compose: allow writing multipart/alternative messages

Add a new :multipart command that can be executed on the composer review
screen. This command takes a MIME type as argument which needs to match
a setting in the new [multipart-converters] section of aerc.conf. A part
can be removed by using the -d flag.

The [multipart-converters] section has MIME types associated with
commands. These commands are executed with sh -c every time the main
email body is updated to generate each part content. The commands are
expected to output valid UTF-8 text.

If a command fails, an explicit error will be printed next to the part
MIME type to allow users to debug their issue but the email may still be
sent anyway with an empty alternative part.

This is mostly intended for people who *really* need to send html
messages for their boss or for corporate reasons. For now, it is
a manual and explicit action to convert a message in such a way.

Here is an example configuration:

  [multipart-converters]
  text/html = pandoc -f markdown -t html

And the associated binding to append an HTML alternative to a message:

  [compose::review]
  H = :multipart text/html<enter>
  hh = :multipart -d text/html<enter>

Link: https://lists.sr.ht/~rjarry/aerc-discuss/%3CCO5KH4W57XNB.2PZLR1CNFK22H%40mashenka%3E
Co-authored-by: Eric McConville <emcconville@emcconville.com>
Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Bence Ferdinandy <bence@ferdinandy.com>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

1 files changed, 34 insertions, 0 deletions

diff --git a/config/converters.go b/config/converters.go
new file mode 100644
index 00000000..8c6b88df
--- /dev/null
+++ b/config/converters.go
@@ -0,0 +1,34 @@
+package config
+
+import (
+	"fmt"
+	"strings"
+
+	"git.sr.ht/~rjarry/aerc/log"
+	"github.com/go-ini/ini"
+)
+
+func (config *AercConfig) parseConverters(file *ini.File) error {
+	converters, err := file.GetSection("multipart-converters")
+	if err != nil {
+		goto out
+	}
+
+	for mimeType, command := range converters.KeysHash() {
+		mimeType = strings.ToLower(mimeType)
+		if mimeType == "text/plain" {
+			return fmt.Errorf(
+				"multipart-converters: text/plain is reserved")
+		}
+		if !strings.HasPrefix(mimeType, "text/") {
+			return fmt.Errorf(
+				"multipart-converters: %q: only text/* MIME types are supported",
+				mimeType)
+		}
+		config.Converters[mimeType] = command
+	}
+
+out:
+	log.Debugf("aerc.conf: [multipart-converters] %#v", config.Converters)
+	return nil
+}