Remember .btf peers that answered, and dial them first on start (#25)

* net: remember .btf peers that answered, and dial them first on start

Discovery depends entirely on the Nostr relays. A node that has been up
for hours knows exactly which peers respond, and throws all of it away on
exit; the next start reaches for the relays again and walks the same
descriptor list, of which roughly 96% is dead. That is what makes a
restart take minutes before the first tunnel opens -- measured today at
about seven.

Every peer that completes a tunnel is now written to btfpeers.json with
its meeting node and encryption key, which is everything needed to redial
it. On start a thread tries those before the relays have answered
anything, stopping once eight are connected. Entries older than a week
are dropped, and the file holds at most fifty.

Cache only: nothing here changes how a first-ever run bootstraps, and a
missing or corrupt file just falls through to relay discovery.

* net: exchange signed .btf descriptors between peers

Discovery has depended entirely on the Nostr relays. Two connected nodes never
told each other who else existed, so a relay outage left a running network
unable to grow, and a node that found one stale peer had no way to learn there
was anything better. The old `addr` gossip went out with the IRC seed.

Peers now trade signed descriptors over a new "btfpeers" message: our own
first, then peers that actually answered us. The receiver checks each Schnorr
signature against the key the `.btf` address decodes to, so the sender is
trusted for nothing -- forging an entry needs somebody else's secret key. That
leaves flooding with self-generated keys, which the per-message caps and the
"peers that answered" ordering are there to blunt.

Unknown commands are ignored (main.cpp), so nodes without this just drop the
message; nothing here is a protocol break.

The cache from the peer-cache change now keeps each peer's descriptor verbatim
alongside the parsed fields, because that signature is what makes a peer
relayable. Entries without one -- resolved through Nostr, or written before
this -- stay dialable, just not passed on. A dial never clears a descriptor
already on file.

* main_gui: implement /port, which net.cpp already documented

net.cpp has said nListenPort is "tunable via /port" since it was written, but
nothing ever parsed the option. Two nodes could not share a machine, which is
exactly what testing peer exchange needs. Sets addrLocalHost.port too, so we
do not advertise a port we never bound.

1 files changed, 30 insertions, 0 deletions

diff --git a/src/nostr.cpp b/src/nostr.cpp
index ec02c2e..482a835 100644
--- a/src/nostr.cpp
+++ b/src/nostr.cpp
@@ -342,6 +342,36 @@ std::string BtfLocalAddress()
     return g_nostrKey.BtfAddress();
 }
 
+// This node's own descriptor, signed and self-certifying -- the same blob
+// PublishDescriptor puts on Nostr, but handed straight to a peer instead.
+//
+// Reads strBtfActiveRelay directly rather than going through BtfActiveRelay():
+// only a rendezvous we actually registered at is worth announcing. Handing out
+// a meeting node we are not reachable at would make every peer that receives
+// it burn a dial, and over peer exchange that mistake travels further than it
+// does over Nostr.
+std::string BtfLocalDescriptor()
+{
+    if (!EnsureNostrKey())
+        return "";
+    string meeting;
+    CRITICAL_BLOCK(cs_activeRelay)
+        meeting = strBtfActiveRelay;
+    if (meeting.empty())
+        return "";
+    return btf::SignDescriptor(g_nostrKey.ctx, g_nostrKey.seckey,
+                               g_nostrKey.EncPubHex(), meeting, (uint64_t)GetTime());
+}
+
+// The shared secp256k1 context, so net.cpp can verify descriptors that arrive
+// from peers. NULL if the identity could not be loaded.
+void* BtfSecpContext()
+{
+    if (!EnsureNostrKey())
+        return NULL;
+    return (void*)g_nostrKey.ctx;
+}
+
 int GetDiscoveredPeerCount()
 {
     int n = 0;