xC: implement buffer completion in the relay

And actually support completion with non-UTF-8 locales.
We used to ignore the encoding conversion result.

xC.c

@@ -13271,12 +13271,6 @@ process_input (struct app_context *ctx, char *user_input)
 // The amount of crap that goes into this is truly insane.
 // It's mostly because of Editline's total ignorance of this task.
 
-static void
-completion_init (struct completion *self)
-{
-	memset (self, 0, sizeof *self);
-}
-
 static void
 completion_free (struct completion *self)
 {
@@ -13295,13 +13289,13 @@ completion_add_word (struct completion *self, size_t start, size_t end)
 	self->words[self->words_len++] = (struct completion_word) { start, end };
 }
 
-static void
+static struct completion
-completion_parse (struct completion *self, const char *line, size_t len)
+completion_make (const char *line, size_t len)
 {
-	self->line = xstrndup (line, len);
+	struct completion self = { .line = xstrndup (line, len) };
 
 	// The first and the last word may be empty
-	const char *s = self->line;
+	const char *s = self.line;
 	while (true)
 	{
 		const char *start = s;
@@ -13309,10 +13303,11 @@ completion_parse (struct completion *self, const char *line, size_t len)
 		const char *end = start + word_len;
 		s = end + strspn (end, WORD_BREAKING_CHARS);
 
-		completion_add_word (self, start - self->line, end - self->line);
+		completion_add_word (&self, start - self.line, end - self.line);
 		if (s == end)
 			break;
 	}
+	return self;
 }
 
 static void
@@ -13486,14 +13481,13 @@ complete_set (struct app_context *ctx, struct completion *data,
 }
 
 static void
-complete_topic (struct app_context *ctx, struct completion *data,
+complete_topic (struct buffer *buffer, struct completion *data,
 	const char *word, struct strv *output)
 {
 	(void) data;
 
 	// TODO: make it work in other server-related buffers, too, i.e. when we're
 	//   completing the third word and the second word is a known channel name
-	struct buffer *buffer = ctx->current_buffer;
 	if (buffer->type != BUFFER_CHANNEL)
 		return;
 
@@ -13509,10 +13503,9 @@ complete_topic (struct app_context *ctx, struct completion *data,
 }
 
 static void
-complete_nicknames (struct app_context *ctx, struct completion *data,
+complete_nicknames (struct buffer *buffer, struct completion *data,
 	const char *word, struct strv *output)
 {
-	struct buffer *buffer = ctx->current_buffer;
 	if (buffer->type == BUFFER_SERVER)
 	{
 		struct user *self_user = buffer->server->irc_user;
@@ -13534,9 +13527,9 @@ complete_nicknames (struct app_context *ctx, struct completion *data,
 	}
 }
 
-static char **
+static struct strv
-complete_word (struct app_context *ctx, struct completion *data,
+complete_word (struct app_context *ctx, struct buffer *buffer,
-	const char *word)
+	struct completion *data, const char *word)
 {
 	char *initial = completion_word (data, 0);
 
@@ -13555,11 +13548,11 @@ complete_word (struct app_context *ctx, struct completion *data,
 	}
 	else if (data->location == 1 && !strcmp (initial, "/topic"))
 	{
-		complete_topic     (ctx, data, word, &words);
+		complete_topic     (buffer, data, word, &words);
-		complete_nicknames (ctx, data, word, &words);
+		complete_nicknames (buffer, data, word, &words);
 	}
 	else
-		complete_nicknames (ctx, data, word, &words);
+		complete_nicknames (buffer, data, word, &words);
 
 	cstr_set (&initial, NULL);
 	LIST_FOR_EACH (struct hook, iter, ctx->completion_hooks)
@@ -13568,17 +13561,12 @@ complete_word (struct app_context *ctx, struct completion *data,
 		hook->complete (hook, data, word, &words);
 	}
 
-	if (words.len == 1)
+	if (words.len <= 2)
-	{
-		// Nothing matched
-		strv_free (&words);
-		return NULL;
-	}
-
-	if (words.len == 2)
 	{
+		// When nothing matches, this copies the sentinel value
 		words.vector[0] = words.vector[1];
 		words.vector[1] = NULL;
+		words.len--;
 	}
 	else
 	{
@@ -13589,7 +13577,7 @@ complete_word (struct app_context *ctx, struct completion *data,
 		else
 			words.vector[0] = xstrndup (words.vector[1], prefix);
 	}
-	return words.vector;
+	return words;
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
@@ -13655,26 +13643,26 @@ locale_to_utf8 (struct app_context *ctx, const char *locale,
 	return str_steal (&utf8);
 }
 
-static void
-utf8_vector_to_locale (struct app_context *ctx, char **vector)
-{
-	for (; *vector; vector++)
-	{
-		char *converted = iconv_xstrdup
-			(ctx->term_from_utf8, *vector, -1, NULL);
-		if (!soft_assert (converted))
-			converted = xstrdup ("");
-
-		cstr_set (vector, converted);
-	}
-}
-
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
+static struct strv
+make_completions (struct app_context *ctx, struct buffer *buffer,
+	const char *line_utf8, size_t start, size_t end)
+{
+	struct completion comp = completion_make (line_utf8, strlen (line_utf8));
+	completion_locate (&comp, start);
+	char *word = xstrndup (line_utf8 + start, end - start);
+	struct strv completions = complete_word (ctx, buffer, &comp, word);
+	free (word);
+	completion_free (&comp);
+	return completions;
+}
+
 /// Takes a line in locale-specific encoding and position of a word to complete,
 /// returns a vector of matches in locale-specific encoding.
 static char **
-make_completions (struct app_context *ctx, char *line, int start, int end)
+make_input_completions
+	(struct app_context *ctx, const char *line, int start, int end)
 {
 	int *fixes[] = { &start, &end };
 	char *line_utf8 = locale_to_utf8 (ctx, line, fixes, N_ELEMENTS (fixes));
@@ -13683,20 +13671,23 @@ make_completions (struct app_context *ctx, char *line, int start, int end)
 
 	hard_assert (start >= 0 && end >= 0 && start <= end);
 
