net: set DNSError.IsTemporary from addrinfoErrno errors

Today it is not possible (AFAICT) to detect if a DNSError if of type EAI_AGAIN, i.e. if it is something temporary that should be retried. This information is available inside addrinfoErrno but when the DNSError is created this information is lost.

This PR fixes this so that the addinfoErrno.Temporary information is added to DNSError as well. With that a user who gets a DNSError can check now is its a temporary error (for errors that resulted from a addrinfoErrno this is EAI_AGAIN).

Change-Id: I64badb2ebd904e41fc2e0755416f7f32560534d8
GitHub-Last-Rev: ced7238a6597039fb23f36f372bd1cf33d60d4a6
GitHub-Pull-Request: golang/go#31676
Reviewed-on: https://go-review.googlesource.com/c/go/+/174557Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/cgo_unix.go

@@ -106,6 +106,7 @@ func cgoLookupServicePort(hints *C.struct_addrinfo, network, service string) (po
 	var res *C.struct_addrinfo
 	gerrno, err := C.getaddrinfo(nil, (*C.char)(unsafe.Pointer(&cservice[0])), hints, &res)
 	if gerrno != 0 {
+		isTemporary := false
 		switch gerrno {
 		case C.EAI_SYSTEM:
 			if err == nil { // see golang.org/issue/6232
@@ -113,8 +114,9 @@ func cgoLookupServicePort(hints *C.struct_addrinfo, network, service string) (po
 			}
 		default:
 			err = addrinfoErrno(gerrno)
+			isTemporary = addrinfoErrno(gerrno).Temporary()
 		}
-		return 0, &DNSError{Err: err.Error(), Name: network + "/" + service}
+		return 0, &DNSError{Err: err.Error(), Name: network + "/" + service, IsTemporary: isTemporary}
 	}
 	defer C.freeaddrinfo(res)
 
@@ -159,6 +161,7 @@ func cgoLookupIPCNAME(network, name string) (addrs []IPAddr, cname string, err e
 	gerrno, err := C.getaddrinfo((*C.char)(unsafe.Pointer(&h[0])), nil, &hints, &res)
 	if gerrno != 0 {
 		isErrorNoSuchHost := false
+		isTemporary := false
 		switch gerrno {
 		case C.EAI_SYSTEM:
 			if err == nil {
@@ -176,9 +179,10 @@ func cgoLookupIPCNAME(network, name string) (addrs []IPAddr, cname string, err e
 			isErrorNoSuchHost = true
 		default:
 			err = addrinfoErrno(gerrno)
+			isTemporary = addrinfoErrno(gerrno).Temporary()
 		}
 
-		return nil, "", &DNSError{Err: err.Error(), Name: name, IsNotFound: isErrorNoSuchHost}
+		return nil, "", &DNSError{Err: err.Error(), Name: name, IsNotFound: isErrorNoSuchHost, IsTemporary: isTemporary}
 	}
 	defer C.freeaddrinfo(res)
 
@@ -299,6 +303,7 @@ func cgoLookupAddrPTR(addr string, sa *C.struct_sockaddr, salen C.socklen_t) (na
 		}
 	}
 	if gerrno != 0 {
+		isTemporary := false
 		switch gerrno {
 		case C.EAI_SYSTEM:
 			if err == nil { // see golang.org/issue/6232
@@ -306,8 +311,9 @@ func cgoLookupAddrPTR(addr string, sa *C.struct_sockaddr, salen C.socklen_t) (na
 			}
 		default:
 			err = addrinfoErrno(gerrno)
+			isTemporary = addrinfoErrno(gerrno).Temporary()
 		}
-		return nil, &DNSError{Err: err.Error(), Name: addr}
+		return nil, &DNSError{Err: err.Error(), Name: addr, IsTemporary: isTemporary}
 	}
 	for i := 0; i < len(b); i++ {
 		if b[i] == 0 {