chainfee: allow specifying min relay feerate from API source

This commit adds a new expected field, `min_relay_feerate`, in the
response body returned from the API source, allowing the API to specify
a min relay feerate to be used instead of the FeePerKwFloor.

This change is backwards compatible as for an old API source which
doesn't specify the `min_relay_feerate`, it will be interpreted as zero.

lnwallet/chainfee/estimator_test.go

@@ -107,17 +107,20 @@ func TestSparseConfFeeSource(t *testing.T) {
 		2: 42,
 		3: 54321,
 	}
-	testJSON := map[string]map[uint32]uint32{
-		"fee_by_block_target": testFees,
+	testMinRelayFee := SatPerKVByte(1000)
+	testResp := WebAPIResponse{
+		MinRelayFeerate:  testMinRelayFee,
+		FeeByBlockTarget: testFees,
 	}
-	jsonResp, err := json.Marshal(testJSON)
+
+	jsonResp, err := json.Marshal(testResp)
 	require.NoError(t, err, "unable to marshal JSON API response")
 	reader := bytes.NewReader(jsonResp)
 
 	// Finally, ensure the expected map is returned without error.
-	fees, err := feeSource.parseResponse(reader)
+	resp, err := feeSource.parseResponse(reader)
 	require.NoError(t, err, "unable to parse API response")
-	require.Equal(t, testFees, fees, "unexpected fee map returned")
+	require.Equal(t, testResp, resp, "unexpected resp returned")
 
 	// Test parsing an improperly formatted JSON API response.
 	badFees := map[string]uint32{"hi": 12345, "hello": 42, "satoshi": 54321}
@@ -131,6 +134,45 @@ func TestSparseConfFeeSource(t *testing.T) {
 	require.Error(t, err, "expected error when parsing bad JSON")
 }
 
+// TestFeeSourceCompatibility checks that when a fee source doesn't return a
+// `min_relay_feerate` field in its response, the floor feerate is used.
+//
+// NOTE: Field `min_relay_feerate` was added in v0.18.3.
+func TestFeeSourceCompatibility(t *testing.T) {
+	t.Parallel()
+
+	// Test that GenQueryURL returns the URL as is.
+	url := "test"
+	feeSource := SparseConfFeeSource{URL: url}
+
+	// Test parsing a properly formatted JSON API response.
+	//
+	// Create the resp without the `min_relay_feerate` field.
+	testFees := map[uint32]uint32{
+		1: 12345,
+	}
+	testResp := struct {
+		// FeeByBlockTarget is a map of confirmation targets to sat/kvb
+		// fees.
+		FeeByBlockTarget map[uint32]uint32 `json:"fee_by_block_target"`
+	}{
+		FeeByBlockTarget: testFees,
+	}
+
+	jsonResp, err := json.Marshal(testResp)
+	require.NoError(t, err, "unable to marshal JSON API response")
+	reader := bytes.NewReader(jsonResp)
+
+	// Ensure the expected map is returned without error.
+	resp, err := feeSource.parseResponse(reader)
+	require.NoError(t, err, "unable to parse API response")
+	require.Equal(t, testResp.FeeByBlockTarget, resp.FeeByBlockTarget,
+		"unexpected resp returned")
+
+	// Expect the floor feerate to be used.
+	require.Equal(t, FeePerKwFloor.FeePerKVByte(), resp.MinRelayFeerate)
+}
+
 // TestWebAPIFeeEstimator checks that the WebAPIFeeEstimator returns fee rates
 // as expected.
 func TestWebAPIFeeEstimator(t *testing.T) {
@@ -194,14 +236,17 @@ func TestWebAPIFeeEstimator(t *testing.T) {
 	// This will create a `feeByBlockTarget` map with the following values,
 	// - 2: 4000 sat/kb
 	// - 6: 2000 sat/kb.
-	feeRateResp := map[uint32]uint32{
+	feeRates := map[uint32]uint32{
 		minTarget: maxFeeRate,
 		maxTarget: minFeeRate,
 	}
+	resp := WebAPIResponse{
+		FeeByBlockTarget: feeRates,
+	}
 
 	// Create a mock fee source and mock its returned map.
 	feeSource := &mockFeeSource{}
-	feeSource.On("GetFeeMap").Return(feeRateResp, nil)
+	feeSource.On("GetFeeInfo").Return(resp, nil)
 
 	estimator, _ := NewWebAPIEstimator(
 		feeSource, false, minFeeUpdateTimeout, maxFeeUpdateTimeout,
@@ -234,7 +279,7 @@ func TestWebAPIFeeEstimator(t *testing.T) {
 
 			exp := SatPerKVByte(tc.expectedFeeRate).FeePerKWeight()
 			require.Equalf(t, exp, est, "target %v failed, fee "+
-				"map is %v", tc.target, feeRateResp)
+				"map is %v", tc.target, feeRate)
 		})
 	}