std.process.Child: implement maxrss on Darwin

Notably the Darwin (XNU) kernel the maxrss field is number of bytes
and not kilobytes (kibibytes) like other platforms (e.g. Linux, BSD).

watchOS and tvOS are not supported because they do not have the ability
to spawn a child process. iOS is enabled but due to OS sandboxing it
should fail with a permission error.

lib/std/child_process.zig

@@ -17,7 +17,6 @@ const Os = std.builtin.Os;
 const TailQueue = std.TailQueue;
 const maxInt = std.math.maxInt;
 const assert = std.debug.assert;
-const is_darwin = builtin.target.isDarwin();
 
 pub const ChildProcess = struct {
     pub const Id = switch (builtin.os.tag) {
@@ -77,7 +76,7 @@ pub const ChildProcess = struct {
     /// requested statistics may or may not be available. If they are
     /// available, then the `resource_usage_statistics` field will be populated
     /// after calling `wait`.
-    /// On Linux, this obtains rusage statistics from wait4().
+    /// On Linux and Darwin, this obtains rusage statistics from wait4().
     request_resource_usage_statistics: bool = false,
 
     /// This is available after calling wait if
@@ -106,12 +105,20 @@ pub const ChildProcess = struct {
                         return null;
                     }
                 },
+                .macos, .ios => {
+                    if (rus.rusage) |ru| {
+                        // Darwin oddly reports in bytes instead of kilobytes.
+                        return @intCast(usize, ru.maxrss);
+                    } else {
+                        return null;
+                    }
+                },
                 else => return null,
             }
         }
 
         const rusage_init = switch (builtin.os.tag) {
-            .linux => @as(?std.os.rusage, null),
+            .linux, .macos, .ios => @as(?std.os.rusage, null),
             .windows => @as(?windows.VM_COUNTERS, null),
             else => {},
         };
@@ -385,11 +392,16 @@ pub const ChildProcess = struct {
 
     fn waitUnwrapped(self: *ChildProcess) !void {
         const res: os.WaitPidResult = res: {
-            if (builtin.os.tag == .linux and self.request_resource_usage_statistics) {
-                var ru: std.os.rusage = undefined;
-                const res = os.wait4(self.id, 0, &ru);
-                self.resource_usage_statistics.rusage = ru;
-                break :res res;
+            if (self.request_resource_usage_statistics) {
+                switch (builtin.os.tag) {
+                    .linux, .macos, .ios => {
+                        var ru: std.os.rusage = undefined;
+                        const res = os.wait4(self.id, 0, &ru);
+                        self.resource_usage_statistics.rusage = ru;
+                        break :res res;
+                    },
+                    else => {},
+                }
             }
 
             break :res os.waitpid(self.id, 0);

lib/std/process.zig

@@ -1093,7 +1093,7 @@ pub const can_execv = switch (builtin.os.tag) {
 
 /// Tells whether spawning child processes is supported (e.g. via ChildProcess)
 pub const can_spawn = switch (builtin.os.tag) {
-    .wasi => false,
+    .wasi, .watchos, .tvos => false,
     else => true,
 };