[PATCH v3 3/5] util/qemu-sockets: Refactor success and failure paths in inet_listen_saddr()

[Juraj Marcin <jmarcin@redhat.com>]

From: Juraj Marcin <jmarcin@redhat.com>

To get a listening socket, we need to first create a socket, try binding
it to a certain port, and lastly starting listening to it. Each of these
operations can fail due to various reasons, one of them being that the
requested address/port is already in use. In such case, the function
tries the same process with a new port number.

This patch refactors the port number loop, so the success path is no
longer buried inside the 'if' statements in the middle of the loop. Now,
the success path is not nested and ends at the end of the iteration
after successful socket creation, binding, and listening. In case any of
the operations fails, it either continues to the next iteration (and the
next port) or jumps out of the loop to handle the error and exits the
function.

Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
---
 util/qemu-sockets.c | 51 ++++++++++++++++++++++++---------------------
 1 file changed, 27 insertions(+), 24 deletions(-)

diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index d15f6aa4b0..a86964786a 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -303,11 +303,20 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
         port_min = inet_getport(e);
         port_max = saddr->has_to ? saddr->to + port_offset : port_min;
         for (p = port_min; p <= port_max; p++) {
+            if (slisten >= 0) {
+                /*
+                 * We have a socket we tried with the previous port. It cannot
+                 * be rebound, we need to close it and create a new one.
+                 */
+                close(slisten);
+                slisten = -1;
+            }
             inet_setport(e, p);
 
             slisten = create_fast_reuse_socket(e);
             if (slisten < 0) {
-                /* First time we expect we might fail to create the socket
+                /*
+                 * First time we expect we might fail to create the socket
                  * eg if 'e' has AF_INET6 but ipv6 kmod is not loaded.
                  * Later iterations should always succeed if first iteration
                  * worked though, so treat that as fatal.
@@ -317,40 +326,38 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
                 } else {
                     error_setg_errno(errp, errno,
                                      "Failed to recreate failed listening socket");
-                    goto listen_failed;
+                    goto fail;
                 }
             }
             socket_created = true;
 
             rc = try_bind(slisten, saddr, e);
             if (rc < 0) {
-                if (errno != EADDRINUSE) {
-                    error_setg_errno(errp, errno, "Failed to bind socket");
-                    goto listen_failed;
+                if (errno == EADDRINUSE) {
+                    /* This port is already used, try the next one */
+                    continue;
                 }
-            } else {
-                if (!listen(slisten, num)) {
-                    goto listen_ok;
-                }
-                if (errno != EADDRINUSE) {
-                    error_setg_errno(errp, errno, "Failed to listen on socket");
-                    goto listen_failed;
+                error_setg_errno(errp, errno, "Failed to bind socket");
+                goto fail;
+            }
+            if (listen(slisten, num)) {
+                if (errno == EADDRINUSE) {
+                    /* This port is already used, try the next one */
+                    continue;
                 }
+                error_setg_errno(errp, errno, "Failed to listen on socket");
+                goto fail;
             }
-            /* Someone else managed to bind to the same port and beat us
-             * to listen on it! Socket semantics does not allow us to
-             * recover from this situation, so we need to recreate the
-             * socket to allow bind attempts for subsequent ports:
-             */
-            close(slisten);
-            slisten = -1;
+            /* We have a listening socket */
+            freeaddrinfo(res);
+            return slisten;
         }
     }
     error_setg_errno(errp, errno,
                      socket_created ?
                      "Failed to find an available port" :
                      "Failed to create a socket");
-listen_failed:
+fail:
     saved_errno = errno;
     if (slisten >= 0) {
         close(slisten);
@@ -358,10 +365,6 @@ listen_failed:
     freeaddrinfo(res);
     errno = saved_errno;
     return -1;
-
-listen_ok:
-    freeaddrinfo(res);
-    return slisten;
 }
 
 #ifdef _WIN32
-- 
2.48.1