This callback is the gateway to the new mime_node_t-based formatters.
This maintains backwards compatibility so the formatters can be
transitioned one at a time.  Once this is done, the formatter
structure can be safely gutted.
---
 notmuch-client.h |    6 ++++++
 notmuch-reply.c  |    2 +-
 notmuch-show.c   |   22 ++++++++++++++++++----
 3 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/notmuch-client.h b/notmuch-client.h
index 8832242..52448ef 100644
--- a/notmuch-client.h
+++ b/notmuch-client.h
@@ -54,8 +54,14 @@
 #define STRINGIFY(s) STRINGIFY_(s)
 #define STRINGIFY_(s) #s
 
+struct mime_node;
+struct notmuch_show_params;
+
 typedef struct notmuch_show_format {
     const char *message_set_start;
+    void (*part) (const void *ctx,
+		  struct mime_node *node, int indent,
+		  struct notmuch_show_params *params);
     const char *message_start;
     void (*message) (const void *ctx,
 		     notmuch_message_t *message,
diff --git a/notmuch-reply.c b/notmuch-reply.c
index 000f6da..33c4ee8 100644
--- a/notmuch-reply.c
+++ b/notmuch-reply.c
@@ -31,7 +31,7 @@ static void
 reply_part_content (GMimeObject *part);
 
 static const notmuch_show_format_t format_reply = {
-    "",
+    "", NULL,
 	"", NULL,
 	    "", NULL, reply_headers_message_part, ">\n",
 	    "",
diff --git a/notmuch-show.c b/notmuch-show.c
index 0200b9c..45c3569 100644
--- a/notmuch-show.c
+++ b/notmuch-show.c
@@ -42,7 +42,7 @@ static void
 format_part_end_text (GMimeObject *part);
 
 static const notmuch_show_format_t format_text = {
-    "",
+    "", NULL,
 	"\fmessage{ ", format_message_text,
 	    "\fheader{\n", format_headers_text, format_headers_message_part_text, "\fheader}\n",
 	    "\fbody{\n",
@@ -85,7 +85,7 @@ static void
 format_part_end_json (GMimeObject *part);
 
 static const notmuch_show_format_t format_json = {
-    "[",
+    "[", NULL,
 	"{", format_message_json,
 	    "\"headers\": {", format_headers_json, format_headers_message_part_json, "}",
 	    ", \"body\": [",
@@ -106,7 +106,7 @@ format_message_mbox (const void *ctx,
 		     unused (int indent));
 
 static const notmuch_show_format_t format_mbox = {
-    "",
+    "", NULL,
         "", format_message_mbox,
             "", NULL, NULL, "",
             "",
@@ -125,7 +125,7 @@ static void
 format_part_content_raw (GMimeObject *part);
 
 static const notmuch_show_format_t format_raw = {
-    "",
+    "", NULL,
 	"", NULL,
 	    "", NULL, format_headers_message_part_text, "\n",
             "",
@@ -744,6 +744,20 @@ show_message (void *ctx,
 	      int indent,
 	      notmuch_show_params_t *params)
 {
+    if (format->part) {
+	void *local = talloc_new (ctx);
+	mime_node_t *root, *part;
+
+	if (mime_node_open (local, message, params->cryptoctx, params->decrypt,
+			    &root) != NOTMUCH_STATUS_SUCCESS)
+	    return;
+	part = mime_node_seek_dfs (root, params->part < 0 ? 0 : params->part);
+	if (part)
+	    format->part (local, part, indent, params);
+	talloc_free (local);
+	return;
+    }
+
     if (params->part <= 0) {
 	fputs (format->message_start, stdout);
 	if (format->message)
-- 
1.7.7.3