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package main
// Turning a draft into a message.
//
// A draft is an ordered list of typed blocks; a mail is bytes. This is the
// conversion, and the shape it produces is derived from the blocks rather than
// chosen by anybody:
//
// text and quote only → text/plain
// code or image present → multipart/alternative { text/plain, text/html }
// files attached → multipart/mixed { <either of the above>, file* }
//
// The first rule is not about capability but about damage. Plain text is
// exactly what prose and quotations are for, and a mail client renders them the
// way every mail client has for thirty years. A code listing is different in
// kind: its meaning is in its whitespace, and a plain-text mail on the wire is
// subject to being rewrapped by anything between here and the reader. An image
// is not text at all. So the presence of either is what makes the *body* MIME,
// and nothing else does.
//
// The second is a different question and composes with the first rather than
// replacing it. mixed means "several things are enclosed", which is exactly
// true of a message with attachments and exactly false of one that is a single
// body in two renderings — which is why an alternative is not wrapped in a
// mixed when nothing is attached, and is when something is. A draft of prose
// with six photos therefore sends its prose as text/plain, inside a mixed:
// attaching a file does not make the body HTML, because it does nothing to the
// body at all.
//
// The plain-text alternative is generated in every case, including when an HTML
// part exists. It costs a few hundred bytes, it is what a reader with a text
// client actually sees, and generating it from the same blocks means the two
// cannot disagree about what the message says.
import (
"bytes"
"database/sql"
"fmt"
"html"
"strings"
message "github.com/emersion/go-message"
)
// renderPlain renders a draft's blocks as the text/plain body of a mail.
//
// Blocks are separated by a blank line, which is the paragraph break of plain
// text. A quote carries its attribution line above it and every line prefixed,
// which is what a mail client will collapse and what the next reply will quote
// in turn.
func renderPlain(d *draft) string {
var parts []string
for _, b := range d.Blocks {
switch b.Kind {
case blockText:
if s := strings.TrimRight(b.Content, "\n"); strings.TrimSpace(s) != "" {
parts = append(parts, s)
}
case blockQuote:
if strings.TrimSpace(b.Content) == "" {
continue
}
var sb strings.Builder
if b.Meta.Attribution != "" {
sb.WriteString(b.Meta.Attribution)
sb.WriteString("\n")
}
sb.WriteString(quoteText(b.Content))
parts = append(parts, sb.String())
case blockCode:
if strings.TrimSpace(b.Content) == "" {
continue
}
// Indented by four spaces, which is the oldest convention for
// "this is a listing" in plain-text mail and survives being
// forwarded through clients that know nothing about Markdown. It
// also stops a line beginning with ">" inside a listing from
// reading as a quotation.
parts = append(parts, indent(b.Content, " "))
case blockImage:
// An image has no plain-text form. Naming it is better than
// silence, which would leave the text reader wondering what the
// other half of the message was about.
name := b.Meta.Alt
if name == "" {
name = "image"
}
parts = append(parts, "["+name+" — attached image]")
case blockFile:
// Nothing. An attachment is enclosed beside the body, and every
// mail client in existence lists what a message carries; writing
// "[foto.jpg — attached]" into the prose would add a line the
// account owner did not write, next to a list the recipient is
// already being shown. The image case above is the exception
// because an inline image is *in* the body, so leaving it out
// would drop content rather than avoid repeating it.
}
}
return strings.Join(parts, "\n\n") + "\n"
}
// renderHTML renders a draft's blocks as the text/html body of a mail.
//
// Deliberately plain markup: paragraphs, blockquotes and <pre>. It carries no
// stylesheet, no fonts and no layout, because a reply is prose and because
// every one of those is a thing the recipient's client will fight with. What
// HTML buys here is a code listing that survives intact and an image shown in
// place, and nothing else is worth the bytes.
func renderHTML(d *draft) string {
var sb strings.Builder
sb.WriteString("<html><body>\n")
for _, b := range d.Blocks {
switch b.Kind {
case blockText:
if strings.TrimSpace(b.Content) == "" {
continue
}
// Blank lines separate paragraphs; single breaks stay breaks.
for _, para := range strings.Split(b.Content, "\n\n") {
if strings.TrimSpace(para) == "" {
continue
}
fmt.Fprintf(&sb, "<p>%s</p>\n",
strings.ReplaceAll(html.EscapeString(strings.TrimSpace(para)), "\n", "<br>\n"))
}
case blockQuote:
if strings.TrimSpace(b.Content) == "" {
continue
}
// type="cite" is what mail clients key their quote styling off,
// and is what mailweb's own message view already styles.
