Use clone3 / CLONE_INTO_CGROUP on Linux

Use clone3 / CLONE_INTO_CGROUP to have the Linux kernel create the process in the
correct cgroup rather than move the process into the cgroup after it is created.

src/Command.zig

@@ -19,6 +19,7 @@ const Command = @This();
 const std = @import("std");
 const builtin = @import("builtin");
 const internal_os = @import("os/main.zig");
+const termio = @import("termio.zig");
 const windows = internal_os.windows;
 const TempDir = internal_os.TempDir;
 const mem = std.mem;
@@ -32,6 +33,8 @@ const EnvMap = std.process.EnvMap;
 
 const PreExecFn = fn (*Command) void;
 
+const log = std.log.scoped(.command);
+
 /// Path to the command to run. This must be an absolute path. This
 /// library does not do PATH lookup.
 path: []const u8,
@@ -61,6 +64,8 @@ stderr: ?File = null,
 /// exec process takes over, such as signal handlers, setsid, setuid, etc.
 pre_exec: ?*const PreExecFn = null,
 
+linux_cgroup: termio.Options.LinuxCgroup = termio.Options.linux_cgroup_default,
+
 /// If set, then the process will be created attached to this pseudo console.
 /// `stdin`, `stdout`, and `stderr` will be ignored if set.
 pseudo_console: if (builtin.os.tag == .windows) ?windows.exp.HPCON else void =
@@ -133,8 +138,11 @@ fn startPosix(self: *Command, arena: Allocator) !void {
     else
         @compileError("missing env vars");
 
-    // Fork
-    const pid = try posix.fork();
+    const pid: linux.pid_t = switch (builtin.os.tag) {
+        .linux => if (self.linux_cgroup) |cgroup| try internal_os.cgroup.cloneInto(cgroup) else try posix.fork(),
+        else => try posix.fork(),
+    };
+
     if (pid != 0) {
         // Parent, return immediately.
         self.pid = @intCast(pid);