Conns: Fix connClose() / connAccept() behavior.

We assume accept handlers may choose to reject a connection and close
it, but connAccept() callers can't distinguish between this state and
other error states requiring connClose().

This makes it safe (and mandatory!) to always call connClose() if
connAccept() fails, and safe for accept handlers to close connections
(which will defer).

src/connection.c

@@ -152,7 +152,7 @@ static void connSocketClose(connection *conn) {
     /* If called from within a handler, schedule the close but
      * keep the connection until the handler returns.
      */
-    if (conn->flags & CONN_FLAG_IN_HANDLER) {
+    if (connHasRefs(conn)) {
         conn->flags |= CONN_FLAG_CLOSE_SCHEDULED;
         return;
     }
@@ -183,10 +183,16 @@ static int connSocketRead(connection *conn, void *buf, size_t buf_len) {
 }
 
 static int connSocketAccept(connection *conn, ConnectionCallbackFunc accept_handler) {
+    int ret = C_OK;
+
     if (conn->state != CONN_STATE_ACCEPTING) return C_ERR;
     conn->state = CONN_STATE_CONNECTED;
-    if (!callHandler(conn, accept_handler)) return C_ERR;
-    return C_OK;
+
+    connIncrRefs(conn);
+    if (!callHandler(conn, accept_handler)) ret = C_ERR;
+    connDecrRefs(conn);
+
+    return ret;
 }
 
 /* Register a write handler, to be called when the connection is writable.

src/connection.h

@@ -45,9 +45,8 @@ typedef enum {
     CONN_STATE_ERROR
 } ConnectionState;
 
-#define CONN_FLAG_IN_HANDLER        (1<<0)      /* A handler execution is in progress */
-#define CONN_FLAG_CLOSE_SCHEDULED   (1<<1)      /* Closed scheduled by a handler */
-#define CONN_FLAG_WRITE_BARRIER     (1<<2)      /* Write barrier requested */
+#define CONN_FLAG_CLOSE_SCHEDULED   (1<<0)      /* Closed scheduled by a handler */
+#define CONN_FLAG_WRITE_BARRIER     (1<<1)      /* Write barrier requested */
 
 typedef void (*ConnectionCallbackFunc)(struct connection *conn);
 
@@ -70,7 +69,8 @@ typedef struct ConnectionType {
 struct connection {
     ConnectionType *type;
     ConnectionState state;
-    int flags;
+    short int flags;
+    short int refs;
     int last_errno;
     void *private_data;
     ConnectionCallbackFunc conn_handler;
@@ -88,6 +88,13 @@ struct connection {
  * connAccept() may directly call accept_handler(), or return and call it
  * at a later time. This behavior is a bit awkward but aims to reduce the need
  * to wait for the next event loop, if no additional handshake is required.
+ *
+ * IMPORTANT: accept_handler may decide to close the connection, calling connClose().
+ * To make this safe, the connection is only marked with CONN_FLAG_CLOSE_SCHEDULED
+ * in this case, and connAccept() returns with an error.
+ *
+ * connAccept() callers must always check the return value and on error (C_ERR)
+ * a connClose() must be called.
  */
 
 static inline int connAccept(connection *conn, ConnectionCallbackFunc accept_handler) {

src/connhelpers.h

@@ -37,46 +37,49 @@
  * implementations (currently sockets in connection.c and TLS in tls.c).
  *
  * Currently helpers implement the mechanisms for invoking connection
- * handlers, tracking in-handler states and dealing with deferred
- * destruction (if invoked by a handler).
+ * handlers and tracking connection references, to allow safe destruction
+ * of connections from within a handler.
  */
 
-/* Called whenever a handler is invoked on a connection and sets the
- * CONN_FLAG_IN_HANDLER flag to indicate we're in a handler context.
+/* Incremenet connection references.
  *
- * An attempt to close a connection while CONN_FLAG_IN_HANDLER is
- * set will result with deferred close, i.e. setting the CONN_FLAG_CLOSE_SCHEDULED
- * instead of destructing it.
+ * Inside a connection handler, we guarantee refs >= 1 so it is always
+ * safe to connClose().
+ *
+ * In other cases where we don't want to prematurely lose the connection,
+ * it can go beyond 1 as well; currently it is only done by connAccept().
  */
-static inline void enterHandler(connection *conn) {
-    conn->flags |= CONN_FLAG_IN_HANDLER;
+static inline void connIncrRefs(connection *conn) {
+    conn->refs++;
 }
 
-/* Called whenever a handler returns. This unsets the CONN_FLAG_IN_HANDLER
- * flag and performs actual close/destruction if a deferred close was
- * scheduled by the handler.
+/* Decrement connection references.
+ *
+ * Note that this is not intended to provide any automatic free logic!
+ * callHandler() takes care of that for the common flows, and anywhere an
+ * explicit connIncrRefs() is used, the caller is expected to take care of
+ * that.
  */
-static inline int exitHandler(connection *conn) {
-    conn->flags &= ~CONN_FLAG_IN_HANDLER;
-    if (conn->flags & CONN_FLAG_CLOSE_SCHEDULED) {
-        connClose(conn);
-        return 0;
-    }
-    return 1;
+
+static inline void connDecrRefs(connection *conn) {
+    conn->refs--;
+}
+
+static inline int connHasRefs(connection *conn) {
+    return conn->refs;
 }
 
 /* Helper for connection implementations to call handlers:
- * 1. Mark the handler in use.
+ * 1. Increment refs to protect the connection.
  * 2. Execute the handler (if set).
- * 3. Mark the handler as NOT in use and perform deferred close if was
- * requested by the handler at any time.
+ * 3. Decrement refs and perform deferred close, if refs==0.
  */
 static inline int callHandler(connection *conn, ConnectionCallbackFunc handler) {
-    conn->flags |= CONN_FLAG_IN_HANDLER;
+    connIncrRefs(conn);
     if (handler) handler(conn);
-    conn->flags &= ~CONN_FLAG_IN_HANDLER;
+    connDecrRefs(conn);
     if (conn->flags & CONN_FLAG_CLOSE_SCHEDULED) {
-        connClose(conn);
+        if (!connHasRefs(conn)) connClose(conn);
         return 0;
     }
     return 1;