lnwallet: add implementation of WebApiFeeSource for external APIs

This enables users to specify an external API for fee estimation.
The API is expected to return fees in the JSON format:
`{
	fee_by_block_target: {
	   a: x,
	   b: y,
	   ...
	   c: z
        }
 }`
 where a, b, c are block targets and x, y, z are fees in sat/kb.
 Note that a, b, c need not be contiguous.

lnwallet/fee_estimator.go

@@ -2,6 +2,7 @@ package lnwallet
 
 import (
 	"encoding/json"
+	"io"
 
 	"github.com/btcsuite/btcd/blockchain"
 	"github.com/btcsuite/btcd/rpcclient"
@@ -474,3 +475,46 @@ type WebAPIFeeSource interface {
 	// specifics are left to the WebAPIFeeSource implementation.
 	ParseResponse(r io.Reader) (map[uint32]uint32, error)
 }
+
+// SparseConfFeeSource is an implementation of the WebAPIFeeSource that utilizes
+// a user-specified fee estimation API for Bitcoin. It expects the response
+// to be in the JSON format: `fee_by_block_target: { ... }` where the value maps
+// block targets to fee estimates (in sat per kilovbyte).
+type SparseConfFeeSource struct {
+	// URL is the fee estimation API specified by the user.
+	URL string
+}
+
+// GenQueryURL generates the full query URL. The value returned by this
+// method should be able to be used directly as a path for an HTTP GET
+// request.
+//
+// NOTE: Part of the WebAPIFeeSource interface.
+func (s SparseConfFeeSource) GenQueryURL() string {
+	return s.URL
+}
+
+// ParseResponse attempts to parse the body of the response generated by the
+// above query URL. Typically this will be JSON, but the specifics are left to
+// the WebAPIFeeSource implementation.
+//
+// NOTE: Part of the WebAPIFeeSource interface.
+func (s SparseConfFeeSource) ParseResponse(r io.Reader) (map[uint32]uint32, error) {
+	type jsonResp struct {
+		FeeByBlockTarget map[uint32]uint32 `json:"fee_by_block_target"`
+	}
+
+	resp := jsonResp{
+		FeeByBlockTarget: make(map[uint32]uint32),
+	}
+	jsonReader := json.NewDecoder(r)
+	if err := jsonReader.Decode(&resp); err != nil {
+		return nil, err
+	}
+
+	return resp.FeeByBlockTarget, nil
+}
+
+// A compile-time assertion to ensure that SparseConfFeeSource implements the
+// WebAPIFeeSource interface.
+var _ WebAPIFeeSource = (*SparseConfFeeSource)(nil)

lnwallet/fee_estimator_test.go

@@ -1,6 +1,9 @@
 package lnwallet_test
 
 import (
+	"bytes"
+	"encoding/json"
+	"reflect"
 	"testing"
 
 	"github.com/btcsuite/btcutil"
@@ -67,8 +70,8 @@ func TestFeeRateTypes(t *testing.T) {
 	}
 }
 
-// TestStaticFeeEstimator checks that the StaticFeeEstimator
-// returns the expected fee rate.
+// TestStaticFeeEstimator checks that the StaticFeeEstimator returns the
+// expected fee rate.
 func TestStaticFeeEstimator(t *testing.T) {
 	t.Parallel()
 
@@ -89,3 +92,55 @@ func TestStaticFeeEstimator(t *testing.T) {
 		t.Fatalf("expected fee rate %v, got %v", feePerKw, feeRate)
 	}
 }
+
+// TestSparseConfFeeSource checks that SparseConfFeeSource generates URLs and
+// parses API responses as expected.
+func TestSparseConfFeeSource(t *testing.T) {
+	t.Parallel()
+
+	// Test that GenQueryURL returns the URL as is.
+	url := "test"
+	feeSource := lnwallet.SparseConfFeeSource{URL: url}
+	queryURL := feeSource.GenQueryURL()
+	if queryURL != url {
+		t.Fatalf("expected query URL of %v, got %v", url, queryURL)
+	}
+
+	// Test parsing a properly formatted JSON API response.
+	// First, create the response as a bytes.Reader.
+	testFees := map[uint32]uint32{
+		1: 12345,
+		2: 42,
+		3: 54321,
+	}
+	testJSON := map[string]map[uint32]uint32{"fee_by_block_target": testFees}
+	jsonResp, err := json.Marshal(testJSON)
+	if err != nil {
+		t.Fatalf("unable to marshal JSON API response: %v", err)
+	}
+	reader := bytes.NewReader(jsonResp)
+
+	// Finally, ensure the expected map is returned without error.
+	fees, err := feeSource.ParseResponse(reader)
+	if err != nil {
+		t.Fatalf("unable to parse API response: %v", err)
+	}
+	if !reflect.DeepEqual(fees, testFees) {
+		t.Fatalf("expected %v, got %v", testFees, fees)
+	}
+
+	// Test parsing an improperly formatted JSON API response.
+	badFees := map[string]uint32{"hi": 12345, "hello": 42, "satoshi": 54321}
+	badJSON := map[string]map[string]uint32{"fee_by_block_target": badFees}
+	jsonResp, err = json.Marshal(badJSON)
+	if err != nil {
+		t.Fatalf("unable to marshal JSON API response: %v", err)
+	}
+	reader = bytes.NewReader(jsonResp)
+
+	// Finally, ensure the improperly formatted fees error.
+	_, err = feeSource.ParseResponse(reader)
+	if err == nil {
+		t.Fatalf("expected ParseResponse to fail")
+	}
+}