gh-156680: Raise the documented error from IPv6Network.next_network() (#156681)

* gh-156680: Raise the documented error from IPv6Network.next_network()

next_network() guarded address-space exhaustion with except
OverflowError, which only int.to_bytes() on the IPv4 path raises.
_BaseV6._string_from_ip_int() raises ValueError instead, so the
handler never ran for IPv6 and the internal 'IPv6 address is too
large' message escaped.

Build the result from an (address, prefix) tuple rather than formatting
and reparsing a string.

Author: fedonman

Doc/whatsnew/3.16.rst

@@ -412,9 +412,9 @@ io
 ipaddress
 ---------
 
-* Add :meth:`~ipaddress.IPv4Network.next_network` and
-  :meth:`~ipaddress.IPv6Network.next_network` methods to find the next nearest
-  network with a specific prefix size.
+* Add :meth:`IPv4Network.next_network() <ipaddress.IPv4Network.next_network>`
+  and :meth:`IPv6Network.next_network() <ipaddress.IPv6Network.next_network>`
+  methods to find the next nearest network with a specific prefix size.
   (Contributed by Faisal Mahmood in :gh:`87027`.)
 
 

Lib/ipaddress.py

@@ -1124,11 +1124,15 @@ def next_network(self, next_prefix=None):
 
         Args:
             next_prefix: The desired next prefix length, if not specified the
-            same self.prefixlen will be used
+            same self.prefixlen will be used.
 
         Returns:
             An IPv(4|6) Network object of the next closest network.
 
+        Raises:
+            ValueError: If next_prefix is outside the range of valid prefix
+            lengths, or if no further network of that size exists.
+
         """
         if next_prefix is None:
             next_prefix = self.prefixlen
@@ -1150,15 +1154,13 @@ def next_network(self, next_prefix=None):
             ((new_netmask._ip & self.network_address._ip) >> bit_shift) + 1
         ) << bit_shift
 
-        try:
-            return self.__class__(
-                f"{self._string_from_ip_int(next_ip)}/{next_prefix}"
-            )
-        except OverflowError:
+        if next_ip > self._ALL_ONES:
             raise ValueError(
                 f"out of address space, cannot make another /{next_prefix} "
                 "network"
-            ) from None
+            )
+
+        return self.__class__((next_ip, next_prefix))
 
 
 class _BaseConstants:

Lib/test/test_ipaddress.py

@@ -1596,9 +1596,15 @@ def testNextNetworkWithBadPrefix(self):
 
     def testNextNetworkOutOfAddressSpace(self):
         ipv4 = ipaddress.IPv4Network('255.255.255.0/24')
-        self.assertRaises(ValueError, ipv4.next_network)
+        self.assertRaisesRegex(
+            ValueError,
+            'out of address space, cannot make another /24 network',
+            ipv4.next_network)
         ipv6 = ipaddress.IPv6Network('ffff:ffff:ffff:ffff:ffff:ffff:ffff:0/112')
-        self.assertRaises(ValueError, ipv6.next_network)
+        self.assertRaisesRegex(
+            ValueError,
+            'out of address space, cannot make another /112 network',
+            ipv6.next_network)
 
     def testFancySubnetting(self):
         self.assertEqual(sorted(self.ipv4_network.subnets(prefixlen_diff=3)),

Misc/NEWS.d/next/Library/2021-01-09-18-40-15.bpo-42861.T7Ge9O.rst

@@ -1,2 +1,3 @@
-Add :meth:`~ipaddress.IPv4Network.next_network` and
-:meth:`~ipaddress.IPv6Network.next_network`.  Patch by Faisal Mahmood.
+Add :meth:`IPv4Network.next_network() <ipaddress.IPv4Network.next_network>`
+and :meth:`IPv6Network.next_network() <ipaddress.IPv6Network.next_network>`.
+Patch by Faisal Mahmood.