libzigc: dup2 and dup3

This commit adds `dup2` and `dup3` based on musl. Zig already has
wrappers for the syscalls, but musl's implementation checks for a rare,
temporary race condition with dup2/3 and `open`. Like musl, this
implementation adds a fallback for dup3 if the syscall isn't available.

Contributes to: #30978

lib/c/unistd.zig

@@ -21,6 +21,8 @@ comptime {
         symbol(&chrootLinux, "chroot");
         symbol(&ctermidLinux, "ctermid");
         symbol(&dupLinux, "dup");
+        symbol(&dup2Linux, "dup2");
+        symbol(&dup3Linux, "dup3");
 
         symbol(&getegidLinux, "getegid");
         symbol(&geteuidLinux, "geteuid");
@@ -101,6 +103,31 @@ fn dupLinux(fd: c_int) callconv(.c) c_int {
     return errno(linux.dup(fd));
 }
 
+fn dup2Linux(old: c_int, new: c_int) callconv(.c) c_int {
+    const busy: usize = @bitCast(-@as(isize, @intFromEnum(linux.E.BUSY)));
+    var res = busy;
+    while (res == busy) res = linux.dup2(old, new);
+    return errno(res);
+}
+
+fn dup3Linux(old: c_int, new: c_int, flags: c_int) callconv(.c) c_int {
+    const busy: usize = @bitCast(-@as(isize, @intFromEnum(linux.E.BUSY)));
+    var res = busy;
+
+    if (@hasField(linux.SYS, "dup3")) {
+        while (res == busy) res = linux.dup3(old, new, @intCast(flags));
+    } else if (@hasField(linux.SYS, "dup2")) {
+        const cloexec: c_int = @bitCast(linux.O{ .CLOEXEC = true });
+        const inval: usize = @bitCast(-@as(isize, @intFromEnum(linux.E.INVAL)));
+        if (old == new or (flags & ~cloexec != 0)) return errno(inval);
+        while (res == busy) res = linux.dup2(old, new);
+        _ = if (res >= 0 and (flags & cloexec == cloexec)) linux.fcntl(new, linux.F.SETFD, linux.FD_CLOEXEC);
+    } else {
+        return errno(@bitCast(-@as(isize, @intFromEnum(linux.E.NOSYS))));
+    }
+    return errno(res);
+}
+
 fn getegidLinux() callconv(.c) linux.gid_t {
     return linux.getegid();
 }

lib/libc/musl/src/unistd/dup2.c

@@ -1,20 +0,0 @@
-#include <unistd.h>
-#include <errno.h>
-#include <fcntl.h>
-#include "syscall.h"
-
-int dup2(int old, int new)
-{
-	int r;
-#ifdef SYS_dup2
-	while ((r=__syscall(SYS_dup2, old, new))==-EBUSY);
-#else
-	if (old==new) {
-		r = __syscall(SYS_fcntl, old, F_GETFD);
-		if (r >= 0) return old;
-	} else {
-		while ((r=__syscall(SYS_dup3, old, new, 0))==-EBUSY);
-	}
-#endif
-	return __syscall_ret(r);
-}

lib/libc/musl/src/unistd/dup3.c

@@ -1,26 +0,0 @@
-#define _GNU_SOURCE
-#include <unistd.h>
-#include <errno.h>
-#include <fcntl.h>
-#include "syscall.h"
-
-int __dup3(int old, int new, int flags)
-{
-	int r;
-#ifdef SYS_dup2
-	if (old==new) return __syscall_ret(-EINVAL);
-	if (flags) {
-		while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
-		if (r!=-ENOSYS) return __syscall_ret(r);
-		if (flags & ~O_CLOEXEC) return __syscall_ret(-EINVAL);
-	}
-	while ((r=__syscall(SYS_dup2, old, new))==-EBUSY);
-	if (r >= 0 && (flags & O_CLOEXEC))
-		__syscall(SYS_fcntl, new, F_SETFD, FD_CLOEXEC);
-#else
-	while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
-#endif
-	return __syscall_ret(r);
-}
-
-weak_alias(__dup3, dup3);

src/libs/musl.zig

@@ -1758,8 +1758,6 @@ const src_files = [_][]const u8{
     "musl/src/time/wcsftime.c",
     "musl/src/time/__year_to_secs.c",
     "musl/src/unistd/alarm.c",
-    "musl/src/unistd/dup2.c",
-    "musl/src/unistd/dup3.c",
     "musl/src/unistd/faccessat.c",
     "musl/src/unistd/fchdir.c",
     "musl/src/unistd/fchown.c",