Blank DNS names would result in ARES_ENOMEM due to bug in query cache

If a blank DNS name is used, the DNS query cache would fail due to an
invalid sanity check.  This can be legitimate such as:

adig -t SOA .

This fixes that situation as well as a few other spots that were
uncovered and adds a test case to validate the behavior to ensure
it won't regress in the future.

Fixes #858
Reported-By: Nodar Chkuaselidze (@nodech)
Authored-By: Brad House (@bradh352)

src/lib/ares_qcache.c

@@ -121,9 +121,11 @@ static char *ares__qcache_calc_key(const ares_dns_record_t *dnsrec)
       name_len--;
     }
 
-    status = ares__buf_append(buf, (const unsigned char *)name, name_len);
-    if (status != ARES_SUCCESS) {
-      goto fail; /* LCOV_EXCL_LINE: OutOfMemory */
+    if (name_len > 0) {
+      status = ares__buf_append(buf, (const unsigned char *)name, name_len);
+      if (status != ARES_SUCCESS) {
+        goto fail; /* LCOV_EXCL_LINE: OutOfMemory */
+      }
     }
   }
 

src/lib/ares_send.c

@@ -62,7 +62,11 @@ static ares_status_t ares_apply_dns0x20(ares_channel_t    *channel,
   }
 
   len = ares_strlen(name);
-  if (len == 0 || len >= sizeof(dns0x20name)) {
+  if (len == 0) {
+    return ARES_SUCCESS;
+  }
+
+  if (len >= sizeof(dns0x20name)) {
     status = ARES_EBADNAME;
     goto done;
   }

src/lib/str/ares__buf.c

@@ -213,10 +213,14 @@ ares_status_t ares__buf_append(ares__buf_t *buf, const unsigned char *data,
 {
   ares_status_t status;
 
-  if (data == NULL || data_len == 0) {
+  if (data == NULL && data_len != 0) {
     return ARES_EFORMERR;
   }
 
+  if (data_len == 0) {
+    return ARES_SUCCESS;
+  }
+
   status = ares__buf_ensure_space(buf, data_len);
   if (status != ARES_SUCCESS) {
     return status;

test/ares-test-internal.cc

@@ -1021,7 +1021,7 @@ TEST_F(LibraryTest, ArrayMisuse) {
 TEST_F(LibraryTest, BufMisuse) {
   EXPECT_EQ(NULL, ares__buf_create_const(NULL, 0));
   ares__buf_reclaim(NULL);
-  EXPECT_NE(ARES_SUCCESS, ares__buf_append(NULL, NULL, 0));
+  EXPECT_NE(ARES_SUCCESS, ares__buf_append(NULL, NULL, 55));
   size_t len = 10;
   EXPECT_EQ(NULL, ares__buf_append_start(NULL, &len));
   EXPECT_EQ(NULL, ares__buf_append_start(NULL, NULL));

test/ares-test-mock.cc

@@ -817,6 +817,28 @@ TEST_P(MockChannelTest, SearchDomains) {
   EXPECT_EQ("{'www.third.gov' aliases=[] addrs=[2.3.4.5]}", ss.str());
 }
 
+// Issue #858
+TEST_P(CacheQueriesTest, BlankName) {
+  DNSPacket rsp;
+  rsp.set_response().set_aa()
+    .add_question(new DNSQuestion(".", T_SOA))
+    .add_answer(new DNSSoaRR(".", 600, "a.root-servers.net", "nstld.verisign-grs.com", 123456, 3600, 3600, 3600, 3600));
+  EXPECT_CALL(server_, OnRequest("", T_SOA))
+    .WillOnce(SetReply(&server_, &rsp));
+
+  QueryResult result;
+  ares_query_dnsrec(channel_, ".", ARES_CLASS_IN, ARES_REC_TYPE_SOA, QueryCallback, &result, NULL);
+  Process();
+  EXPECT_TRUE(result.done_);
+  EXPECT_EQ(0, result.timeouts_);
+
+  QueryResult cacheresult;
+  ares_query_dnsrec(channel_, ".", ARES_CLASS_IN, ARES_REC_TYPE_SOA, QueryCallback, &cacheresult, NULL);
+  Process();
+  EXPECT_TRUE(cacheresult.done_);
+  EXPECT_EQ(0, cacheresult.timeouts_);
+}
+
 TEST_P(CacheQueriesTest, SearchDomainsCache) {
   DNSPacket nofirst;
   nofirst.set_response().set_aa().set_rcode(NXDOMAIN)