sb.WriteString("<blockquote type=\"cite\">\n")
if b.Meta.Attribution != "" {
fmt.Fprintf(&sb, "<p>%s</p>\n", html.EscapeString(b.Meta.Attribution))
}
for _, para := range strings.Split(b.Content, "\n\n") {
if strings.TrimSpace(para) == "" {
continue
}
fmt.Fprintf(&sb, "<p>%s</p>\n",
strings.ReplaceAll(html.EscapeString(strings.TrimSpace(para)), "\n", "<br>\n"))
}
sb.WriteString("</blockquote>\n")
case blockCode:
if strings.TrimSpace(b.Content) == "" {
continue
}
// The one place the markup earns its keep: <pre> is why a draft
// with a listing is sent as HTML at all.
fmt.Fprintf(&sb, "<pre>%s</pre>\n",
html.EscapeString(strings.TrimRight(b.Content, "\n")))
case blockImage:
// Inline images are carried as related parts and referenced by
// Content-ID; until that lands an image block renders as its
// description rather than as a broken reference. An attached file
// is a different thing and is not this — see blockFile below and
// writeMixedBody.
alt := b.Meta.Alt
if alt == "" {
alt = "image"
}
fmt.Fprintf(&sb, "<p>[%s]</p>\n", html.EscapeString(alt))
case blockFile:
// Nothing, for the reason given in renderPlain: an attachment is
// not body text in either rendering, and the two must agree about
// what the message says.
}
}
sb.WriteString("</body></html>\n")
return sb.String()
}
// indent prefixes every line of s.
func indent(s, prefix string) string {
lines := strings.Split(strings.TrimRight(s, "\n"), "\n")
for i, line := range lines {
if line == "" {
continue
}
lines[i] = prefix + line
}
return strings.Join(lines, "\n")
}
// ============================================================================
// Assembling the message
// ============================================================================
// buildDraftMessage renders a draft as the raw bytes of a mail to one recipient
// set.
//
// The recipients are passed in rather than read off the draft because choosing
// which set to use is the act being authorised, and it happens at the call site
// where that authorisation was given. This function addresses what it is told
// to address.
//
// It returns the bytes and the Message-ID it generated, so the caller can
// record what the draft became.
//
// Named for a draft rather than for a reply because most drafts are replies and
// not all of them are: one composed from nothing threads no message and quotes
// nobody, and is otherwise identical. The threading headers come off the draft
// and are simply absent there.
func buildDraftMessage(db *sql.DB, d *draft, from string, set recipientSet) ([]byte, string, error) {
if len(set.To) == 0 {
// Refused rather than sent with an empty To:, which some servers
// accept and deliver to nobody, leaving every local trace saying the
// mail went out.
return nil, "", fmt.Errorf(
"no recipients: a message must name at least one address in To. " +
"A set holding only Cc addresses cannot be sent; move one of them " +
"to To")
}
var buf bytes.Buffer
h := draftHeader(d, from, set)
messageID, err := h.MessageID()
if err != nil {
// A message with no readable Message-ID is still worth sending; what
// is lost is the ability to tie the Sent copy back to this draft.
messageID = ""
}
// The attachments' bytes are read only here. Everything else that touches a
// draft works from the metadata, so a page render costs a filename and this
// costs the file.
attachments, err := loadDraftAttachments(db, d)
if err != nil {
return nil, "", err
}
// The same question — plain or alternative — asked once, whether the body
// is the whole message or a part of one. A draft that gained an attachment
// must not change what its body says.
rich := d.RichFormat()
if len(attachments) == 0 {
var err error
if rich {
err = writeAlternativeBody(&buf, h, renderPlain(d), renderHTML(d))
} else {
err = writePlainBody(&buf, h, renderPlain(d))
}
if err != nil {
return nil, "", err
}
return buf.Bytes(), messageID, nil
}
writeBody := func(mw *message.Writer) error {
if rich {
return writeAlternativeInto(mw, renderPlain(d), renderHTML(d))
}
return writePlainInto(mw, renderPlain(d))
}
if err := writeMixedBody(&buf, h, writeBody, attachments); err != nil {
return nil, "", err
}
return buf.Bytes(), messageID, nil
}
// loadDraftAttachments reads the bytes of everything a draft encloses.
//
// A file block whose asset has gone is an error rather than an omission. The
// two are created in one transaction and deleted in one, so a block without its
// asset means something went wrong that nobody has seen — and the failure to
// avoid is sending a mail that quietly lacks the invoice it was written to
// carry. The draft is left untouched and says what is missing.
func loadDraftAttachments(db *sql.DB, d *draft) ([]draftAsset, error) {
var out []draftAsset
for _, b := range d.Attachments() {
if b.AssetID == 0 {
return nil, fmt.Errorf(
"the attachment %q has no stored file, so the message was not sent; "+
"remove the attachment and add it again", b.Content)
}
asset, err := loadDraftAsset(db, d.ID, b.AssetID)
if err != nil {
return nil, fmt.Errorf("attachment %q: %w", b.Content, err)
}
out = append(out, *asset)
}
return out, nil
}
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