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>

3 files changed, 50 insertions, 0 deletions

diff --git a/config/aerc.conf b/config/aerc.conf
index ecae5b90..05ebbf41 100644
--- a/config/aerc.conf
+++ b/config/aerc.conf
@@ -359,6 +359,17 @@
 #
 #no-attachment-warning=
 
+[multipart-converters]
+#
+# Converters allow to generate multipart/alternative messages by converting the
+# main text/plain part into any other MIME type. Only exact MIME types are
+# accepted. The commands are invoked with sh -c and are expected to output
+# valid UTF-8 text.
+#
+# Example (obviously, this requires that you write your main text/plain body
+# using the markdown syntax):
+#text/html=pandoc -f markdown -t html --standalone
+
 [filters]
 #
 # Filters allow you to pipe an email body through a shell command to render
diff --git a/config/config.go b/config/config.go
index c4794cc1..b5cc0d60 100644
--- a/config/config.go
+++ b/config/config.go
@@ -18,6 +18,7 @@ type AercConfig struct {
 	Bindings        BindingConfig
 	ContextualBinds []BindingConfigContext
 	Compose         ComposeConfig
+	Converters      map[string]string
 	Accounts        []AccountConfig  `ini:"-"`
 	Filters         []FilterConfig   `ini:"-"`
 	Viewer          ViewerConfig     `ini:"-"`
@@ -140,6 +141,7 @@ func LoadConfigFromFile(root *string, accts []string) (*AercConfig, error) {
 		Viewer:          defaultViewerConfig(),
 		Statusline:      defaultStatuslineConfig(),
 		Compose:         defaultComposeConfig(),
+		Converters:      make(map[string]string),
 		Templates:       defaultTemplatesConfig(),
 		Openers:         make(map[string][]string),
 	}
@@ -153,6 +155,9 @@ func LoadConfigFromFile(root *string, accts []string) (*AercConfig, error) {
 	if err := config.parseCompose(file); err != nil {
 		return nil, err
 	}
+	if err := config.parseConverters(file); err != nil {
+		return nil, err
+	}
 	if err := config.parseViewer(file); err != nil {
 		return nil, err
 	}
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
+}