Add unsubscribe command

The unsubscribe command, available when in a message viewer context,
enables users to easily unsubscribe from mailing lists.

When the command is executed, aerc looks for a List-Unsubscribe header
as defined in RFC 2369. If found, aerc will attempt to present the user
with a suitable interface for completing the request. Currently, mailto
and http(s) URLs are supported. In the case of a HTTP(S) URL, aerc will
open the link in a browser. For mailto links, a new composer tab will be
opened with a message filled out according to the URL. The message is
not sent automatically in order to provide the user a chance to review
it first.

Closes #101

3 files changed, 150 insertions, 0 deletions

diff --git a/commands/msg/unsubscribe.go b/commands/msg/unsubscribe.go
new file mode 100644
index 0000000..d4a7e9a
--- /dev/null
+++ b/commands/msg/unsubscribe.go
@@ -0,0 +1,103 @@
+package msg
+
+import (
+	"bufio"
+	"errors"
+	"net/url"
+	"strings"
+
+	"git.sr.ht/~sircmpwn/aerc/lib"
+	"git.sr.ht/~sircmpwn/aerc/widgets"
+)
+
+// Unsubscribe helps people unsubscribe from mailing lists by way of the
+// List-Unsubscribe header.
+type Unsubscribe struct{}
+
+func init() {
+	register(Unsubscribe{})
+}
+
+// Aliases returns a list of aliases for the :unsubscribe command
+func (Unsubscribe) Aliases() []string {
+	return []string{"unsubscribe"}
+}
+
+// Complete returns a list of completions
+func (Unsubscribe) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+// Execute runs the Unsubscribe command
+func (Unsubscribe) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) != 1 {
+		return errors.New("Usage: unsubscribe")
+	}
+	widget := aerc.SelectedTab().(widgets.ProvidesMessage)
+	headers := widget.SelectedMessage().RFC822Headers
+	if !headers.Has("list-unsubscribe") {
+		return errors.New("No List-Unsubscribe header found")
+	}
+	methods := parseUnsubscribeMethods(headers.Get("list-unsubscribe"))
+	aerc.Logger().Printf("found %d unsubscribe methods", len(methods))
+	for _, method := range methods {
+		aerc.Logger().Printf("trying to unsubscribe using %v", method)
+		switch method.Scheme {
+		case "mailto":
+			return unsubscribeMailto(aerc, method)
+		case "http", "https":
+			return unsubscribeHTTP(method)
+		default:
+			aerc.Logger().Printf("skipping unrecognized scheme: %s", method.Scheme)
+		}
+	}
+	return errors.New("no supported unsubscribe methods found")
+}
+
+// parseUnsubscribeMethods reads the list-unsubscribe header and parses it as a
+// list of angle-bracket <> deliminated URLs. See RFC 2369.
+func parseUnsubscribeMethods(header string) (methods []*url.URL) {
+	r := bufio.NewReader(strings.NewReader(header))
+	for {
+		// discard until <
+		_, err := r.ReadSlice('<')
+		if err != nil {
+			return
+		}
+		// read until <
+		m, err := r.ReadSlice('>')
+		if err != nil {
+			return
+		}
+		m = m[:len(m)-1]
+		if u, err := url.Parse(string(m)); err == nil {
+			methods = append(methods, u)
+		}
+	}
+}
+
+func unsubscribeMailto(aerc *widgets.Aerc, u *url.URL) error {
+	widget := aerc.SelectedTab().(widgets.ProvidesMessage)
+	acct := widget.SelectedAccount()
+	composer := widgets.NewComposer(aerc.Config(), acct.AccountConfig(),
+		acct.Worker())
+	composer.Defaults(map[string]string{
+		"To":      u.Opaque,
+		"Subject": u.Query().Get("subject"),
+	})
+	composer.SetContents(strings.NewReader(u.Query().Get("body")))
+	tab := aerc.NewTab(composer, "unsubscribe")
+	composer.OnSubjectChange(func(subject string) {
+		if subject == "" {
+			tab.Name = "unsubscribe"
+		} else {
+			tab.Name = subject
+		}
+		tab.Content.Invalidate()
+	})
+	return nil
+}
+
+func unsubscribeHTTP(u *url.URL) error {
+	return lib.OpenFile(u.String())
+}
diff --git a/commands/msg/unsubscribe_test.go b/commands/msg/unsubscribe_test.go
new file mode 100644
index 0000000..e4e6f25
--- /dev/null
+++ b/commands/msg/unsubscribe_test.go
@@ -0,0 +1,41 @@
+package msg
+
+import (
+	"testing"
+)
+
+func TestParseUnsubscribe(t *testing.T) {
+	type tc struct {
+		hdr      string
+		expected []string
+	}
+	cases := []*tc{
+		&tc{"", []string{}},
+		&tc{"invalid", []string{}},
+		&tc{"<https://example.com>, <http://example.com>", []string{
+			"https://example.com", "http://example.com",
+		}},
+		&tc{"<https://example.com> is a URL", []string{
+			"https://example.com",
+		}},
+		&tc{"<mailto:user@host?subject=unsubscribe>, <https://example.com>",
+			[]string{
+				"mailto:user@host?subject=unsubscribe", "https://example.com",
+			}},
+		&tc{"<>, <https://example> ", []string{
+			"", "https://example",
+		}},
+	}
+	for _, c := range cases {
+		result := parseUnsubscribeMethods(c.hdr)
+		if len(result) != len(c.expected) {
+			t.Errorf("expected %d methods but got %d", len(c.expected), len(result))
+			continue
+		}
+		for idx := 0; idx < len(result); idx++ {
+			if result[idx].String() != c.expected[idx] {
+				t.Errorf("expected %v but got %v", c.expected[idx], result[idx])
+			}
+		}
+	}
+}
diff --git a/doc/aerc.1.scd b/doc/aerc.1.scd
index 0b86f75..aa2e5ba 100644
--- a/doc/aerc.1.scd
+++ b/doc/aerc.1.scd
@@ -85,6 +85,12 @@ message list, the message in the message viewer, etc).
 *unread*
 	Marks the selected message as unread.
 
+*unsubscribe*
+	Attempt to automatically unsubscribe the user from the mailing list through
+	use of the List-Unsubscribe header. If supported, aerc may open a compose
+	window pre-filled with the unsubscribe information or open the unsubscribe
+	URL in a web browser.
+
 ## MESSAGE LIST COMMANDS
 
 *cf* <folder>