fix: reject unsupported rsync client stderr mode

src/client/client_cli.c

@@ -91,10 +91,12 @@ static int set_stderr_mode(const char* value) {
     log_set_stderr_mode(LOG_STDERR_ERRORS);
   else if (strcmp(value, "all") == 0 || strcmp(value, "a") == 0)
     log_set_stderr_mode(LOG_STDERR_ALL);
-  else if (strcmp(value, "client") == 0 || strcmp(value, "c") == 0)
-    log_set_stderr_mode(LOG_STDERR_CLIENT);
-  else {
-    log_message(LOG_LEVEL_ERROR, "--stderr must be errors, all, or client");
+  else if (strcmp(value, "client") == 0 || strcmp(value, "c") == 0) {
+    log_message(LOG_LEVEL_ERROR,
+                "--stderr=client is not supported: FastSync has no client message channel");
+    return -1;
+  } else {
+    log_message(LOG_LEVEL_ERROR, "--stderr must be errors or all");
     return -1;
   }
   return 0;

src/client/usage.c

@@ -58,7 +58,7 @@ void print_usage(void) {
   printf("  --stats             Print transfer statistics at end\n");
   printf("  --max-depth <n>     Maximum directory depth (0=unlimited)\n");
   printf("  --log-file <path>   Write log messages to file\n");
-  printf("  --stderr=MODE       Route logging to stderr: errors, all, or client\n");
+  printf("  --stderr=MODE       Route logging to stderr: errors or all\n");
   printf("  --partial           Keep partial files on interrupted transfer\n");
   printf("  --partial-dir <dir> Directory for partial files\n");
   printf("  --fastsync-server-path <path>\n");