From f2de83684feee03e7e697c5860626e2251f8f7a2 Mon Sep 17 00:00:00 2001
From: Imanol-Mikel Barba <ibarba@fb.com>
Date: Wed, 5 May 2021 04:48:40 +0100
Subject: [PATCH] Adding IsNotFound to DNSError returned from cgo reverse
 resolution

The UNIX cgo resolver code didn't set IsNotFound on the DNSError returned when the `getnameinfo` call used for reverse resolution in `cgoNameinfoPTR` returns `EAI_NONAME`. This diff sets IsNotFound to true in that case.

EAI_NONAME is set by `getnameinfo` when the address resolution fails, and is defined in Solaris, OpenBSD, AIX, Darwin, Linux, NetBSD and DragonflyBSD according to online manpages. I have only tested on Darwin though.

```
$ ./all.bash
[...]
ALL TESTS PASSED
```

Then, I made a small go program to verify the behaviour:
```
package main

import (
  "fmt"
  "net"
)

func main() {
  names, err := net.LookupAddr("::2")
  fmt.Printf("names: %+v\n", names)

  if dnsErr, ok := err.(*net.DNSError); ok {
    fmt.Printf("error: %+v\n", *dnsErr)
  }
}
```

Before changes:
```
./dnstest
names: []
error: {Err:nodename nor servname provided, or not known Name:::2 Server: IsTimeout:false IsTemporary:false IsNotFound:false}
```

After changes:
```
$ ./dnstest
names: []
error: {Err:no such host Name:::2 Server: IsTimeout:false IsTemporary:false IsNotFound:true}
```
---
 src/net/cgo_unix.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/net/cgo_unix.go b/src/net/cgo_unix.go
index 2ea86e074fb943..a9067c877b82d8 100644
--- a/src/net/cgo_unix.go
+++ b/src/net/cgo_unix.go
@@ -305,17 +305,21 @@ func cgoLookupAddrPTR(addr string, sa *C.struct_sockaddr, salen C.socklen_t) (na
 		}
 	}
 	if gerrno != 0 {
+		isErrorNoSuchHost := false
 		isTemporary := false
 		switch gerrno {
 		case C.EAI_SYSTEM:
 			if err == nil { // see golang.org/issue/6232
 				err = syscall.EMFILE
 			}
+		case C.EAI_NONAME:
+			err = errNoSuchHost
+			isErrorNoSuchHost = true
 		default:
 			err = addrinfoErrno(gerrno)
 			isTemporary = addrinfoErrno(gerrno).Temporary()
 		}
-		return nil, &DNSError{Err: err.Error(), Name: addr, IsTemporary: isTemporary}
+		return nil, &DNSError{Err: err.Error(), Name: addr, IsTemporary: isTemporary, IsNotFound: isErrorNoSuchHost}
 	}
 	for i := 0; i < len(b); i++ {
 		if b[i] == 0 {