1 files changed, 53 insertions, 0 deletions
diff --git a/js/xpconnect/tests/unit/test_js_memory_telemetry.js b/js/xpconnect/tests/unit/test_js_memory_telemetry.js
new file mode 100644
index 0000000000..4c44f2e48c
--- /dev/null
+++ b/js/xpconnect/tests/unit/test_js_memory_telemetry.js
@@ -0,0 +1,53 @@
+"use strict";
+
+add_task(function test_compartment_realm_counts() {
+  const compsSystem = "MEMORY_JS_COMPARTMENTS_SYSTEM";
+  const compsUser = "MEMORY_JS_COMPARTMENTS_USER";
+  const realmsSystem = "MEMORY_JS_REALMS_SYSTEM";
+  const realmsUser = "MEMORY_JS_REALMS_USER";
+
+  Cu.forceShrinkingGC();
+
+  Services.telemetry.gatherMemory();
+  let snapshot1 = Services.telemetry.getSnapshotForHistograms("main", true).parent;
+
+  // We can't hard code exact counts, but we can check some basic invariants:
+  //
+  // * Compartments must contain at least one realm, so there must be more
+  //   realms than compartments.
+  // * There must be at least one system realm.
+
+  Assert.ok(snapshot1[realmsSystem].sum <= snapshot1[compsSystem].sum,
+            "Number of system compartments can't exceed number of system realms");
+  Assert.ok(snapshot1[realmsUser].sum <= snapshot1[compsUser].sum,
+            "Number of user compartments can't exceed number of user realms");
+  Assert.ok(snapshot1[realmsSystem].sum > 0,
+            "There must be at least one system realm");
+
+  // Now we create a bunch of sandboxes (more than one to be more resilient
+  // against GCs happening in the meantime), so we can check:
+  //
+  // * There are now more realms and user compartments than before. Not system
+  //   compartments, because system realms share a compartment.
+  // * The system compartment contains multiple realms.
+
+  let systemPrincipal = Services.scriptSecurityManager.getSystemPrincipal();
+  let arr = [];
+  for (let i = 0; i < 5; i++) {
+    arr.push(Cu.Sandbox(null));
+    arr.push(Cu.Sandbox(systemPrincipal));
+  }
+
+  Services.telemetry.gatherMemory();
+  let snapshot2 = Services.telemetry.getSnapshotForHistograms("main", true).parent;
+
+  for (let k of [realmsSystem, realmsUser, compsUser]) {
+    Assert.ok(snapshot2[k].sum > snapshot1[k].sum,
+              "There must be more compartments/realms now: " + k);
+  }
+
+  Assert.ok(snapshot2[realmsSystem].sum > snapshot2[compsSystem].sum,
+            "There must be more system realms than system compartments now");
+
+  arr[0].x = 10; // Ensure the JS engine keeps |arr| alive until this point.
+});