Initial DAP implementation

So far only basic debugging functionality is implemented (breakpoints,
stepping, stack traces/variables).

This is not tested against an established DAP client. My intention is to
create a custom GUI frontend and extend the protocol to support
xsystem4-specific features, such as inspecting the scene. (This work is
underway.)

Instruction references are hex-encoded address strings. Clients do not
need to obtain them from DAP requests. This is outside the spec, but it
is needed for a binary-only debugger.

Functions that write directly to stdout (e.g. printf) should generally
not be used any more.  Instead, use either sys_message (which respects
sys_silent) or log_message (which will send the message to the debugger
if DAP is enabled). stderr can be used as normal.

src/msgqueue.c

@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2021 kichikuou <KichikuouChrome@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ */
+
+#include <stdlib.h>
+#include "msgqueue.h"
+
+struct msgq_elem {
+	void *msg;
+	struct msgq_elem *next;
+};
+
+struct msgq *msgq_new(void) {
+	struct msgq *q = calloc(1, sizeof(struct msgq));
+	q->mutex = SDL_CreateMutex();
+	q->cond_nonempty = SDL_CreateCond();
+	return q;
+}
+
+void msgq_free(struct msgq *q) {
+	SDL_DestroyMutex(q->mutex);
+	SDL_DestroyCond(q->cond_nonempty);
+	free(q);
+}
+
+void msgq_enqueue(struct msgq *q, void *msg) {
+	struct msgq_elem *e = malloc(sizeof(struct msgq_elem));
+	e->msg = msg;
+	e->next = NULL;
+
+	SDL_LockMutex(q->mutex);
+	if (!q->head) {
+		q->head = q->last = e;
+	} else {
+		q->last->next = e;
+		q->last = e;
+	}
+	SDL_UnlockMutex(q->mutex);
+	SDL_CondSignal(q->cond_nonempty);
+}
+
+void *msgq_dequeue(struct msgq *q) {
+	SDL_LockMutex(q->mutex);
+	while (!q->head)
+		SDL_CondWait(q->cond_nonempty, q->mutex);
+
+	struct msgq_elem *e = q->head;
+	q->head = e->next;
+	if (!e->next)
+		q->last = NULL;
+
+	SDL_UnlockMutex(q->mutex);
+
+	void *msg = e->msg;
+	free(e);
+	return msg;
+}
+
+void *msgq_dequeue_timeout(struct msgq *q, uint32_t timeout_ms) {
+	SDL_LockMutex(q->mutex);
+
+	while (!q->head && SDL_CondWaitTimeout(q->cond_nonempty, q->mutex, timeout_ms) == 0)
+		;
+
+	if (!q->head) {  // timed out
+		SDL_UnlockMutex(q->mutex);
+		return NULL;
+	}
+
+	struct msgq_elem *e = q->head;
+	q->head = e->next;
+	if (!e->next)
+		q->last = NULL;
+
+	SDL_UnlockMutex(q->mutex);
+
+	void *msg = e->msg;
+	free(e);
+	return msg;
+}