httpbakery/agent: support authinfo in environment variables

This makes it straightforward for client programs to support agent
authentication without needing an explicit command line flag.

httpbakery/agent/agent.go

@@ -7,8 +7,11 @@
 package agent
 
 import (
+	"encoding/json"
 	"errors"
+	"io/ioutil"
 	"net/url"
+	"os"
 	"strings"
 
 	"github.com/juju/loggo"
@@ -40,6 +43,32 @@ type Agent struct {
 	Username string `json:"username" yaml:"username"`
 }
 
+var ErrNoAuthInfo = errgo.New("no bakery agent info found in environment")
+
+// AuthInfoFromEnvironment returns an AuthInfo derived
+// from environment variables.
+//
+// It recognizes the following variable:
+// BAKERY_AGENT_FILE - path to a file containing agent authentication
+//    info in JSON format (as marshaled by the AuthInfo type).
+//
+// If BAKERY_AGENT_FILE is not set, ErrNoAuthInfo will be returned.
+func AuthInfoFromEnvironment() (*AuthInfo, error) {
+	agentFile := os.Getenv("BAKERY_AGENT_FILE")
+	var ai AuthInfo
+	data, err := ioutil.ReadFile(agentFile)
+	if err != nil {
+		return nil, errgo.Mask(err)
+	}
+	if err := json.Unmarshal(data, &ai); err != nil {
+		return nil, errgo.Notef(err, "cannot unmarshal agent information from %q: %v", agentFile)
+	}
+	if ai.Key == nil {
+		return nil, errgo.Newf("no private key found in %q", agentFile)
+	}
+	return &ai, nil
+}
+
 // SetUpAuth sets up agent authentication on the given client.
 // If this is called several times on the same client, earlier
 // calls will take precedence over later calls when there's

httpbakery/agent/agent_test.go

@@ -1,9 +1,13 @@
 package agent_test
 
 import (
+	"encoding/json"
+	"io/ioutil"
 	"net/http"
+	"os"
 
 	"github.com/juju/testing"
+	jc "github.com/juju/testing/checkers"
 	"golang.org/x/net/context"
 	gc "gopkg.in/check.v1"
 	"gopkg.in/errgo.v1"
@@ -117,6 +121,36 @@ func (s *agentSuite) TestSetUpAuth(c *gc.C) {
 	c.Assert(err, gc.Equals, nil)
 }
 
+func (s *agentSuite) TestAuthInfoFromEnvironment(c *gc.C) {
+	defer os.Setenv("BAKERY_AGENT_FILE", "")
+
+	f, err := ioutil.TempFile("", "")
+	c.Assert(err, gc.Equals, nil)
+	defer os.Remove(f.Name())
+	defer f.Close()
+
+	authInfo := &agent.AuthInfo{
+		Key: bakery.MustGenerateKey(),
+		Agents: []agent.Agent{{
+			URL:      "https://0.1.2.3/x",
+			Username: "bob",
+		}, {
+			URL:      "https://0.2.3.4",
+			Username: "charlie",
+		}},
+	}
+	data, err := json.Marshal(authInfo)
+	_, err = f.Write(data)
+	c.Assert(err, gc.Equals, nil)
+	f.Close()
+
+	os.Setenv("BAKERY_AGENT_FILE", f.Name())
+
+	authInfo1, err := agent.AuthInfoFromEnvironment()
+	c.Assert(err, gc.Equals, nil)
+	c.Assert(authInfo1, jc.DeepEquals, authInfo)
+}
+
 func AgentHandlers(h AgentHandler) []httprequest.Handler {
 	return reqServer.Handlers(func(p httprequest.Params) (agentHandlers, context.Context, error) {
 		return agentHandlers{h}, p.Context, nil