // reading that resolved no memory field at all still arrives with one key.
+    // Why not `length === 0`: any reading that resolved at least one field also
+    // carries the signal label, so a lone label key means nothing was measured.
     if (!Object.keys(details).some((key) => key !== PRESSURE_SIGNAL_KEY)) {
       return false
     }

src/main/crash-reporting/system-memory-details.ts

@@ -136,7 +136,7 @@ function cgroupCeilingBelowHostRam(details: CrashReportDetails): boolean {
   return total === undefined || lowest < total
 }
 
-/** The cgroup's own stall is what systemd-oomd reads; the host figure is the fallback. */
+/** Our own cgroup's stall is nearest the kill; a busy host with a calm cgroup is a sibling's. */
 function stallIsHigh(details: CrashReportDetails): boolean {
   const fullAvg10 =
     numericDetail(details, 'CgroupStallFullAvg10Pct') ?? numericDetail(details, 'StallFullAvg10Pct')