resourcecoordinator: support 10.14 hosts with 10.15 SDK

The size of a structure changed between 10.14 and 10.15 in such a way
that the 10.15 structure can't be passed directly to a 10.14 host
over mach IPC. Deal with that by using the compatible prefix of the
structure.

Bug: 1023913
Change-Id: Ic2d5091f5e720097182436f42d94df6bf9a92fcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1967436
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724732}

services/resource_coordinator/public/cpp/memory_instrumentation/os_metrics_mac.cc

@@ -50,8 +50,34 @@ struct ChromeTaskVMInfo {
 #else
 using ChromeTaskVMInfo = task_vm_info;
 #endif  // MAC_OS_X_VERSION_10_11
-mach_msg_type_number_t ChromeTaskVMInfoCount =
-    sizeof(ChromeTaskVMInfo) / sizeof(natural_t);
+
+// Don't simply use sizeof(task_vm_info) / sizeof(natural_t):
+// In the 10.15 SDK, this structure is 87 32-bit words long, and in
+// mach_types.defs:
+//
+//   type task_info_t    = array[*:87] of integer_t;
+//
+// However in the 10.14 SDK, this structure is 42 32-bit words, and in
+// mach_types.defs:
+//
+//   type task_info_t    = array[*:52] of integer_t;
+//
+// As a result, the 10.15 SDK's task_vm_info won't fit inside the 10.14 SDK's
+// task_info_t, so the *rest of the system* (on 10.14 and earlier) can't handle
+// calls that request the full 10.15 structure. We have to request a prefix of
+// it that 10.14 and earlier can handle by limiting the length we request. The
+// rest of the fields just get ignored, but we don't use them anyway.
+
+constexpr mach_msg_type_number_t ChromeTaskVMInfoCount =
+    TASK_VM_INFO_REV2_COUNT;
+
+// The count field is in units of natural_t, which is the machine's word size
+// (64 bits on all modern machines), but the task_info_t array is in units of
+// integer_t, which is 32 bits.
+constexpr mach_msg_type_number_t MAX_MIG_SIZE_FOR_1014 =
+    52 / (sizeof(natural_t) / sizeof(integer_t));
+static_assert(ChromeTaskVMInfoCount <= MAX_MIG_SIZE_FOR_1014,
+              "task_vm_info must be small enough for 10.14 MIG interfaces");
 
 using VMRegion = mojom::VmRegion;