chore: increase min-capacity-sat to 10k

The old value of 1000 sat was too small to cover the dust reserves.
This lead to the situation when trying to open a channel with minimal
amount, the channels got refused because they were not able cover the
commitment fees.

For this reason the minimal capacity should be increased to i.e. 10k
satoshi, as the technical minimum that also accounts for fees and
reserves is somewhere around 6k sat.

lightningd/options.c

@@ -548,8 +548,8 @@ static const struct config testnet_config = {
 
 	.use_dns = true,
 
-	/* Sets min_effective_htlc_capacity - at 1000$/BTC this is 1ct */
-	.min_capacity_sat = 1000,
+	/* Sets min_effective_htlc_capacity - at 1000$/BTC this is 10ct */
+	.min_capacity_sat = 10000,
 };
 
 /* aka. "Dude, where's my coins?" */
@@ -614,8 +614,8 @@ static const struct config mainnet_config = {
 
 	.use_dns = true,
 
-	/* Sets min_effective_htlc_capacity - at 1000$/BTC this is 1ct */
-	.min_capacity_sat = 1000,
+	/* Sets min_effective_htlc_capacity - at 1000$/BTC this is 10ct */
+	.min_capacity_sat = 10000,
 };
 
 static void check_config(struct lightningd *ld)

tests/test_connection.py

@@ -131,11 +131,13 @@ def test_bad_opening(node_factory):
 def test_opening_tiny_channel(node_factory):
     # Test custom min-capacity-sat parameters
     #
-    #       ----> [l2] (1000) - default
+    #       o---> [l2] (1000)   - old default (too little for reserves)
     #      /
-    #  [l1] ----> [l3] (3000) - less than own initial commit tx
+    #  [l1]-----> [l3] (~6000)  - technical minimal value that wont be rejected
     #      \
-    #       ----> [l4] (~6000) - enough to cover own initial commit tx
+    #       o---> [l4] (~10000) - the current default
+    #        \
+    #         o-> [l5] (20000)  - a node with a higher minimal value
     #
     # For each:
     #  1. Try to establish channel 1sat smaller than min_capacity_sat
@@ -151,42 +153,50 @@ def test_opening_tiny_channel(node_factory):
     min_commit_tx_fees = 5430
     min_for_funder = min_commit_tx_fees + dustlimit + 1
 
-    l2_min_capacity = 1000
-    l3_min_capacity = 3000
-    l4_min_capacity = min_for_funder
+    l2_min_capacity = 1000  # the old default of 1k sats
+    l3_min_capacity = min_for_funder  # the absolute technical minimum
+    l4_min_capacity = 10000  # the current default
+    l5_min_capacity = 20000  # a server with more than default minimum
 
-    l1 = node_factory.get_node()
-    l2 = node_factory.get_node()
+    # Outgoing node must have smallest min value, so inbound side of test channels wont be rejected
+    l1 = node_factory.get_node(options={'min-capacity-sat': 1000})
+    l2 = node_factory.get_node(options={'min-capacity-sat': l2_min_capacity})
     l3 = node_factory.get_node(options={'min-capacity-sat': l3_min_capacity})
     l4 = node_factory.get_node(options={'min-capacity-sat': l4_min_capacity})
+    l5 = node_factory.get_node(options={'min-capacity-sat': l5_min_capacity})
 
     l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
     l1.rpc.connect(l3.info['id'], 'localhost', l3.port)
     l1.rpc.connect(l4.info['id'], 'localhost', l4.port)
+    l1.rpc.connect(l5.info['id'], 'localhost', l5.port)
 
-    # Open channel with less than default 1000 sats should be rejected
-    with pytest.raises(RpcError, match=r'channel capacity is 999sat, which is below 1000000msat'):
+    # Open channel with one less than 1000 sats should be rejected at l2
+    with pytest.raises(RpcError, match=r'channel capacity is .*sat, which is below .*sat'):
         l1.fund_channel(l2, l2_min_capacity + reserves - 1)
-    # Open a channel with exactly the minimal amount for the fundee.
+    # Open a channel with exactly the minimal amount for the fundee,
     # This will raise an exception at l1, as the funder cannot afford fees for initial_commit_tx.
+    # Note: The old default of 1k sat is below the technical minimum when accounting for dust reserves
+    # This is why this must fail, for this reason the default will be raised to 10k sat.
     with pytest.raises(RpcError, match=r'Funder cannot afford fee on initial commitment transaction'):
         l1.fund_channel(l2, l2_min_capacity + reserves)
 
-    # Open channel with less than custom 3000 sats should be rejected at l3
-    with pytest.raises(RpcError, match=r'channel capacity is 2999sat, which is below 3000000msat'):
+    # Open channel with one less than technical minimum should be rejected at l3
+    with pytest.raises(RpcError, match=r'channel capacity is .*sat, which is below .*sat'):
         l1.fund_channel(l3, l3_min_capacity + reserves - 1)
-    with pytest.raises(RpcError, match=r'Funder cannot afford fee on initial commitment transaction'):
-        l1.fund_channel(l3, l3_min_capacity + reserves)
+    # When amount technical minimum matches exactly, own initial_commit_tx fees can now be covered
+    l1.fund_channel(l3, l3_min_capacity + reserves)
 
-    # Open channel with less than custom ~6000 sats should be rejected at l4
+    # Open channel with one less than default 10k sats should be rejected at l4
     with pytest.raises(RpcError, match=r'channel capacity is .*, which is below .*msat'):
         l1.fund_channel(l4, l4_min_capacity + reserves - 1)
-    # When amount exactly matches, own initial_commit_tx fees can now be covered
+    # This must be possible with enough capacity
     l1.fund_channel(l4, l4_min_capacity + reserves)
 
-    # Also fund channels with minimal funder amount that should not be rejected by own daemon
-    l1.fund_channel(l2, min_for_funder)
-    l1.fund_channel(l3, min_for_funder)
+    # Open channel with less than minimum should be rejected at l5
+    with pytest.raises(RpcError, match=r'channel capacity is .*, which is below .*msat'):
+        l1.fund_channel(l5, l5_min_capacity + reserves - 1)
+    # bigger channels must not be affected
+    l1.fund_channel(l5, l5_min_capacity * 10)
 
 
 def test_second_channel(node_factory):

wallet/test/test_utils.c

@@ -22,5 +22,5 @@ const struct config test_config = {
 	.rescan = 30,
 	.max_fee_multiplier = 10,
 	.use_dns = true,
-	.min_capacity_sat = 1000,
+	.min_capacity_sat = 10000,
 };