[PATCH v1] migration: Wait for cpr.sock file to appear before connecting

[Jaehoon Kim <jhkim@linux.ibm.com>]

When the source VM attempts to connect to the destination VM's Unix
domain socket(cpr.sock) during CPR transfer, the socket file might not
yet be exist if the destination side hasn't completed the bind
operation. This can lead to connection failures when running tests with
the qtest framework.

To address this, add cpr_validate_socket_path(), which wait for the
socket file to appear. This avoids intermittent qtest failures caused by
early connection attempts.

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
---
 include/migration/cpr.h  |  1 +
 migration/cpr-transfer.c | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/migration/cpr.h b/include/migration/cpr.h
index 7561fc75ad..cc9384b4f9 100644
--- a/include/migration/cpr.h
+++ b/include/migration/cpr.h
@@ -23,6 +23,7 @@ MigMode cpr_get_incoming_mode(void);
 void cpr_set_incoming_mode(MigMode mode);
 bool cpr_is_incoming(void);
 
+bool cpr_validate_socket_path(const char *path, Error **errp);
 int cpr_state_save(MigrationChannel *channel, Error **errp);
 int cpr_state_load(MigrationChannel *channel, Error **errp);
 void cpr_state_close(void);
diff --git a/migration/cpr-transfer.c b/migration/cpr-transfer.c
index e1f140359c..3088ed323f 100644
--- a/migration/cpr-transfer.c
+++ b/migration/cpr-transfer.c
@@ -17,6 +17,33 @@
 #include "migration/vmstate.h"
 #include "trace.h"
 
+#define CPR_MAX_RETRIES     50     /* Retry for up to 5 seconds */
+#define CPR_RETRY_DELAY_US  100000 /* 100 ms per retry */
+
+bool cpr_validate_socket_path(const char *path, Error **errp)
+{
+    struct stat st;
+    int retries = CPR_MAX_RETRIES;
+
+    do {
+        if (!stat(path, &st) && S_ISSOCK(st.st_mode)) {
+            return true;
+        }
+
+        if (errno == ENOENT) {
+            usleep(CPR_RETRY_DELAY_US);
+        } else {
+            error_setg_errno(errp, errno,
+                "Unable to check status of socket path '%s'", path);
+            return false;
+        }
+    } while (--retries > 0);
+
+    error_setg(errp, "Socket path '%s' not found after %d retries",
+                                            path, CPR_MAX_RETRIES);
+    return false;
+}
+
 QEMUFile *cpr_transfer_output(MigrationChannel *channel, Error **errp)
 {
     MigrationAddress *addr = channel->addr;
@@ -28,6 +55,14 @@ QEMUFile *cpr_transfer_output(MigrationChannel *channel, Error **errp)
         QIOChannel *ioc = QIO_CHANNEL(sioc);
         SocketAddress *saddr = &addr->u.socket;
 
+        /*
+         * Verify that the cpr.sock Unix domain socket file exists and is ready
+         * before proceeding with the connection.
+         */
+        if (!cpr_validate_socket_path(addr->u.socket.u.q_unix.path, errp)) {
+            return NULL;
+        }
+
         if (qio_channel_socket_connect_sync(sioc, saddr, errp) < 0) {
             return NULL;
         }
-- 
2.49.0