doc/GOSSIP_STORE.md: document the gossip_store file format.

Mainly for @vincenzopalazzo who is thinking of writing a Rust version!

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

doc/GOSSIP_STORE.md

@@ -0,0 +1,146 @@
+# gossip_store: Direct Access To Lightning Gossip
+
+Hi!
+
+The lightning_gossipd dameon stores the gossip messages, along with
+some internal data, in a file called the "gossip_store".  Various
+plugins and daemons access this (in a read-only manner), and the
+format is documented here.
+
+## The File Header
+
+```
+u8 version;
+```
+
+The gossmap header consists of one byte.  The top 3 bits are the major
+version: if these are not all zero, you need to re-read this (updated)
+document to see what changed.  The lower 5 bits are the minor version,
+which won't worry you: currently they will be 11.
+
+After the file header comes a number of records.
+
+## The Record Header
+
+```
+be16 flags;
+be16 len;
+be32 crc;
+be32 timestamp;
+```
+
+Each record consists of a header and a message.  The header is
+big-endian, containing flags, the length (of the following body), the
+crc32c (of the following message, starting with the timestamp field in
+the header) and a timestamp extracted from certain messages (zero
+where not relevant, but ignore it in those cases).
+
+The flags currently defined are:
+
+```
+#define DELETED		 0x8000
+#define PUSH		 0x4000
+#define RATELIMIT	 0x2000
+```
+
+Deleted fields should be ignored: on restart, they will be removed as
+the gossip_store is rewritten.
+
+The push flag indicates gossip which is generated locally: this is
+important for gossip timestamp filtering, where peers request gossip
+and we always send our own gossip messages even if the timestamp
+wasn't within their request.
+
+The ratelimit flag indicates that this gossip message came too fast:
+we record it, but don't relay it to peers.
+
+Other flags should be ignored.
+
+## The Message
+
+Each messages consists of a 16-bit big-endian "type" field (for
+efficiency, an implementation may read this along with the header),
+and optional data.  Some messages are defined by the BOLT 7 gossip
+protocol, others are for internal use.  Unknown message types should be
+skipped over.
+
+### BOLT 7 Messages
+
+These are the messages which gossipd has validated, and ensured are in
+order.
+
+* `channel_announcement` (256): a complete, validated channel announcement.  This will always come before any `channel_update` which refers to it, or `node_announcement` which refers to a node.
+* `channel_update` (258): a complete, validated channel update.  Note that you can see multiple of these (old ones will be deleted as they are replaced though).
+* `node_announcement` (257): a complete, validated node announcement.  Note that you can also see multiple of these (old ones will be deleted as they are replaced).
+
+### Internal Gossip Daemon Messages
+
+These messages contain additional data, which may be useful.
+
+* `gossip_store_channel_amount` (4101)
+  * `satoshis`: u64
+
+This always immediately follows `channel_announcement` messages, and
+contains the actual capacity of the channel.
+
+* `gossip_store_private_channel` (4104)
+  * `amount_sat`: u64
+  * `len`: u16
+  * `announcement`: u8[len]
+
+This contains information about a private (could be made public
+later!) channel, with announcement in the same format as a normal
+`channel_announcement` with invalid signatures.
+
+* `gossip_store_private_update` (4102)
+  * `len`: u16
+  * `update`: u8[len]
+
+This contains a private `channel_update` (i.e. for a channel described
+by `gossip_store_private_channel`.
+
+* `gossip_store_delete_chan` (4103)
+  * `scid`: u64
+
+This is added when a channel is deleted.  You won't often see this if
+you're reading the file once (as the channel record header will have
+been marked `deleted` first), but useful if you are polling the file
+for updates.
+
+* `gossip_store_ended` (4105)
+  * `equivalent_offset`: u64
+
+This is only ever added as the final entry in the gossip_store.  It
+means the file has been deleted (usually because lightningd has been
+restarted), and you should re-open it.  As an optimization, the
+`equivalent_offset` in the new file reflects the point at which the
+new gossip_store is equivalent to this one (with deleted records
+removed).  However, if lightningd has been restarted multiple times it
+is possible that this offset is not valid, so it's really only useful
+if you're actively monitoring the file.
+
+* `gossip_store_chan_dying` (4106)
+  * `scid`: u64
+  * `blockheight`: u32
+
+This is placed in the gossip_store file when a funding transaction is
+spent.  `blockheight` is set to 12 blocks beyond the block containing
+the spend: at this point, gossipd will delete the channel.
+
+## Using the Gossip Store File
+
+- Always check the major version number!  We will increment it if the format
+  changes in a way that breaks readers.
+- Ignore unknown flags in the header.
+- Ignore message types you don't know.
+- You don't need to check the messages, as they have been validated.
+- It is possible to see a partially-written record at the end.  Ignore it.
+
+If you are keeping the file open to watch for changes:
+
+- The file is append-only, so you can simply try reading more records 
+  using inotify (or equivalent) or simply checking every few seconds.
+- If you see a `gossip_store_ended` message, reopen the file.
+
+Happy hacking!
+Rusty.