linux: emulate wait4 using waitid

Author: lunagl

lib/std/os/linux.zig

@@ -1586,21 +1586,62 @@ pub fn unlinkat(dirfd: i32, path: [*:0]const u8, flags: u32) usize {
     return syscall3(.unlinkat, @as(usize, @bitCast(@as(isize, dirfd))), @intFromPtr(path), flags);
 }
 
-pub fn waitpid(pid: pid_t, status: *u32, flags: u32) usize {
-    return syscall4(.wait4, @as(usize, @bitCast(@as(isize, pid))), @intFromPtr(status), flags, 0);
+pub fn waitpid(pid: pid_t, status: ?*u32, flags: u32) usize {
+    return wait4(pid, status, flags, null);
 }
 
-pub fn wait4(pid: pid_t, status: *u32, flags: u32, usage: ?*rusage) usize {
-    return syscall4(
-        .wait4,
-        @as(usize, @bitCast(@as(isize, pid))),
-        @intFromPtr(status),
-        flags,
+pub fn wait4(pid: pid_t, status: ?*u32, flags: u32, usage: ?*rusage) usize {
+    if (@hasField(SYS, "wait4")) {
+        return syscall4(
+            .wait4,
+            @as(usize, @bitCast(@as(isize, pid))),
+            @intFromPtr(status),
+            flags,
+            @intFromPtr(usage),
+        );
+    }
+
+    // Emulate wait4 using waitid; adapted from musl/src/internal/emulate_wait4.c
+
+    const id_type: P = if (pid < -1 or pid == 0) .PGID else if (pid == -1) .ALL else .PID;
+    const id = if (pid < -1) -pid else pid;
+
+    var info: siginfo_t = undefined;
+    info.fields.common.first.piduid.pid = 0;
+
+    const r = syscall5(
+        .waitid,
+        @intFromEnum(id_type),
+        @bitCast(@as(isize, id)),
+        @intFromPtr(&info),
+        flags | W.EXITED,
         @intFromPtr(usage),
     );
+    if (@clz(r) == 0) return r; // r < 0
+    // WNOHANG passed and no children in waitable state
+    if (info.fields.common.first.piduid.pid == 0) return 0;
+
+    if (status) |s| {
+        const si_status: u32 = @bitCast(info.fields.common.second.sigchld.status);
+        s.* = switch (info.code) {
+            // CLD_EXITED
+            1 => (si_status & 0xff) << 8,
+            // CLD_KILLED
+            2 => si_status & 0x7f,
+            // CLD_DUMPED
+            3 => si_status & 0x7f | 0x80,
+            // CLD_TRAPPED, CLD_STOPPED
+            // see ptrace(2); the high bits of si_status can contain PTRACE_EVENT_ values which must be preserved
+            4, 5 => (si_status << 8) + 0x7f,
+            // CLD_CONTINUED
+            6 => 0xffff,
+            else => unreachable,
+        };
+    }
+    return @bitCast(@as(isize, info.fields.common.first.piduid.pid));
 }
 
-pub fn waitid(id_type: P, id: i32, infop: *siginfo_t, flags: u32) usize {
+pub fn waitid(id_type: P, id: pid_t, infop: *siginfo_t, flags: u32) usize {
     return syscall5(.waitid, @intFromEnum(id_type), @as(usize, @bitCast(@as(isize, id))), @intFromPtr(infop), flags, 0);
 }