fedoracoreos: Add FetchStream API

Most callers will want this.  Additionally, if we ever add
more integrity for the stream such as GPG signing, this will
be the place to do it.

xref coreos/fedora-coreos-tracker#774

example/example.go

@@ -4,10 +4,7 @@
 package main
 
 import (
-	"encoding/json"
 	"fmt"
-	"io/ioutil"
-	"net/http"
 	"os"
 
 	"github.com/coreos/stream-metadata-go/fedoracoreos"
@@ -54,21 +51,9 @@ func printAMI(fcosstable stream.Stream) error {
 }
 
 func run() error {
-	streamurl := fedoracoreos.GetStreamURL(fedoracoreos.StreamStable)
-	resp, err := http.Get(streamurl.String())
+	fcosstable, err := fedoracoreos.FetchStream(fedoracoreos.StreamStable)
 	if err != nil {
-		return err
-	}
-	body, err := ioutil.ReadAll(resp.Body)
-	resp.Body.Close()
-	if err != nil {
-		return err
-	}
-
-	var fcosstable stream.Stream
-	err = json.Unmarshal(body, &fcosstable)
-	if err != nil {
-		return err
+		return fmt.Errorf("failed to fetch stream: %w", err)
 	}
 	if len(os.Args) != 2 {
 		return fmt.Errorf("usage: example aws-ami|download-iso")

fedoracoreos/fcos.go

@@ -4,10 +4,14 @@
 package fedoracoreos
 
 import (
+	"encoding/json"
 	"fmt"
+	"io/ioutil"
+	"net/http"
 	"net/url"
 
 	"github.com/coreos/stream-metadata-go/fedoracoreos/internals"
+	"github.com/coreos/stream-metadata-go/stream"
 )
 
 const (
@@ -25,3 +29,24 @@ func GetStreamURL(stream string) url.URL {
 	u.Path = fmt.Sprintf("streams/%s.json", stream)
 	return u
 }
+
+// FetchStream downloads the Fedora CoreOS stream data and parses it.
+func FetchStream(streamName string) (stream.Stream, error) {
+	streamurl := GetStreamURL(streamName)
+	resp, err := http.Get(streamurl.String())
+	if err != nil {
+		return stream.Stream{}, err
+	}
+	body, err := ioutil.ReadAll(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return stream.Stream{}, err
+	}
+
+	var res stream.Stream
+	err = json.Unmarshal(body, &res)
+	if err != nil {
+		return stream.Stream{}, err
+	}
+	return res, nil
+}