resolv: Do not wait for non-existing second DNS response after error (bug 30081)

In single-request mode, there is no second response after an error
because the second query has not been sent yet.  Waiting for it
introduces an unnecessary timeout.

Reviewed-by: DJ Delorie <dj@redhat.com>
(cherry picked from commit af625987d6)

NEWS

@@ -9,6 +9,7 @@ Version 2.41
 
 The following bugs are resolved with this release:
 
+  [30081] resolv: Do not wait for non-existing second DNS response after error
   [31890] resolv: Allow short error responses to match any DNS query
 
 Version 2.40

resolv/Makefile

@@ -106,6 +106,7 @@ tests += \
   tst-resolv-nondecimal \
   tst-resolv-res_init-multi \
   tst-resolv-search \
+  tst-resolv-semi-failure \
   tst-resolv-short-response \
   tst-resolv-trailing \
 
@@ -300,6 +301,8 @@ $(objpfx)tst-resolv-nondecimal: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-qtypes: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-rotate: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-search: $(objpfx)libresolv.so $(shared-thread-library)
+$(objpfx)tst-resolv-semi-failure: $(objpfx)libresolv.so \
+  $(shared-thread-library)
 $(objpfx)tst-resolv-short-response: $(objpfx)libresolv.so \
   $(shared-thread-library)
 $(objpfx)tst-resolv-trailing: $(objpfx)libresolv.so $(shared-thread-library)

resolv/res_send.c

@@ -1238,7 +1238,7 @@ send_dg(res_state statp,
 			  *resplen2 = 0;
 			  return resplen;
 			}
-			if (buf2 != NULL)
+			if (buf2 != NULL && !single_request)
 			  {
 			    /* No data from the first reply.  */
 			    resplen = 0;

resolv/tst-resolv-semi-failure.c

@@ -0,0 +1,133 @@
+/* Test parallel failure/success responses (bug 30081).
+   Copyright (C) 2024 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 <resolv.h>
+#include <support/check.h>
+#include <support/resolv_test.h>
+#include <support/check_nss.h>
+
+/* The rcode in the initial response.  */
+static volatile int rcode;
+
+/* Whether to fail the initial A query (!fail_aaaa) or the initial
+   AAAA query (fail_aaaa).  */
+static volatile bool fail_aaaa;
+
+static void
+response (const struct resolv_response_context *ctx,
+          struct resolv_response_builder *b,
+          const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  /* Handle the failing query.  */
+  if ((fail_aaaa && qtype == T_AAAA) && ctx->server_index == 0)
+    {
+      struct resolv_response_flags flags = {.rcode = rcode};
+      resolv_response_init (b, flags);
+      return;
+    }
+
+  /* Otherwise produce a response.  */
+  resolv_response_init (b, (struct resolv_response_flags) {});
+  resolv_response_add_question (b, qname, qclass, qtype);
+  resolv_response_section (b, ns_s_an);
+  resolv_response_open_record (b, qname, qclass, qtype, 0);
+  switch (qtype)
+    {
+    case T_A:
+      {
+        char ipv4[4] = {192, 0, 2, 17};
+        resolv_response_add_data (b, &ipv4, sizeof (ipv4));
+      }
+      break;
+    case T_AAAA:
+      {
+        char ipv6[16]
+          = {0x20, 0x01, 0xd, 0xb8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1};
+        resolv_response_add_data (b, &ipv6, sizeof (ipv6));
+      }
+      break;
+    default:
+      FAIL_EXIT1 ("unexpected TYPE%d query", qtype);
+    }
+  resolv_response_close_record (b);
+}
+
+static void
+check_one (void)
+{
+
+  /* The buggy 1-second query timeout results in 30 seconds of delay,
+     which triggers are test timeout failure.  */
+  for (int i = 0;  i < 30; ++i)
+    {
+      static const struct addrinfo hints =
+        {
+          .ai_family = AF_UNSPEC,
+          .ai_socktype = SOCK_STREAM,
+        };
+      struct addrinfo *ai;
+      int ret = getaddrinfo ("www.example", "80", &hints, &ai);
+      const char *expected;
+      if (ret == 0 && ai->ai_next != NULL)
+        expected = ("address: STREAM/TCP 192.0.2.17 80\n"
+                    "address: STREAM/TCP 2001:db8::1 80\n");
+      else
+        /* Only one response because the AAAA lookup failure is
+           treated as an ignoreable error.  */
+        expected = "address: STREAM/TCP 192.0.2.17 80\n";
+      check_addrinfo ("www.example", ai, ret, expected);
+      if (ret == 0)
+        freeaddrinfo (ai);
+    }
+}
+
+static int
+do_test (void)
+{
+  for (int do_single_lookup = 0; do_single_lookup < 2; ++do_single_lookup)
+    {
+      struct resolv_test *aux = resolv_test_start
+        ((struct resolv_redirect_config)
+         {
+           .response_callback = response,
+         });
+
+      if (do_single_lookup)
+        _res.options |= RES_SNGLKUP;
+
+      for (int do_fail_aaaa = 0; do_fail_aaaa < 2; ++do_fail_aaaa)
+        {
+          fail_aaaa = do_fail_aaaa;
+
+          rcode = 2; /* SERVFAIL.  */
+          check_one ();
+
+          rcode = 4; /* NOTIMP.  */
+          check_one ();
+
+          rcode = 5; /* REFUSED.  */
+          check_one ();
+        }
+
+      resolv_test_end (aux);
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>

resolv/tst-resolv-short-response.c

@@ -81,6 +81,18 @@ check_one (void)
       check_hostent ("www.example", gethostbyname2 ("www.example", AF_INET6),
                      "name: www.example\n"
                      "address: 2001:db8::1\n");
+      static const struct addrinfo hints =
+        {
+          .ai_family = AF_UNSPEC,
+          .ai_socktype = SOCK_STREAM,
+        };
+      struct addrinfo *ai;
+      int ret = getaddrinfo ("www.example", "80", &hints, &ai);
+      check_addrinfo ("www.example", ai, ret,
+                      "address: STREAM/TCP 192.0.2.17 80\n"
+                      "address: STREAM/TCP 2001:db8::1 80\n");
+      if (ret == 0)
+        freeaddrinfo (ai);
     }
 }