Fix AXFR-style IXFR with multiple messages. (#1151)

The inbound xfr code is conflating the expected rdtype in responses with
the incremental/replacement response style.  This causes a problem when
an AXFR-style IXFR response spans multiple messages, as resetting the
style to AXFR (replacement) also changed the expected type in the
question section of future responses to AXFR.

This change separates out the style from the expected rdtype.

dns/xfr.py

@@ -83,8 +83,13 @@ def __init__(
         if rdtype == dns.rdatatype.IXFR:
             if serial is None:
                 raise ValueError("a starting serial must be supplied for IXFRs")
-        elif is_udp:
-            raise ValueError("is_udp specified for AXFR")
+            self.incremental = True
+        elif rdtype == dns.rdatatype.AXFR:
+            if is_udp:
+                raise ValueError("is_udp specified for AXFR")
+            self.incremental = False
+        else:
+            raise ValueError("rdtype is not IXFR or AXFR")
         self.serial = serial
         self.is_udp = is_udp
         (_, _, self.origin) = txn_manager.origin_information()
@@ -103,8 +108,7 @@ def process_message(self, message: dns.message.Message) -> bool:
         Returns `True` if the transfer is complete, and `False` otherwise.
         """
         if self.txn is None:
-            replacement = self.rdtype == dns.rdatatype.AXFR
-            self.txn = self.txn_manager.writer(replacement)
+            self.txn = self.txn_manager.writer(not self.incremental)
         rcode = message.rcode()
         if rcode != dns.rcode.NOERROR:
             raise TransferError(rcode)
@@ -131,7 +135,7 @@ def process_message(self, message: dns.message.Message) -> bool:
                 raise dns.exception.FormError("first RRset is not an SOA")
             answer_index = 1
             self.soa_rdataset = rdataset.copy()  # pyright: ignore
-            if self.rdtype == dns.rdatatype.IXFR:
+            if self.incremental:
                 assert self.soa_rdataset is not None
                 soa = cast(dns.rdtypes.ANY.SOA.SOA, self.soa_rdataset[0])
                 if soa.serial == self.serial:
@@ -168,7 +172,7 @@ def process_message(self, message: dns.message.Message) -> bool:
                 #
                 # Every time we see an origin SOA delete_mode inverts
                 #
-                if self.rdtype == dns.rdatatype.IXFR:
+                if self.incremental:
                     self.delete_mode = not self.delete_mode
                 #
                 # If this SOA Rdataset is equal to the first we saw
@@ -177,8 +181,7 @@ def process_message(self, message: dns.message.Message) -> bool:
                 # part of the response.
                 #
                 if rdataset == self.soa_rdataset and (
-                    self.rdtype == dns.rdatatype.AXFR
-                    or (self.rdtype == dns.rdatatype.IXFR and self.delete_mode)
+                    (not self.incremental) or self.delete_mode
                 ):
                     #
                     # This is the final SOA
@@ -187,7 +190,7 @@ def process_message(self, message: dns.message.Message) -> bool:
                     if self.expecting_SOA:
                         # We got an empty IXFR sequence!
                         raise dns.exception.FormError("empty IXFR sequence")
-                    if self.rdtype == dns.rdatatype.IXFR and self.serial != soa.serial:
+                    if self.incremental and self.serial != soa.serial:
                         raise dns.exception.FormError("unexpected end of IXFR sequence")
                     self.txn.replace(name, rdataset)
                     self.txn.commit()
@@ -199,7 +202,7 @@ def process_message(self, message: dns.message.Message) -> bool:
                     #
                     self.expecting_SOA = False
                     soa = cast(dns.rdtypes.ANY.SOA.SOA, rdataset[0])
-                    if self.rdtype == dns.rdatatype.IXFR:
+                    if self.incremental:
                         if self.delete_mode:
                             # This is the start of an IXFR deletion set
                             if soa.serial != self.serial:
@@ -220,7 +223,7 @@ def process_message(self, message: dns.message.Message) -> bool:
                 # SOA RR, but saw something else, so this must be an
                 # AXFR response.
                 #
-                self.rdtype = dns.rdatatype.AXFR
+                self.incremental = False
                 self.expecting_SOA = False
                 self.delete_mode = False
                 self.txn.rollback()

tests/test_xfr.py

@@ -263,6 +263,30 @@ class Server(object):
 @ 3600 IN SOA foo bar 1 2 3 4 5
 """
 
+ixfr_axfr1 = """id 1
+opcode QUERY
+rcode NOERROR
+flags AA
+;QUESTION
+example. IN IXFR
+;ANSWER
+@ 3600 IN SOA foo bar 1 2 3 4 5
+@ 3600 IN NS ns1
+@ 3600 IN NS ns2
+"""
+ixfr_axfr2 = """id 1
+opcode QUERY
+rcode NOERROR
+flags AA
+;QUESTION
+example. IN IXFR
+;ANSWER
+bar.foo 300 IN MX 0 blaz.foo
+ns1 3600 IN A 10.0.0.1
+ns2 3600 IN A 10.0.0.2
+@ 3600 IN SOA foo bar 1 2 3 4 5
+"""
+
 
 def test_basic_axfr():
     z = dns.versioned.Zone("example.")
@@ -394,6 +418,19 @@ def test_ixfr_is_axfr():
     assert z == ez
 
 
+def test_ixfr_is_axfr_two_parts():
+    z = dns.versioned.Zone("example.")
+    m1 = dns.message.from_text(ixfr_axfr1, origin=z.origin, one_rr_per_rrset=True)
+    m2 = dns.message.from_text(ixfr_axfr2, origin=z.origin, one_rr_per_rrset=True)
+    with dns.xfr.Inbound(z, dns.rdatatype.IXFR, serial=0xFFFFFFFF) as xfr:
+        done = xfr.process_message(m1)
+        assert not done
+        done = xfr.process_message(m2)
+        assert done
+    ez = dns.zone.from_text(base, "example.")
+    assert z == ez
+
+
 def test_ixfr_requires_serial():
     z = dns.zone.from_text(base, "example.", zone_factory=dns.versioned.Zone)
     with pytest.raises(ValueError):