Enforce maximum name length for parse_name() and add unit tests

Summary:
Adds unit tests for the maximum length of a query name for the seeder's
`parse_name()` function.

Currently, parse_name() has a logical flaw that prevents testing above the
maximum query name length.  The length of a query name is the sum of its labels
plus 1 for each `.`.  The maximum length of a query name is 255 characters. This
is not strictly enforced in parse_name() and returns a misleading error when it
occurs. This patch fixes this and adds a unit test for it.

Depends on D5328

Test Plan:
  make check
  ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D5329

src/seeder/dns.cpp

@@ -78,6 +78,11 @@ int parse_name(const uint8_t **inpos, const uint8_t *inend,
         }
         // add dot in output
         if (!init) {
+            // The maximum size of a query name is 255. The buffer must have
+            // room for at least the '.' and a valid non-'.' character
+            if (bufused > 253) {
+                return -1;
+            }
             if (bufused == bufsize - 1) {
                 return -2;
             }
@@ -103,7 +108,7 @@ int parse_name(const uint8_t **inpos, const uint8_t *inend,
         }
         // copy label
         while (octet) {
-            if (*inpos == inend) {
+            if (*inpos == inend || bufused > 254) {
                 return -1;
             }
             if (bufused == bufsize - 1) {

src/seeder/test/dns_tests.cpp

@@ -119,4 +119,19 @@ BOOST_AUTO_TEST_CASE(parse_name_label_tests) {
     CheckParseNameError("www." + maxLengthLabel + "a.com", -1);
 }
 
+BOOST_AUTO_TEST_CASE(parse_name_qname_length_tests) {
+    const std::string maxLengthLabel(MAX_LABEL_LENGTH, 'a');
+
+    // Check behavior for a name that is the maximum length
+    std::string maxLengthQName = maxLengthLabel + '.' + maxLengthLabel + '.' +
+                                 maxLengthLabel + '.' + maxLengthLabel;
+    BOOST_CHECK_EQUAL(maxLengthQName.size(), MAX_QUERY_NAME_LENGTH);
+    CheckParseName(maxLengthQName);
+
+    // Check that a query name that is too long causes an error
+    // Allocates an extra large buffer to guarantee an error is not caused by
+    // the buffer size
+    CheckParseNameError(maxLengthQName + "a", -1, 2 * maxLengthQName.size());
+}
+
 BOOST_AUTO_TEST_SUITE_END()