mainchan.c: rewrite handling of open-failure aborts.

This is another case where a stale pointer bug could have arisen from
a toplevel callback going off after an object was freed.

But here, just adding delete_callbacks_for_context wouldn't help. The
actual context parameter for the callback wasn't mainchan itself; it
was a tiny separate object, allocated to hold just the parameters of
the function the callback wanted to call. So if _those_ parameters
became stale before the callback was triggered, then even
delete_callbacks_for_context wouldn't have been able to help.

Also, mainchan itself would have been freed moments after this
callback was queued, so moving it to be a callback on mainchan itself
wouldn't help.

Solution: move the callback right out to Ssh, by introducing a new
ssh_sw_abort_deferred() which is just like ssh_sw_abort but does its
main work in a toplevel callback. Then ssh.c's existing call to
delete_callbacks_for_context will clean it up if necessary.

mainchan.c

@@ -334,33 +334,13 @@ static void mainchan_ready(mainchan *mc)
     queue_idempotent_callback(&mc->ppl->ic_process_queue);
 }
 
-struct mainchan_open_failure_abort_ctx {
-    Ssh *ssh;
-    char *abort_message;
-};
-
-static void mainchan_open_failure_abort(void *vctx)
-{
-    struct mainchan_open_failure_abort_ctx *ctx =
-        (struct mainchan_open_failure_abort_ctx *)vctx;
-    ssh_sw_abort(
-        ctx->ssh, "Server refused to open main channel: %s",
-        ctx->abort_message);
-    sfree(ctx->abort_message);
-    sfree(ctx);
-}
-
 static void mainchan_open_failure(Channel *chan, const char *errtext)
 {
     assert(chan->vt == &mainchan_channelvt);
     mainchan *mc = container_of(chan, mainchan, chan);
 
-    struct mainchan_open_failure_abort_ctx *ctx =
-        snew(struct mainchan_open_failure_abort_ctx);
-
-    ctx->ssh = mc->ppl->ssh;
-    ctx->abort_message = dupstr(errtext);
-    queue_toplevel_callback(mainchan_open_failure_abort, ctx);
+    ssh_sw_abort_deferred(mc->ppl->ssh,
+                          "Server refused to open main channel: %s", errtext);
 }
 
 static size_t mainchan_send(Channel *chan, bool is_stderr,

ssh.c

@@ -133,6 +133,8 @@ struct Ssh {
 
     Pinger *pinger;
 
+    char *deferred_abort_message;
+
     bool need_random_unref;
 };
 
@@ -555,6 +557,24 @@ void ssh_user_close(Ssh *ssh, const char *fmt, ...)
     }
 }
 
+void ssh_deferred_abort_callback(void *vctx)
+{
+    Ssh *ssh = (Ssh *)vctx;
+    char *msg = ssh->deferred_abort_message;
+    ssh->deferred_abort_message = NULL;
+    ssh_sw_abort(ssh, msg);
+    sfree(msg);
+}
+
+void ssh_sw_abort_deferred(Ssh *ssh, const char *fmt, ...)
+{
+    if (!ssh->deferred_abort_message) {
+        GET_FORMATTED_MSG;
+        ssh->deferred_abort_message = msg;
+        queue_toplevel_callback(ssh_deferred_abort_callback, ssh);
+    }
+}
+
 static void ssh_socket_log(Plug *plug, int type, SockAddr *addr, int port,
                            const char *error_msg, int error_code)
 {
@@ -915,6 +935,8 @@ static void ssh_free(Backend *be)
 	ssh_gss_cleanup(ssh->gss_state.libs);
 #endif
 
+    sfree(ssh->deferred_abort_message);
+
     delete_callbacks_for_context(ssh); /* likely to catch ic_out_raw */
 
     need_random_unref = ssh->need_random_unref;

ssh.h

@@ -407,6 +407,7 @@ void ssh_remote_error(Ssh *ssh, const char *fmt, ...);
 void ssh_remote_eof(Ssh *ssh, const char *fmt, ...);
 void ssh_proto_error(Ssh *ssh, const char *fmt, ...);
 void ssh_sw_abort(Ssh *ssh, const char *fmt, ...);
+void ssh_sw_abort_deferred(Ssh *ssh, const char *fmt, ...);
 void ssh_user_close(Ssh *ssh, const char *fmt, ...);
 
 /* Bit positions in the SSH-1 cipher protocol word */