lib-http: guard against hshared use-after-free

This fixes a race condition where the http_client_host_shared_idle_timeout()
function would get called with an already freed hshared argument.

Specifically, the situation arises from the hshared idle timeout calling
http_client_host_shared_free(), which removes the timeout and then proceeds to
free the client queue.  The client queue freeing code indirectly calls
http_client_host_shared_check_idle(), which notices that there is no idle
timeout and allocates one.

The backtrace at the point of this new timeout allocation:

    frame #3: 0x00007f0c775897f0 libdovecot.so.0`timeout_add_to(...) ioloop.c:280
    frame #4: 0x00007f0c7751a45f libdovecot.so.0`http_client_host_shared_check_idle(hshared=<unavailable>) at http-client-host.c:69
    frame #5: 0x00007f0c7750de89 libdovecot.so.0`http_client_request_error(_req=<unavailable>, status=9000, error="") at http-client-request.c:1525
    frame #6: 0x00007f0c77517f38 libdovecot.so.0`http_client_queue_fail_full(queue=0x000055e13cff0e10, status=9000, error="", all=<unavailable>) at http-client-queue.c:183
    frame #7: 0x00007f0c77518baa libdovecot.so.0`http_client_queue_free(queue=0x000055e13cff0e10) at http-client-queue.c:141
    frame #8: 0x00007f0c7751a8bc libdovecot.so.0`http_client_host_free_shared(_host=<unavailable>) at http-client-host.c:391
    frame #9: 0x00007f0c7751ab4c libdovecot.so.0`http_client_host_shared_free(_hshared=0x00007ffdac109e48) at http-client-host.c:294
    frame #10: 0x00007f0c7751ace8 libdovecot.so.0`http_client_host_shared_idle_timeout(hshared=<unavailable>) at http-client-host.c:40
    frame #11: 0x00007f0c7758a1a4 libdovecot.so.0`io_loop_handle_timeouts at ioloop.c:682
    frame #12: 0x00007f0c7758a089 libdovecot.so.0`io_loop_handle_timeouts(ioloop=0x000055e13cfc8d80) at ioloop.c:696
    frame #13: 0x00007f0c7758befc libdovecot.so.0`io_loop_handler_run_internal(ioloop=0x000055e13cfc8d80) at ioloop-select.c:126
    frame #14: 0x00007f0c7758a56d libdovecot.so.0`io_loop_handler_run(ioloop=<unavailable>) at ioloop.c:767
    frame #15: 0x00007f0c7758a798 libdovecot.so.0`io_loop_run(ioloop=0x000055e13cfc8d80) at ioloop.c:740
    frame #16: 0x00007f0c774f61eb libdovecot.so.0`master_service_run(service=0x000055e13cfc8c10, callback=<unavailable>) at master-service.c:782
    frame #17: 0x000055e13b48e3a5 stats`main(argc=<unavailable>, argv=<unavailable>) at main.c:99
    frame #18: 0x00007f0c771092e1 libc.so.6`__libc_start_main + 241
    frame #19: 0x000055e13b48e41a stats`_start + 42

Author: stephanbosch

src/lib-http/http-client-host.c

@@ -44,6 +44,8 @@ http_client_host_shared_check_idle(
 	struct http_client_host *host;
 	int timeout = 0;
 
+	if (hshared->destroyed)
+		return;
 	if (hshared->to_idle != NULL)
 		return;
 
@@ -266,6 +268,10 @@ void http_client_host_shared_free(struct http_client_host_shared **_hshared)
 	struct http_client_host *host;
 	const char *hostname = hshared->name;
 
+	if (hshared->destroyed)
+		return;
+	hshared->destroyed = TRUE;
+
 	e_debug(hshared->event, "Host destroy");
 
 	timeout_remove(&hshared->to_idle);

src/lib-http/http-client-private.h

@@ -344,6 +344,7 @@ struct http_client_host_shared {
 	/* timeouts */
 	struct timeout *to_idle;
 
+	bool destroyed:1;	/* shared host object is being destroyed */
 	bool unix_local:1;
 	bool explicit_ip:1;
 };