safely handle failure to open hosts, services, resolv.conf files

previously, transient failures like fd exhaustion or other
resource-related errors were treated the same as non-existence of
these files, leading to fallbacks or false-negative results. in
particular:

- failure to open hosts resulted in fallback to dns, possibly yielding
  EAI_NONAME for a hostname that should be defined locally, or an
  unwanted result from dns that the hosts file was intended to
  replace.

- failure to open services resulted in EAI_SERVICE.

- failure to open resolv.conf resulted in querying localhost rather
  than the configured nameservers.

now, only permanent errors trigger the fallback behaviors above; all
other errors are reportable to the caller as EAI_SYSTEM.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/network/lookup_name.c b/src/network/lookup_name.c
index 0225a934..df9e623e 100644
--- a/src/network/lookup_name.c
+++ b/src/network/lookup_name.c
@@ -9,6 +9,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <pthread.h>
+#include <errno.h>
 #include "lookup.h"
 #include "stdio_impl.h"
 #include "syscall.h"
@@ -51,7 +52,14 @@ static int name_from_hosts(struct address buf[static MAXADDRS], char canon[stati
 	int cnt = 0;
 	unsigned char _buf[1032];
 	FILE _f, *f = __fopen_rb_ca("/etc/hosts", &_f, _buf, sizeof _buf);
-	if (!f) return 0;
+	if (!f) switch (errno) {
+	case ENOENT:
+	case ENOTDIR:
+	case EACCES:
+		return 0;
+	default:
+		return EAI_SYSTEM;
+	}
 	while (fgets(line, sizeof line, f) && cnt < MAXADDRS) {
 		char *p, *z;