1 files changed, 240 insertions, 0 deletions
diff --git a/srv/mailinglist/mailinglist.go b/srv/mailinglist/mailinglist.go
new file mode 100644
index 0000000..03067b2
--- /dev/null
+++ b/srv/mailinglist/mailinglist.go
@@ -0,0 +1,240 @@
+// Package mailinglist manages the list of subscribed emails and allows emailing
+// out to them.
+package mailinglist
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"html/template"
+	"io"
+	"strings"
+
+	"github.com/google/uuid"
+	"github.com/tilinna/clock"
+)
+
+var (
+	// ErrAlreadyVerified is used when the email is already fully subscribed.
+	ErrAlreadyVerified = errors.New("email is already subscribed")
+)
+
+// MailingList is able to subscribe, unsubscribe, and iterate through emails.
+type MailingList interface {
+
+	// May return ErrAlreadyVerified.
+	BeginSubscription(email string) error
+
+	// May return ErrNotFound or ErrAlreadyVerified.
+	FinalizeSubscription(subToken string) error
+
+	// May return ErrNotFound.
+	Unsubscribe(unsubToken string) error
+
+	// Publish blasts the mailing list with an update about a new blog post.
+	Publish(postTitle, postURL string) error
+}
+
+// Params are parameters used to initialize a new MailingList. All fields are
+// required unless otherwise noted.
+type Params struct {
+	Store  Store
+	Mailer Mailer
+	Clock  clock.Clock
+
+	// URL of the page which should be navigated to in order to finalize a
+	// subscription.
+	FinalizeSubURL string
+
+	// URL of the page which should be navigated to in order to remove a
+	// subscription.
+	UnsubURL string
+}
+
+type mailingList struct {
+	params Params
+}
+
+var beginSubTpl = template.Must(template.New("beginSub").Parse(`
+Welcome to the Mediocre Blog mailing list! By subscribing to this mailing list
+you are signing up to receive an email everytime a new blog post is published.
+
+In order to complete your subscription please navigate to the following link:
+
+{{ .SubLink }}
+
+This mailing list is built and run using my own hardware and software, and I
+solemnly swear that you'll never receive an email from it unless there's a new
+blog post.
+
+If you did not initiate this email, and/or do not wish to subscribe to the
+mailing list, then simply delete this email and pretend that nothing ever
+happened.
+
+- Brian
+`))
+
+func (m *mailingList) BeginSubscription(email string) error {
+
+	emailRecord, err := m.params.Store.Get(email)
+
+	if errors.Is(err, ErrNotFound) {
+		emailRecord = Email{
+			Email:     email,
+			SubToken:  uuid.New().String(),
+			CreatedAt: m.params.Clock.Now(),
+		}
+
+		if err := m.params.Store.Set(emailRecord); err != nil {
+			return fmt.Errorf("storing pending email: %w", err)
+		}
+
+	} else if err != nil {
+		return fmt.Errorf("finding existing email record: %w", err)
+
+	} else if !emailRecord.VerifiedAt.IsZero() {
+		return ErrAlreadyVerified
+	}
+
+	body := new(bytes.Buffer)
+	err = beginSubTpl.Execute(body, struct {
+		SubLink string
+	}{
+		SubLink: fmt.Sprintf("%s?subToken=%s", m.params.FinalizeSubURL, emailRecord.SubToken),
+	})
+
+	if err != nil {
+		return fmt.Errorf("executing beginSubTpl: %w", err)
+	}
+
+	err = m.params.Mailer.Send(
+		email,
+		"Mediocre Blog - Please verify your email address",
+		body.String(),
+	)
+
+	if err != nil {
+		return fmt.Errorf("sending email: %w", err)
+	}
+
+	return nil
+}
+
+func (m *mailingList) FinalizeSubscription(subToken string) error {
+	emailRecord, err := m.params.Store.GetBySubToken(subToken)
+
+	if err != nil {
+		return fmt.Errorf("retrieving email record: %w", err)
+
+	} else if !emailRecord.VerifiedAt.IsZero() {
+		return ErrAlreadyVerified
+	}
+
+	emailRecord.VerifiedAt = m.params.Clock.Now()
+	emailRecord.UnsubToken = uuid.New().String()
+
+	if err := m.params.Store.Set(emailRecord); err != nil {
+		return fmt.Errorf("storing verified email: %w", err)
+	}
+
+	return nil
+}
+
+func (m *mailingList) Unsubscribe(unsubToken string) error {
+	emailRecord, err := m.params.Store.GetByUnsubToken(unsubToken)
+
+	if err != nil {
+		return fmt.Errorf("retrieving email record: %w", err)
+	}
+
+	if err := m.params.Store.Delete(emailRecord.Email); err != nil {
+		return fmt.Errorf("deleting email record: %w", err)
+	}
+
+	return nil
+}
+
+var publishTpl = template.Must(template.New("publish").Parse(`
+A new post has been published to the Mediocre Blog!
+
+{{ .PostTitle }}
+{{ .PostURL }}
+
+If you're interested then please check it out!
+
+If you'd like to unsubscribe from this mailing list then visit the following
+link instead:
+
+{{ .UnsubURL }}
+
+- Brian
+`))
+
+type multiErr []error
+
+func (m multiErr) Error() string {
+	if len(m) == 0 {
+		panic("multiErr with no members")
+	}
+
+	b := new(strings.Builder)
+	fmt.Fprintln(b, "The following errors were encountered:")
+	for _, err := range m {
+		fmt.Fprintf(b, "\t- %s\n", err.Error())
+	}
+
+	return b.String()
+}
+
+func (m *mailingList) Publish(postTitle, postURL string) error {
+
+	var mErr multiErr
+
+	iter := m.params.Store.GetAll()
+	for {
+		emailRecord, err := iter()
+		if errors.Is(err, io.EOF) {
+			break
+
+		} else if err != nil {
+			mErr = append(mErr, fmt.Errorf("iterating through email records: %w", err))
+			break
+
+		} else if emailRecord.VerifiedAt.IsZero() {
+			continue
+		}
+
+		body := new(bytes.Buffer)
+		err = publishTpl.Execute(body, struct {
+			PostTitle string
+			PostURL   string
+			UnsubURL  string
+		}{
+			PostTitle: postTitle,
+			PostURL:   postURL,
+			UnsubURL:  fmt.Sprintf("%s?unsubToken=%s", m.params.UnsubURL, emailRecord.UnsubToken),
+		})
+
+		if err != nil {
+			mErr = append(mErr, fmt.Errorf("rendering publish email template for %q: %w", emailRecord.Email, err))
+			continue
+		}
+
+		err = m.params.Mailer.Send(
+			emailRecord.Email,
+			fmt.Sprintf("Mediocre Blog - New Post! - %s", postTitle),
+			body.String(),
+		)
+
+		if err != nil {
+			mErr = append(mErr, fmt.Errorf("sending email to %q: %w", emailRecord.Email, err))
+			continue
+		}
+	}
+
+	if len(mErr) > 0 {
+		return mErr
+	}
+
+	return nil
+}