code review 13532045: net: defend against broken getaddrinfo on Linux

src/pkg/net/cgo_unix.go

 // Copyright 2011 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.
 
 // +build !netgo
 // +build darwin dragonfly freebsd linux netbsd openbsd
 
 package net
 
 /*
@@ skipping 81 matching lines @@
         hints.ai_socktype = C.SOCK_STREAM
 
         h := C.CString(name)
         defer C.free(unsafe.Pointer(h))
         gerrno, err := C.getaddrinfo(h, nil, &hints, &res)
         if gerrno != 0 {
                 var str string
                 if gerrno == C.EAI_NONAME {
                         str = noSuchHost
                 } else if gerrno == C.EAI_SYSTEM {
+                        if err == nil {
+                                // err should not be nil, but sometimes getaddrinfo returns
+                                // gerrno == C.EAI_SYSTEM with err == nil on Linux.
+                                // The report claims that it happens when we have too many
+                                // open files, so use syscall.EMFILE (too many open files in system).
+                                // Most system calls would return ENFILE (too many open files),
+                                // so at the least EMFILE should be easy to recognize if this
+                                // comes up again. golang.org/issue/6232.
+                                err = syscall.EMFILE
+                        }
                         str = err.Error()
                 } else {
                         str = C.GoString(C.gai_strerror(gerrno))
                 }
                 return nil, "", &DNSError{Err: str, Name: name}, true
         }
         defer C.freeaddrinfo(res)
         if res != nil {
                 cname = C.GoString(res.ai_canonname)
                 if cname == "" {
@@ skipping 30 matching lines @@
 func cgoLookupCNAME(name string) (cname string, err error, completed bool) {
         _, cname, err, completed = cgoLookupIPCNAME(name)
         return
 }
 
 func copyIP(x IP) IP {
         y := make(IP, len(x))
         copy(y, x)
         return y
 }