Remote instance dereferencing (#70)

Remote instances are now dereferenced when they post to an inbox on a GtS instance.

    Dereferencing will be done first by checking the /api/v1/instance endpoint of an instance.
    If that doesn't work, /.well-known/nodeinfo will be checked.
    If that doesn't work, only a minimal representation of the instance will be stored.

A new field was added to the Instance database model. To create it:

alter table instances add column contact_account_username text;

internal/transport/finger.go

@@ -0,0 +1,48 @@
+package transport
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+)
+
+func (t *transport) Finger(c context.Context, targetUsername string, targetDomain string) ([]byte, error) {
+	l := t.log.WithField("func", "Finger")
+	urlString := fmt.Sprintf("https://%s/.well-known/webfinger?resource=acct:%s@%s", targetDomain, targetUsername, targetDomain)
+	l.Debugf("performing GET to %s", urlString)
+
+	iri, err := url.Parse(urlString)
+	if err != nil {
+		return nil, fmt.Errorf("Finger: error parsing url %s: %s", urlString, err)
+	}
+
+	l.Debugf("performing GET to %s", iri.String())
+
+	req, err := http.NewRequest("GET", iri.String(), nil)
+	if err != nil {
+		return nil, err
+	}
+	req = req.WithContext(c)
+	req.Header.Add("Accept", "application/json")
+	req.Header.Add("Accept", "application/jrd+json")
+	req.Header.Add("Date", t.clock.Now().UTC().Format("Mon, 02 Jan 2006 15:04:05")+" GMT")
+	req.Header.Add("User-Agent", fmt.Sprintf("%s %s", t.appAgent, t.gofedAgent))
+	req.Header.Set("Host", iri.Host)
+	t.getSignerMu.Lock()
+	err = t.getSigner.SignRequest(t.privkey, t.pubKeyID, req, nil)
+	t.getSignerMu.Unlock()
+	if err != nil {
+		return nil, err
+	}
+	resp, err := t.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("GET request to %s failed (%d): %s", iri.String(), resp.StatusCode, resp.Status)
+	}
+	return ioutil.ReadAll(resp.Body)
+}