net: use libc (not cgo) for DNS on macOS

Change the macOS implementation to use libc calls.
Using libc calls directly is what we do for all the runtime and os syscalls.
Doing so here as well improves consistency and also makes it possible
to cross-compile (from non-Mac systems) macOS binaries that use the
native name resolver.

Fixes #12524.

Change-Id: I011f4fcc5c50fbb5396e494889765dcbb9342336
Reviewed-on: https://go-review.googlesource.com/c/go/+/446178
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/internal/syscall/unix/asm_darwin.s

@@ -6,3 +6,15 @@
 
 TEXT ·libc_getentropy_trampoline(SB),NOSPLIT,$0-0
 	JMP	libc_getentropy(SB)
+
+TEXT ·libc_getaddrinfo_trampoline(SB),NOSPLIT,$0-0
+	JMP	libc_getaddrinfo(SB)
+
+TEXT ·libc_freeaddrinfo_trampoline(SB),NOSPLIT,$0-0
+	JMP	libc_freeaddrinfo(SB)
+
+TEXT ·libc_getnameinfo_trampoline(SB),NOSPLIT,$0-0
+	JMP	libc_getnameinfo(SB)
+
+TEXT ·libc_gai_strerror_trampoline(SB),NOSPLIT,$0-0
+	JMP	libc_gai_strerror(SB)

src/internal/syscall/unix/getentropy_darwin.go

@@ -8,7 +8,6 @@ package unix
 
 import (
 	"internal/abi"
-	"syscall"
 	"unsafe"
 )
 
@@ -27,6 +26,3 @@ func GetEntropy(p []byte) error {
 	}
 	return nil
 }
-
-//go:linkname syscall_syscall syscall.syscall
-func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)

src/internal/syscall/unix/net_darwin.go

@@ -0,0 +1,116 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"internal/abi"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	AI_CANONNAME = 0x2
+	AI_ALL       = 0x100
+	AI_V4MAPPED  = 0x800
+	AI_MASK      = 0x1407
+
+	EAI_AGAIN    = 2
+	EAI_NONAME   = 8
+	EAI_SYSTEM   = 11
+	EAI_OVERFLOW = 14
+
+	NI_NAMEREQD = 4
+)
+
+type Addrinfo struct {
+	Flags     int32
+	Family    int32
+	Socktype  int32
+	Protocol  int32
+	Addrlen   uint32
+	Canonname *byte
+	Addr      *syscall.RawSockaddr
+	Next      *Addrinfo
+}
+
+//go:cgo_import_dynamic libc_getaddrinfo getaddrinfo "/usr/lib/libSystem.B.dylib"
+func libc_getaddrinfo_trampoline()
+
+func Getaddrinfo(hostname, servname *byte, hints *Addrinfo, res **Addrinfo) (int, error) {
+	gerrno, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_getaddrinfo_trampoline),
+		uintptr(unsafe.Pointer(hostname)),
+		uintptr(unsafe.Pointer(servname)),
+		uintptr(unsafe.Pointer(hints)),
+		uintptr(unsafe.Pointer(res)),
+		0,
+		0)
+	var err error
+	if errno != 0 {
+		err = errno
+	}
+	return int(gerrno), err
+}
+
+//go:cgo_import_dynamic libc_freeaddrinfo freeaddrinfo "/usr/lib/libSystem.B.dylib"
+func libc_freeaddrinfo_trampoline()
+
+func Freeaddrinfo(ai *Addrinfo) {
+	syscall_syscall6(abi.FuncPCABI0(libc_freeaddrinfo_trampoline),
+		uintptr(unsafe.Pointer(ai)),
+		0, 0, 0, 0, 0)
+}
+
+//go:cgo_import_dynamic libc_getnameinfo getnameinfo "/usr/lib/libSystem.B.dylib"
+func libc_getnameinfo_trampoline()
+
+func Getnameinfo(sa *syscall.RawSockaddr, salen int, host *byte, hostlen int, serv *byte, servlen int, flags int) (int, error) {
+	gerrno, _, errno := syscall_syscall9(abi.FuncPCABI0(libc_getnameinfo_trampoline),
+		uintptr(unsafe.Pointer(sa)),
+		uintptr(salen),
+		uintptr(unsafe.Pointer(host)),
+		uintptr(hostlen),
+		uintptr(unsafe.Pointer(serv)),
+		uintptr(servlen),
+		uintptr(flags),
+		0,
+		0)
+	var err error
+	if errno != 0 {
+		err = errno
+	}
+	return int(gerrno), err
+}
+
+//go:cgo_import_dynamic libc_gai_strerror gai_strerror "/usr/lib/libSystem.B.dylib"
+func libc_gai_strerror_trampoline()
+
+func GaiStrerror(ecode int) string {
+	r1, _, _ := syscall_syscall(abi.FuncPCABI0(libc_gai_strerror_trampoline),
+		uintptr(ecode),
+		0, 0)
+	return GoString((*byte)(unsafe.Pointer(r1)))
+}
+
+func GoString(p *byte) string {
+	if p == nil {
+		return ""
+	}
+	x := unsafe.Slice(p, 1e9)
+	for i, c := range x {
+		if c == 0 {
+			return string(x[:i])
+		}
+	}
+	return ""
+}
+
+//go:linkname syscall_syscall syscall.syscall
+func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscall6 syscall.syscall6
+func syscall_syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscall9 syscall.syscall9
+func syscall_syscall9(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)

src/net/cgo_resnew.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build cgo && !netgo && (darwin || (linux && !android) || netbsd || solaris)
+//go:build cgo && !netgo && ((linux && !android) || netbsd || solaris)
 
 package net
 

src/net/cgo_sockold.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build cgo && !netgo && (aix || darwin || dragonfly || freebsd || netbsd || openbsd)
+//go:build cgo && !netgo && (aix || dragonfly || freebsd || netbsd || openbsd)
 
 package net