viewer: allow filters to provide their own paging

Allow filter commands to declare that they will provide their own paging
functionality. To do so, a filter command must start with an exclamation
mark "!". For example:

	text/html = ! w3m -I UTF-8 -T text/html

This will effectively run w3m as the main process running in the
embedded terminal of the part viewer. It will provide interactive access
to w3m and will effectively allow navigating using w3m paging and
coloring.

Implements: https://todo.sr.ht/~rjarry/aerc/250
Signed-off-by: Robin Jarry <[email protected]>
Tested-by: Inwit <[email protected]>

app/msgviewer.go

@@ -452,14 +452,6 @@ func NewPartViewer(
 		pagerin io.WriteCloser
 		term    *Terminal
 	)
-	pagerCmd, err := CmdFallbackSearch(config.PagerCmds(), false)
-	if err != nil {
-		acct.PushError(fmt.Errorf("could not start pager: %w", err))
-		return nil, err
-	}
-	cmd := opt.SplitArgs(pagerCmd)
-	pager = exec.Command(cmd[0], cmd[1:]...)
-
 	info := msg.MessageInfo()
 	mime := part.FullMIMEType()
 
@@ -492,9 +484,21 @@ func NewPartViewer(
 				log.Tracef("command %v", f.Command)
 			}
 		}
-		if filter != nil {
+		if filter == nil {
+			continue
+		}
+		if !f.NeedsPager {
+			pager = filter
 			break
 		}
+		pagerCmd, err := CmdFallbackSearch(config.PagerCmds(), false)
+		if err != nil {
+			acct.PushError(fmt.Errorf("could not start pager: %w", err))
+			return nil, err
+		}
+		cmd := opt.SplitArgs(pagerCmd)
+		pager = exec.Command(cmd[0], cmd[1:]...)
+		break
 	}
 	var noFilter *ui.Grid
 	if filter != nil {
@@ -524,8 +528,14 @@ func NewPartViewer(
 		if config.General.EnableOSC8 {
 			filter.Env = append(filter.Env, "AERC_OSC8_URLS=1")
 		}
-		log.Debugf("<%s> part=%v %s: %v | %v",
-			info.Envelope.MessageId, curindex, mime, filter, pager)
+		if pager == filter {
+			log.Debugf("<%s> part=%v %s: %v",
+				info.Envelope.MessageId, curindex, mime, filter)
+		} else {
+			log.Debugf("<%s> part=%v %s: %v | %v",
+				info.Envelope.MessageId, curindex, mime, filter, pager)
+		}
+		var err error
 		if pagerin, err = pager.StdinPipe(); err != nil {
 			return nil, err
 		}
@@ -598,26 +608,36 @@ func (pv *PartViewer) attemptCopy() {
 	if strings.EqualFold(pv.part.MIMEType, "text") {
 		pv.source = parse.StripAnsi(pv.hyperlinks(pv.source))
 	}
-	pv.filter.Stdin = pv.source
-	pv.filter.Stdout = pv.pagerin
-	pv.filter.Stderr = pv.pagerin
-	err := pv.filter.Start()
-	if err != nil {
-		log.Errorf("error running filter: %v", err)
-		return
+	if pv.filter != pv.pager {
+		// Filter is a separate process that needs to output to the pager.
+		pv.filter.Stdin = pv.source
+		pv.filter.Stdout = pv.pagerin
+		pv.filter.Stderr = pv.pagerin
+		err := pv.filter.Start()
+		if err != nil {
+			log.Errorf("error running filter: %v", err)
+			return
+		}
 	}
 	go func() {
 		defer log.PanicHandler()
 		defer atomic.StoreInt32(&pv.copying, 0)
-		err = pv.filter.Wait()
-		if err != nil {
-			log.Errorf("error waiting for filter: %v", err)
-			return
+		var err error
+		if pv.filter == pv.pager {
+			// Filter already implements its own paging.
+			_, err = io.Copy(pv.pagerin, pv.source)
+			if err != nil {
+				log.Errorf("io.Copy: %s", err)
+			}
+		} else {
+			err = pv.filter.Wait()
+			if err != nil {
+				log.Errorf("filter.Wait: %v", err)
+			}
 		}
 		err = pv.pagerin.Close()
 		if err != nil {
 			log.Errorf("error closing pager pipe: %v", err)
-			return
 		}
 	}()
 }
@@ -627,6 +647,11 @@ func (pv *PartViewer) writeMailHeaders() {
 	if !config.Viewer.ShowHeaders || info.RFC822Headers == nil {
 		return
 	}
+	if pv.filter == pv.pager {
+		// Filter already implements its own paging.
+		// Piping another filter into it will cause mayhem.
+		return
+	}
 	var file io.WriteCloser
 
 	for _, f := range config.Filters {

config/aerc.conf

@@ -768,6 +768,7 @@ message/delivery-status=colorize
 message/rfc822=colorize
 #text/html=pandoc -f html -t plain | colorize
 #text/html=html | colorize
+#text/html=! w3m -T text/html -I UTF-8
 #text/*=bat -fP --file-name="$AERC_FILENAME"
 #application/x-sh=bat -fP -l sh
 #image/*=catimg -w $(tput cols) -

config/filters.go

@@ -18,11 +18,12 @@ const (
 )
 
 type FilterConfig struct {
-	Type    FilterType
-	Filter  string
-	Command string
-	Header  string
-	Regex   *regexp.Regexp
+	Type       FilterType
+	Filter     string
+	Command    string
+	NeedsPager bool
+	Header     string
+	Regex      *regexp.Regexp
 }
 
 var Filters []*FilterConfig
@@ -34,9 +35,16 @@ func parseFilters(file *ini.File) error {
 	}
 
 	for _, key := range filters.Keys() {
+		pager := true
+		cmd := key.Value()
+		if strings.HasPrefix(cmd, "!") {
+			cmd = strings.TrimLeft(cmd, "! \t")
+			pager = false
+		}
 		filter := FilterConfig{
-			Command: key.Value(),
-			Filter:  key.Name(),
+			Command:    cmd,
+			NeedsPager: pager,
+			Filter:     key.Name(),
 		}
 
 		switch {

doc/aerc-config.5.scd

@@ -1011,6 +1011,13 @@ filters need to be able to read from standard input. Many programs support
 reading from stdin by putting _-_ instead of a path to a file. You can also
 chain together multiple filters by piping with _|_.
 
+Some filter commands may require interactive user input. If a filter command
+starts with an exclamation mark _!_, the configured *pager* will *not* be used.
+Instead, the filter command will be executed as the main process in the embedded
+terminal of the part viewer. The filter command standard input, output and error
+will be set to the terminal TTY. The filter is expected to implement its own
+paging.
+
 aerc ships with some default filters installed in the libexec directory (usually
 _/usr/libexec/aerc/filters_). Note that these may have additional dependencies
 that aerc does not have alone.
@@ -1100,6 +1107,12 @@ _text/html_
 	text/html=pandoc -f html -t plain
 	```
 
+	Use w3m internal pager to interactively view an HTML part with coloring:
+
+	```
+	text/html=! w3m -I UTF-8 -T text/html
+	```
+
 _text/calendar_
 	Parse calendar invites: