gossmap: make API more robust against future changes.

Many changes to gossmap (including the pending ones!) don't actually
concern readers, as long as they obey certain rules:

1. Ignore unknown messages.
2. Treat all 16 upper bits of length as flags, ignore unknown ones.

So now we split the version byte into MAJOR and MINOR, and you can
ignore MINOR changes.

We don't expose the internal version (for creating the map)
programmatically: you should really hardcode what major version you
understand!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-client/pyln/client/gossmap.py

@@ -9,13 +9,12 @@ import io
 import struct
 
 # These duplicate constants in lightning/common/gossip_store.h
-GOSSIP_STORE_VERSIONS = [0x09, 0x0a]
+GOSSIP_STORE_MAJOR_VERSION = (0 << 5)
+GOSSIP_STORE_MAJOR_VERSION_MASK = 0xE0
 GOSSIP_STORE_LEN_DELETED_BIT = 0x80000000
 GOSSIP_STORE_LEN_PUSH_BIT = 0x40000000
 GOSSIP_STORE_LEN_RATELIMIT_BIT = 0x20000000
-GOSSIP_STORE_LEN_MASK = (~(GOSSIP_STORE_LEN_PUSH_BIT
-                           | GOSSIP_STORE_LEN_DELETED_BIT
-                           | GOSSIP_STORE_LEN_RATELIMIT_BIT))
+GOSSIP_STORE_LEN_MASK = (0x0000FFFF)
 
 # These duplicate constants in lightning/gossipd/gossip_store_wiregen.h
 WIRE_GOSSIP_STORE_PRIVATE_CHANNEL = 4104
@@ -174,7 +173,7 @@ class Gossmap(object):
         self.channels: Dict[ShortChannelId, GossmapChannel] = {}
         self._last_scid: Optional[str] = None
         version = self.store_file.read(1)[0]
-        if version not in GOSSIP_STORE_VERSIONS:
+        if (version & GOSSIP_STORE_MAJOR_VERSION_MASK) != GOSSIP_STORE_MAJOR_VERSION:
             raise ValueError("Invalid gossip store version {}".format(version))
         self.bytes_read = 1
         self.refresh()