resolv: Add tests for getaddrinfo returning EAI_AGAIN [BZ #16849]

This patch adds two tests that verify correct behavior of getaddrinfo
when DNS resolution fails with a temporary error. Both tests ensure
that getaddrinfo returns EAI_AGAIN in cases where no valid address can
be resolved due to network or resolver failure.

* tst-getaddrinfo-eai-again.c
  Runs inside the glibc test-container without any DNS server
  configured. The test performs queries using AF_INET, AF_INET6,
  and AF_UNSPEC and verifies that getaddrinfo returns EAI_AGAIN
  when resolution fails.

* tst-getaddrinfo-eai-again-timeout.c
  Runs outside of the container but uses the resolv_test framework
  to simulate network failures. The test covers two failure modes:
  - No response from the server (resolv_response_drop)
  - Zero-length reply from the server
  In both cases, getaddrinfo is expected to return EAI_AGAIN.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

resolv/Makefile

@@ -80,7 +80,10 @@ routines_no_fortify += \
   # routines_no_fortify
 
 tests = tst-aton tst-leaks tst-inet_ntop
-tests-container = tst-leaks2
+tests-container += \
+  tst-getaddrinfo-eai-again \
+  tst-leaks2 \
+  # tests-container
 
 tests-internal += tst-inet_aton_exact
 
@@ -136,6 +139,7 @@ tests-static += tst-ns_rr_cursor
 # These tests need libdl.
 ifeq (yes,$(build-shared))
 tests += \
+  tst-getaddrinfo-eai-again-timeout \
   tst-resolv-ai_idn \
   tst-resolv-ai_idn-latin1 \
   tst-resolv-ai_idn-nolibidn2 \
@@ -280,6 +284,8 @@ $(objpfx)mtrace-tst-resolv-res_ninit.out: $(objpfx)tst-resolv-res_ninit.out
 
 $(objpfx)tst-bug18665-tcp: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-bug18665: $(objpfx)libresolv.so $(shared-thread-library)
+$(objpfx)tst-getaddrinfo-eai-again-timeout: \
+  $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-ai_idn: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-ai_idn-latin1: \
   $(objpfx)libresolv.so $(shared-thread-library)

resolv/tst-getaddrinfo-eai-again-timeout.c

@@ -0,0 +1,122 @@
+/* Test for BZ #16849. Verify that getaddrinfo correctly returns
+   EAI_AGAIN when DNS resolution fails due to timeout or malformed
+   responses.
+
+   This test uses two simulated failure modes:
+   - The DNS server does not respond at all (resolv_response_drop).
+   - The DNS server responds with a zero-length packet.
+
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <netdb.h>
+#include <resolv.h>
+#include <stdbool.h>
+#include <support/check.h>
+#include <support/resolv_test.h>
+
+/* Track whether the callbacks were actually invoked.  */
+static volatile bool response_called_drop = false;
+static volatile bool response_called_zero_len = false;
+
+/* Simulate a DNS server that sends a zero-length response.  */
+static void
+response_zero_len (const struct resolv_response_context *ctx,
+                   struct resolv_response_builder *b,
+                   const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  response_called_zero_len = true;
+  /* Do nothing — zero-length reply.  */
+}
+
+/* Simulate a DNS server that drops the request.  */
+static void
+response_drop (const struct resolv_response_context *ctx,
+               struct resolv_response_builder *b,
+               const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  response_called_drop = true;
+  resolv_response_drop (b);
+}
+
+/* Query getaddrinfo for multiple families and expect EAI_AGAIN.  */
+static void
+query_host (const char *host_name)
+{
+  int family[] = { AF_INET, AF_INET6, AF_UNSPEC };
+  const char *family_names[] = { "AF_INET", "AF_INET6", "AF_UNSPEC" };
+
+  for (int i = 0; i < 3; i++)
+    {
+      struct addrinfo hints =
+        {
+          .ai_socktype = 0,
+          .ai_protocol = 0,
+          .ai_family = family[i],
+          .ai_flags = 0,
+        };
+      struct addrinfo *result;
+      int res = getaddrinfo (host_name, NULL, &hints, &result);
+      if (res != EAI_AGAIN)
+        FAIL_EXIT1 ("getaddrinfo (%s, %s) returned %s, expected EAI_AGAIN",
+                    host_name, family_names[i], gai_strerror (res));
+    }
+}
+
+/* Simulate DNS server dropping all queries.  */
+static void
+test_drop (void)
+{
+  struct resolv_test *aux = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response_drop,
+     });
+  /* Reduce default timeout to make the test run faster.  */
+  _res.retrans = 1;
+  _res.retry = 1;
+  query_host ("site.example");
+  resolv_test_end (aux);
+}
+
+/* Simulate DNS server sending zero-length responses.  */
+static void
+test_zero_len_packet (void)
+{
+  struct resolv_test *aux = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response_zero_len,
+     });
+  query_host ("site.example");
+  resolv_test_end (aux);
+}
+
+static int
+do_test (void)
+{
+  test_drop ();
+  test_zero_len_packet ();
+
+  if (!response_called_drop)
+    FAIL_EXIT1 ("response_drop callback was not called");
+  if (!response_called_zero_len)
+    FAIL_EXIT1 ("response_zero_len callback was not called");
+  return 0;
+}
+
+#include <support/test-driver.c>

resolv/tst-getaddrinfo-eai-again.c

@@ -0,0 +1,56 @@
+/* Test for BZ #16849. Verify that getaddrinfo correctly returns
+   EAI_AGAIN error code if DNS query fails due to a network failure.
+
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <netdb.h>
+#include <support/check.h>
+
+/* Query getaddrinfo with various address families and verify that
+   it returns EAI_AGAIN when DNS resolution fails.  */
+static void
+query_host (const char *host_name)
+{
+  int family[] = { AF_INET, AF_INET6, AF_UNSPEC };
+  const char *family_names[] = { "AF_INET", "AF_INET6", "AF_UNSPEC" };
+
+  for (int i = 0; i < 3; i++)
+    {
+      struct addrinfo hints =
+        {
+          .ai_socktype = 0,
+          .ai_protocol = 0,
+          .ai_family = family[i],
+          .ai_flags = 0,
+        };
+      struct addrinfo *result;
+      int res = getaddrinfo (host_name, NULL, &hints, &result);
+      if (res != EAI_AGAIN)
+        FAIL_EXIT1 ("getaddrinfo (%s, %s) returned %s, expected EAI_AGAIN",
+                    host_name, family_names[i], gai_strerror (res));
+    }
+}
+
+static int
+do_test (void)
+{
+  query_host ("site.example");
+  return 0;
+}
+
+#include <support/test-driver.c>