addrman: ensure old versions don't parse peers.dat

Even though the format of `peers.dat` was changed in an incompatible
way (old software versions <0.21 cannot understand the new file format),
it is not guaranteed that old versions will fail to parse it. There is a
chance that old versions parse its contents as garbage and use it.

Old versions expect the "key size" field to be 32 and fail the parsing
if it is not. Thus, we put something other than 32 in it. This will make
versions between 0.11.0 and 0.20.1 deterministically fail on the new
format. Versions prior to https://github.com/bitcoin/bitcoin/pull/5941
(<0.11.0) will still parse it as garbage.

Also, introduce a way to increment the `peers.dat` format in a way that
does not necessary make older versions refuse to read it.

1 files changed, 2 insertions, 2 deletions

diff --git a/doc/release-notes.md b/doc/release-notes.md
index d3983b1689..0331328ff5 100644
--- a/doc/release-notes.md
+++ b/doc/release-notes.md
@@ -65,8 +65,8 @@ format of this file has been changed in a backwards-incompatible way in order to
 accommodate the storage of Tor v3 and other BIP155 addresses. This means that if
 the file is modified by 0.21.0 or newer then older versions will not be able to
 read it. Those old versions, in the event of a downgrade, will log an error
-message that deserialization has failed and will continue normal operation
-as if the file was missing, creating a new empty one. (#19954)
+message "Incorrect keysize in addrman deserialization" and will continue normal
+operation as if the file was missing, creating a new empty one. (#19954)
 
 Notable changes
 ===============