viewer: allow piping full headers in a filter

Allow defining a .headers special filter command that will be used only
to process email headers (when [viewer].show-headers=true).

Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Kt Programs <ktprograms@gmail.com>

4 files changed, 65 insertions, 9 deletions

diff --git a/config/aerc.conf b/config/aerc.conf
index 1ce4810e..b09a7ab7 100644
--- a/config/aerc.conf
+++ b/config/aerc.conf
@@ -437,6 +437,12 @@ message/rfc822=colorize
 #subject,~Git(hub|lab)=lolcat -f
 #from,thatguywhodoesnothardwraphismessages=wrap -w 100 | colorize
 
+# This special filter is only used to post-process email headers when
+# [viewer].show-headers=true
+# By default, headers are piped directly into the pager.
+#
+#.headers=colorize
+
 [openers]
 #
 # Openers allow you to specify the command to use for the :open action on a
diff --git a/config/filters.go b/config/filters.go
index 0b7a1cb2..113a6de6 100644
--- a/config/filters.go
+++ b/config/filters.go
@@ -13,6 +13,7 @@ type FilterType int
 const (
 	FILTER_MIMETYPE FilterType = iota
 	FILTER_HEADER
+	FILTER_HEADERS
 )
 
 type FilterConfig struct {
@@ -57,6 +58,8 @@ func parseFilters(file *ini.File) error {
 			if err != nil {
 				return err
 			}
+		case filter.Filter == ".headers":
+			filter.Type = FILTER_HEADERS
 		default:
 			filter.Type = FILTER_MIMETYPE
 		}
diff --git a/doc/aerc-config.5.scd b/doc/aerc-config.5.scd
index cc0a5f80..22c3a40a 100644
--- a/doc/aerc-config.5.scd
+++ b/doc/aerc-config.5.scd
@@ -644,6 +644,10 @@ The following variables are defined in the filter command environment:
 Note that said email body is converted into UTF-8 before being passed to
 filters.
 
+If *show-headers* is enabled, only the currently viewed part body is piped into
+the filter command. A special _.headers_ filter command can be defined to post
+process the full headers.
+
 ## EXAMPLES
 
 _text/plain_
@@ -708,6 +712,13 @@ _text/\*_
 	text/\*=bat -fP --file-name="$AERC_FILENAME" --style=plain
 	```
 
+_.headers_
+	Colorize email headers when *show-headers* is _true_.
+
+	```
+	.headers=colorize
+	```
+
 _message/delivery-status_
 	When not being able to deliver the provider might send such emails:
 
diff --git a/widgets/msgviewer.go b/widgets/msgviewer.go
index d33e61cc..d2fc0d80 100644
--- a/widgets/msgviewer.go
+++ b/widgets/msgviewer.go
@@ -689,9 +689,45 @@ func (pv *PartViewer) attemptCopy() {
 func (pv *PartViewer) writeMailHeaders() {
 	info := pv.msg.MessageInfo()
 	if config.Viewer.ShowHeaders && info.RFC822Headers != nil {
-		// header need to bypass the filter, else we run into issues
-		// with the filter messing with newlines etc.
-		// hence all writes in this block go directly to the pager
+		var file io.WriteCloser
+
+		for _, f := range config.Filters {
+			if f.Type != config.FILTER_HEADERS {
+				continue
+			}
+			log.Debugf("<%s> piping headers in filter: %s",
+				info.Envelope.MessageId, f.Command)
+			filter := exec.Command("sh", "-c", f.Command)
+			if pv.filter != nil {
+				// inherit from filter env
+				filter.Env = pv.filter.Env
+			}
+
+			stdin, err := filter.StdinPipe()
+			if err == nil {
+				filter.Stdout = pv.pagerin
+				filter.Stderr = pv.pagerin
+				err := filter.Start()
+				if err == nil {
+					//nolint:errcheck // who cares?
+					defer filter.Wait()
+					file = stdin
+				} else {
+					log.Errorf(
+						"failed to start header filter: %v",
+						err)
+				}
+			} else {
+				log.Errorf("failed to create pipe: %v", err)
+			}
+			break
+		}
+		if file == nil {
+			file = pv.pagerin
+		} else {
+			defer file.Close()
+		}
+
 		fields := info.RFC822Headers.Fields()
 		for fields.Next() {
 			var value string
@@ -702,22 +738,22 @@ func (pv *PartViewer) writeMailHeaders() {
 			}
 			field := fmt.Sprintf(
 				"%s: %s\n", fields.Key(), value)
-			_, err = pv.pagerin.Write([]byte(field))
+			_, err = file.Write([]byte(field))
 			if err != nil {
-				log.Errorf("failed to write to stdin of pager: %v", err)
+				log.Errorf("failed to write headers: %v", err)
 			}
 		}
 		// virtual header
 		if len(info.Labels) != 0 {
 			labels := fmtHeader(info, "Labels", "", "", "", "")
-			_, err := pv.pagerin.Write([]byte(fmt.Sprintf("Labels: %s\n", labels)))
+			_, err := file.Write([]byte(fmt.Sprintf("Labels: %s\n", labels)))
 			if err != nil {
-				log.Errorf("failed to write to stdin of pager: %v", err)
+				log.Errorf("failed to write to labels: %v", err)
 			}
 		}
-		_, err := pv.pagerin.Write([]byte{'\n'})
+		_, err := file.Write([]byte{'\n'})
 		if err != nil {
-			log.Errorf("failed to write to stdin of pager: %v", err)
+			log.Errorf("failed to write empty line: %v", err)
 		}
 	}
 }