Make asynchronous agent_query() requests cancellable.

Now, instead of returning a boolean indicating whether the query has
completed or is still pending, agent_query() returns NULL to indicate
that the query _has_ completed, and if it hasn't, it returns a pointer
to a context structure representing the pending query, so that the
latter can be used to cancel the query if (for example) you later
decide you need to free the thing its callback was using as a context.

This should fix a potential race-condition segfault if you overload an
agent forwarding channel and then close it abruptly. (Which nobody
will be doing for sensible purposes, of course! But I ran across this
while stress-testing other aspects of agent forwarding.)

unix/uxagentc.c

@@ -23,8 +23,8 @@ int agent_exists(void)
     return FALSE;
 }
 
-static tree234 *agent_connections;
-struct agent_connection {
+static tree234 *agent_pending_queries;
+struct agent_pending_query {
     int fd;
     char *retbuf;
     char sizebuf[4];
@@ -34,8 +34,8 @@ struct agent_connection {
 };
 static int agent_conncmp(void *av, void *bv)
 {
-    struct agent_connection *a = (struct agent_connection *) av;
-    struct agent_connection *b = (struct agent_connection *) bv;
+    agent_pending_query *a = (agent_pending_query *) av;
+    agent_pending_query *b = (agent_pending_query *) bv;
     if (a->fd < b->fd)
 	return -1;
     if (a->fd > b->fd)
@@ -45,7 +45,7 @@ static int agent_conncmp(void *av, void *bv)
 static int agent_connfind(void *av, void *bv)
 {
     int afd = *(int *) av;
-    struct agent_connection *b = (struct agent_connection *) bv;
+    agent_pending_query *b = (agent_pending_query *) bv;
     if (afd < b->fd)
 	return -1;
     if (afd > b->fd)
@@ -59,7 +59,7 @@ static int agent_connfind(void *av, void *bv)
  * (conn->retbuf non-NULL and filled with something useful) or has
  * failed totally (conn->retbuf is NULL).
  */
-static int agent_try_read(struct agent_connection *conn)
+static int agent_try_read(agent_pending_query *conn)
 {
     int ret;
 
@@ -90,13 +90,21 @@ static int agent_try_read(struct agent_connection *conn)
     return 1;
 }
 
+void agent_cancel_query(agent_pending_query *conn)
+{
+    uxsel_del(conn->fd);
+    close(conn->fd);
+    del234(agent_pending_queries, conn);
+    sfree(conn);
+}
+
 static int agent_select_result(int fd, int event)
 {
-    struct agent_connection *conn;
+    agent_pending_query *conn;
 
     assert(event == 1);		       /* not selecting for anything but R */
 
-    conn = find234(agent_connections, &fd, agent_connfind);
+    conn = find234(agent_pending_queries, &fd, agent_connfind);
     if (!conn) {
 	uxsel_del(fd);
 	return 1;
@@ -111,21 +119,19 @@ static int agent_select_result(int fd, int event)
      * of that passes to the callback.)
      */
     conn->callback(conn->callback_ctx, conn->retbuf, conn->retlen);
-    uxsel_del(fd);
-    close(fd);
-    del234(agent_connections, conn);
-    sfree(conn);
+    agent_cancel_query(conn);
     return 0;
 }
 
-int agent_query(void *in, int inlen, void **out, int *outlen,
-		void (*callback)(void *, void *, int), void *callback_ctx)
+agent_pending_query *agent_query(
+    void *in, int inlen, void **out, int *outlen,
+    void (*callback)(void *, void *, int), void *callback_ctx)
 {
     char *name;
     int sock;
     struct sockaddr_un addr;
     int done;
-    struct agent_connection *conn;
+    agent_pending_query *conn;
 
     name = getenv("SSH_AUTH_SOCK");
     if (!name)
@@ -155,7 +161,7 @@ int agent_query(void *in, int inlen, void **out, int *outlen,
 	done += ret;
     }
 
-    conn = snew(struct agent_connection);
+    conn = snew(agent_pending_query);
     conn->fd = sock;
     conn->retbuf = conn->sizebuf;
     conn->retsize = 4;
@@ -179,7 +185,7 @@ int agent_query(void *in, int inlen, void **out, int *outlen,
         *out = conn->retbuf;
         *outlen = conn->retlen;
         sfree(conn);
-        return 1;
+        return NULL;
     }
 
     /*
@@ -188,15 +194,15 @@ int agent_query(void *in, int inlen, void **out, int *outlen,
      * response hasn't been received yet, and call the callback when
      * select_result comes back to us.
      */
-    if (!agent_connections)
-	agent_connections = newtree234(agent_conncmp);
-    add234(agent_connections, conn);
+    if (!agent_pending_queries)
+	agent_pending_queries = newtree234(agent_conncmp);
+    add234(agent_pending_queries, conn);
 
     uxsel_set(sock, 1, agent_select_result);
-    return 0;
+    return conn;
 
     failure:
     *out = NULL;
     *outlen = 0;
-    return 1;
+    return NULL;
 }