Added a simple client library and main file

This is cleaner than making requests to the distributor using curl piped through jq etc. This also demonstrates that requesting information from the distributor really is a small amount of code with few dependencies.

client/client.go

@@ -0,0 +1,98 @@
+// Copyright 2023 Google LLC. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package client contains a simple RESTful client that retrieves information from
+// a distributor at a known URL.
+package client
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strconv"
+
+	"github.com/transparency-dev/distributor/api"
+)
+
+// LogID is the globally unique name for a log.
+type LogID string
+
+// NewRestDistributor constructs a new client.
+func NewRestDistributor(baseURL string, client *http.Client) *RestDistributor {
+	return &RestDistributor{
+		baseURL: baseURL,
+		client:  client,
+	}
+}
+
+// RestDistributor is a client that fetches data via RESTful HTTP calls.
+type RestDistributor struct {
+	baseURL string
+	client  *http.Client
+}
+
+// GetLogs returns all logs that the distributor knows about.
+func (d *RestDistributor) GetLogs() ([]LogID, error) {
+	u, err := url.Parse(d.baseURL + api.HTTPGetLogs)
+	if err != nil {
+		return nil, err
+	}
+	r := make([]LogID, 0)
+	bs, err := d.fetchData(u)
+	if err != nil {
+		return nil, err
+	}
+	if err := json.Unmarshal(bs, &r); err != nil {
+		return nil, err
+	}
+	return r, nil
+}
+
+// GetCheckpointN returns the freshest checkpoint for the log that at least N witnesses
+// have provided signatures for.
+func (d *RestDistributor) GetCheckpointN(l LogID, n uint) ([]byte, error) {
+	u, err := url.Parse(d.baseURL + fmt.Sprintf(api.HTTPGetCheckpointN, l, strconv.Itoa(int(n))))
+	if err != nil {
+		return nil, err
+	}
+	return d.fetchData(u)
+}
+
+// GetCheckpointWitness returns the latest checkpoint that a named witness has provided
+// for the given log.
+func (d *RestDistributor) GetCheckpointWitness(l LogID, w string) ([]byte, error) {
+	u, err := url.Parse(d.baseURL + fmt.Sprintf(api.HTTPCheckpointByWitness, l, w))
+	if err != nil {
+		return nil, err
+	}
+	return d.fetchData(u)
+}
+
+func (d *RestDistributor) fetchData(u *url.URL) ([]byte, error) {
+	resp, err := d.client.Get(u.String())
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read body: %v", err)
+	}
+	if resp.StatusCode != 200 {
+		return nil, fmt.Errorf("bad status response (%s): %q", resp.Status, body)
+	}
+	return body, nil
+}

cmd/client/client.go

@@ -0,0 +1,49 @@
+// Copyright 2023 Google LLC. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// client is a simple client that demonstrates fetching data from a distributor.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"net/http"
+
+	"github.com/golang/glog"
+	"github.com/transparency-dev/distributor/client"
+)
+
+var (
+	baseURL = flag.String("base_url", "https://api.transparency.dev", "The base URL of the distributor")
+	n       = flag.Uint("n", 2, "The desired number of witness signatures for each log")
+)
+
+func main() {
+	flag.Parse()
+
+	d := client.NewRestDistributor(*baseURL, http.DefaultClient)
+
+	logs, err := d.GetLogs()
+	if err != nil {
+		glog.Exitf("Failed to enumerate logs: %v", err)
+	}
+	for _, l := range logs {
+		cp, err := d.GetCheckpointN(l, *n)
+		if err != nil {
+			glog.Warningf("Could not get checkpoint.%d for log %s: %v", *n, l, err)
+			continue
+		}
+		fmt.Printf("Checkpoint.%d for log %s:\n'''\n%s\n'''\n\n", *n, l, cp)
+	}
+}