Introduce socket shutdown into connection type, used if a fork is active (#11376)

Introduce socket `shutdown()` into connection type, and use it
on normal socket if a fork is active. This allows us to close
client connections when there are child processes sharing the
file descriptors.

Fixes #10077. The reason is that since the `fork()` child is holding
the file descriptors, the `close` in `unlinkClient -> connClose`
isn't sufficient. The client will not realize that the connection is
disconnected until the child process ends.

Let's try to be conservative and only use shutdown when the fork is active.

src/connection.h

@@ -80,9 +80,10 @@ typedef struct ConnectionType {
     int (*addr)(connection *conn, char *ip, size_t ip_len, int *port, int remote);
     int (*listen)(connListener *listener);
 
-    /* create/close connection */
+    /* create/shutdown/close connection */
     connection* (*conn_create)(void);
     connection* (*conn_create_accepted)(int fd, void *priv);
+    void (*shutdown)(struct connection *conn);
     void (*close)(struct connection *conn);
 
     /* connect & accept */
@@ -240,6 +241,10 @@ static inline int connSetWriteHandlerWithBarrier(connection *conn, ConnectionCal
     return conn->type->set_write_handler(conn, func, barrier);
 }
 
+static inline void connShutdown(connection *conn) {
+    conn->type->shutdown(conn);
+}
+
 static inline void connClose(connection *conn) {
     conn->type->close(conn);
 }

src/networking.c

@@ -1448,6 +1448,8 @@ void unlinkClient(client *c) {
                 }
             }
         }
+        /* Only use shutdown when the fork is active and we are the parent. */
+        if (server.child_type) connShutdown(c->conn);
         connClose(c->conn);
         c->conn = NULL;
     }

src/socket.c

@@ -125,6 +125,12 @@ static int connSocketConnect(connection *conn, const char *addr, int port, const
  * move here as we implement additional connection types.
  */
 
+static void connSocketShutdown(connection *conn) {
+    if (conn->fd == -1) return;
+
+    shutdown(conn->fd, SHUT_RDWR);
+}
+
 /* Close the connection and free resources. */
 static void connSocketClose(connection *conn) {
     if (conn->fd != -1) {
@@ -388,9 +394,10 @@ static ConnectionType CT_Socket = {
     .addr = connSocketAddr,
     .listen = connSocketListen,
 
-    /* create/close connection */
+    /* create/shutdown/close connection */
     .conn_create = connCreateSocket,
     .conn_create_accepted = connCreateAcceptedSocket,
+    .shutdown = connSocketShutdown,
     .close = connSocketClose,
 
     /* connect & accept */

src/tls.c

@@ -748,6 +748,19 @@ static int connTLSListen(connListener *listener) {
     return listenToPort(listener);
 }
 
+static void connTLSShutdown(connection *conn_) {
+    tls_connection *conn = (tls_connection *) conn_;
+
+    if (conn->ssl) {
+        if (conn->c.state == CONN_STATE_CONNECTED)
+            SSL_shutdown(conn->ssl);
+        SSL_free(conn->ssl);
+        conn->ssl = NULL;
+    }
+
+    connectionTypeTcp()->shutdown(conn_);
+}
+
 static void connTLSClose(connection *conn_) {
     tls_connection *conn = (tls_connection *) conn_;
 
@@ -1103,9 +1116,10 @@ static ConnectionType CT_TLS = {
     .addr = connTLSAddr,
     .listen = connTLSListen,
 
-    /* create/close connection */
+    /* create/shutdown/close connection */
     .conn_create = connCreateTLS,
     .conn_create_accepted = connCreateAcceptedTLS,
+    .shutdown = connTLSShutdown,
     .close = connTLSClose,
 
     /* connect & accept */

src/unix.c

@@ -103,6 +103,10 @@ static void connUnixAcceptHandler(aeEventLoop *el, int fd, void *privdata, int m
     }
 }
 
+static void connUnixShutdown(connection *conn) {
+    connectionTypeTcp()->shutdown(conn);
+}
+
 static void connUnixClose(connection *conn) {
     connectionTypeTcp()->close(conn);
 }
@@ -162,9 +166,10 @@ static ConnectionType CT_Unix = {
     .addr = connUnixAddr,
     .listen = connUnixListen,
 
-    /* create/close connection */
+    /* create/shutdown/close connection */
     .conn_create = connCreateUnix,
     .conn_create_accepted = connCreateAcceptedUnix,
+    .shutdown = connUnixShutdown,
     .close = connUnixClose,
 
     /* connect & accept */

tests/unit/introspection.tcl

@@ -54,6 +54,44 @@ start_server {tags {"introspection"}} {
         # After killing `me`, the first ping will throw an error
         assert_error "*I/O error*" {r ping}
         assert_equal "PONG" [r ping]
+
+        $rd1 close
+        $rd2 close
+        $rd3 close
+        $rd4 close
+    }
+
+    test "CLIENT KILL close the client connection during bgsave" {
+        # Start a slow bgsave, trigger an active fork.
+        r flushall
+        r set k v
+        r config set rdb-key-save-delay 10000000
+        r bgsave
+        wait_for_condition 1000 10 {
+            [s rdb_bgsave_in_progress] eq 1
+        } else {
+            fail "bgsave did not start in time"
+        }
+
+        # Kill (close) the connection
+        r client kill skipme no
+
+        # In the past, client connections needed to wait for bgsave
+        # to end before actually closing, now they are closed immediately.
+        assert_error "*I/O error*" {r ping} ;# get the error very quickly
+        assert_equal "PONG" [r ping]
+
+        # Make sure the bgsave is still in progress
+        assert_equal [s rdb_bgsave_in_progress] 1
+
+        # Stop the child before we proceed to the next test
+        r config set rdb-key-save-delay 0
+        r flushall
+        wait_for_condition 1000 10 {
+            [s rdb_bgsave_in_progress] eq 0
+        } else {
+            fail "bgsave did not stop in time"
+        }
     }
 
     test {MONITOR can log executed commands} {