-	struct completion c;
+	struct strv completions =
-	completion_init (&c);
+		make_completions (ctx, ctx->current_buffer, line_utf8, start, end);
-	completion_parse (&c, line, strlen (line));
-	completion_locate (&c, start);
-	char *word = xstrndup (line + start, end - start);
-	char **completions = complete_word (ctx, &c, word);
-	free (word);
-	completion_free (&c);
-
-	if (completions)
-		utf8_vector_to_locale (ctx, completions);
-
 	free (line_utf8);
-	return completions;
+	if (!completions.len)
+	{
+		strv_free (&completions);
+		return NULL;
+	}
+	for (size_t i = 0; i < completions.len; i++)
+	{
+		char *converted = iconv_xstrdup
+			(ctx->term_from_utf8, completions.vector[i], -1, NULL);
+		if (!soft_assert (converted))
+			converted = xstrdup ("?");
+		cstr_set (&completions.vector[i], converted);
+	}
+	return completions.vector;
 }
 
 // --- Common code for user actions --------------------------------------------
@@ -14381,7 +14372,7 @@ app_readline_completion (const char *text, int start, int end)
 	// Don't iterate over filenames and stuff
 	rl_attempted_completion_over = true;
 
-	return make_completions (g_ctx, rl_line_buffer, start, end);
+	return make_input_completions (g_ctx, rl_line_buffer, start, end);
 }
 
 static int
@@ -14425,8 +14416,6 @@ static unsigned char
 on_editline_complete (EditLine *editline, int key)
 {
 	(void) key;
-	(void) editline;
-
 	struct app_context *ctx = g_ctx;
 
 	// First prepare what Readline would have normally done for us...
@@ -14440,7 +14429,7 @@ on_editline_complete (EditLine *editline, int key)
 	while (el_start && !strchr (WORD_BREAKING_CHARS, copy[el_start - 1]))
 		el_start--;
 
-	char **completions = make_completions (ctx, copy, el_start, el_end);
+	char **completions = make_input_completions (ctx, copy, el_start, el_end);
 
 	// XXX: possibly incorrect wrt. shift state encodings
 	copy[el_end] = '\0';
@@ -15178,12 +15167,56 @@ client_message_buffer_name (const struct relay_command_message *m)
 	}
 }
 
+static void
+client_process_buffer_complete (struct client *c, uint32_t seq,
+	struct buffer *buffer, struct relay_command_data_buffer_complete *req)
+{
+	struct str *line = &req->text;
+	uint32_t end = req->position;
+	if (line->len < end || line->len != strlen (line->str))
+	{
+		relay_prepare_error (c->ctx, seq, "Invalid arguments");
+		goto out;
+	}
+
+	uint32_t start = end;
+	while (start && !strchr (WORD_BREAKING_CHARS, line->str[start - 1]))
+		start--;
+
+	struct strv completions =
+		make_completions (c->ctx, buffer, line->str, start, end);
+	if (completions.len > UINT32_MAX)
+	{
+		relay_prepare_error (c->ctx, seq, "Internal error");
+		goto out_internal;
+	}
+
+	struct relay_event_data_response *e =
+		&relay_prepare (c->ctx)->data.response;
+	e->event = RELAY_EVENT_RESPONSE;
+	e->command_seq = seq;
+	e->data.command = RELAY_COMMAND_BUFFER_COMPLETE;
+
+	struct relay_response_data_buffer_complete *resp =
+		&e->data.buffer_complete;
+	resp->start = start;
+	resp->completions_len = completions.len;
+	resp->completions = xcalloc (completions.len, sizeof *resp->completions);
+	for (size_t i = 0; i < completions.len; i++)
+		resp->completions[i] = str_from_cstr (completions.vector[i]);
+
+out_internal:
+	strv_free (&completions);
+out:
+	relay_send (c);
+}
+
 static void
 client_process_buffer_log
 	(struct client *c, uint32_t seq, struct buffer *buffer)
 {
-	struct relay_event_message *m = relay_prepare (c->ctx);
+	struct relay_event_data_response *e =
-	struct relay_event_data_response *e = &m->data.response;
+		&relay_prepare (c->ctx)->data.response;
 	e->event = RELAY_EVENT_RESPONSE;
 	e->command_seq = seq;
 	e->data.command = RELAY_COMMAND_BUFFER_LOG;
@@ -15254,9 +15287,8 @@ client_process_message (struct client *c,
 		reset_autoaway (c->ctx);
 		break;
 	case RELAY_COMMAND_BUFFER_COMPLETE:
-		// TODO: Run the completion machinery.
+		client_process_buffer_complete (c, m->command_seq, buffer,
-		relay_prepare_error (c->ctx, m->command_seq, "Not implemented");
+			&m->data.buffer_complete);
-		relay_send (c);
 		break;
 	case RELAY_COMMAND_BUFFER_INPUT:
 		(void) process_input_utf8 (c->ctx,