Mailcap on slave terminals seems to work right.

Mailcap is AFAIK specific to UNIX like systems, so using shared memory
and semaphores is allowed. Using IPC it was easy.
IMHO, without shared memory it is much harder to do.
The code needs clenup, checking for header files etc.

src/mime/backend/mailcap.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE /* stpcpy */
 /* RFC1524 (mailcap file) implementation */
 
 /* This file contains various functions for implementing a fair subset of
@@ -34,13 +35,19 @@
 #include "mime/mime.h"
 #include "osdep/osdep.h"		/* For exe() */
 #include "session/session.h"
+#include "terminal/hardio.h"
 #include "terminal/terminal.h"
 #include "util/file.h"
 #include "util/hash.h"
 #include "util/lists.h"
 #include "util/memory.h"
+#include "util/sem.h"
 #include "util/string.h"
 
+extern int master_sem;
+extern int slave_sem;
+extern unsigned char *shared_mem;
+
 struct mailcap_hash_item {
 	/* The entries associated with the type */
 	struct list_head entries; /* -> struct mailcap_entry */
@@ -634,7 +641,7 @@ get_mailcap_entry(unsigned char *type)
 }
 
 static struct mime_handler *
-get_mime_handler_mailcap(unsigned char *type, struct terminal *term)
+get_mime_handler_mailcap_common(unsigned char *type)
 {
 	struct mailcap_entry *entry;
 	struct mime_handler *handler;
@@ -661,6 +668,87 @@ get_mime_handler_mailcap(unsigned char *type, struct terminal *term)
 	return handler;
 }
 
+static struct mime_handler *
+get_mime_handler_mailcap(unsigned char *type, struct terminal *term)
+{
+	struct mime_handler *handler;
+	unsigned char *desc, *data;
+	int block, len;
+
+	if (!term || term->master)
+		return get_mime_handler_mailcap_common(type);
+
+	len = strlen(type) + 1;
+	data = fmem_alloc(2 + len);
+	if (!data)
+		return NULL;
+	data[0] = 0;
+	data[1] = 3; /* Mailcap */
+	memcpy(data + 2, type, len);
+	hard_write(term->fdout, data, len + 2);
+	fmem_free(data);
+
+	sem_signal(slave_sem);
+	sem_wait(master_sem);
+	if (!shared_mem || !*shared_mem)
+		return NULL;
+	desc = strchr(shared_mem, '\0') + 1;
+	block = (shared_mem[4095] > 0);
+	handler = init_mime_handler(shared_mem, desc, mailcap_mime_module.name,
+				    get_mailcap_ask(), block);
+	if (handler) handler->copiousoutput = shared_mem[4095] & 1;
+	return handler;
+}
+
+/* Called by slaves. */
+void
+get_slave_mailcap(unsigned char *type)
+{
+	sem_wait(slave_sem);
+	if (shared_mem) {
+		struct mailcap_entry *entry;
+		unsigned char *program;
+		unsigned char *current;
+		int len;
+
+		if (!get_mailcap_enable()
+	    	|| (!mailcap_map && !init_mailcap_map())) {
+			shared_mem[0] = '\0';
+			goto end;
+		}
+
+		entry = get_mailcap_entry(type);
+		if (!entry) {
+			shared_mem[0] = '\0';
+			goto end;
+		}
+
+		program = format_command(entry->command, type, entry->copiousoutput);
+		if (!program) {
+			shared_mem[0] = '\0';
+			goto end;
+		}
+		len = strlen(program) + 1;
+		if (entry->description)
+			len += strlen(entry->description);
+		len++;
+		if (len > 4095) { /* size of the shared memory block */
+			mem_free(program);
+			shared_mem[0] = '\0';
+			goto end;
+		}
+		current = stpcpy(shared_mem, program);
+		mem_free(program);
+		if (entry->description)
+			current = stpcpy(current, entry->description);
+		else
+			*++current = '\0';
+		shared_mem[4095] = (entry->needsterminal << 1) | (entry->copiousoutput << 0);
+		done_mailcap(NULL);
+	}
+end:
+	sem_signal(master_sem);
+}
 
 const struct mime_backend mailcap_mime_backend = {
 	/* get_content_type: */	NULL,

src/mime/backend/mailcap.h

@@ -8,4 +8,6 @@
 extern const struct mime_backend mailcap_mime_backend;
 extern struct module mailcap_mime_module;
 
+void get_slave_mailcap(unsigned char *type);
+
 #endif

src/terminal/kbd.c

@@ -25,6 +25,7 @@
 #include "intl/gettext/libintl.h"
 #include "main/select.h"
 #include "main/timer.h"
+#include "mime/backend/mailcap.h"
 #include "osdep/ascii.h"
 #include "osdep/osdep.h"
 #include "terminal/hardio.h"
@@ -560,6 +561,12 @@ has_nul_byte:
 		add_char_to_string(&path, ch);
 	}
 
+	if (fg == 3) {
+		/* Mailcap */
+		get_slave_mailcap(path.source);
+		goto next_round;
+	}
+
 	if (!init_string(&delete)) {
 		done_string(&path);
 		goto free_and_return;
@@ -622,8 +629,9 @@ has_nul_byte:
 	}
 
 nasty_thing:
-	done_string(&path);
 	done_string(&delete);
+next_round:
+	done_string(&path);
 	assert(ITRM_OUT_QUEUE_SIZE - p > 0);
 	memmove(buf, buf + p, ITRM_OUT_QUEUE_SIZE - p);
 	bytes_read -= p;