fix(oneshot): retry script execution when ETXTBSY occurs

When multiple test threads run concurrently, one thread may fork while another still has the script file open for writing, causing exec to fail with ETXTBSY. Added a wait loop that probes the script with a no-op argument until the file is safe to execute, preventing spurious test failures.

Auto-committed-on: dragonfly

Author: senamakel

crates/tinybox-docker/src/oneshot/test.rs

@@ -29,15 +29,33 @@ fn canon(p: &Path) -> String {
 fn fake(dir: &Path, body: &str) -> (DockerCli, PathBuf) {
     let out = dir.join("argv.out");
     let script = format!(
-        "#!/bin/sh\n: > '{out}'\nfor a in \"$@\"; do printf '%s\\0' \"$a\" >> '{out}'; done\n{body}\n",
+        "#!/bin/sh\n[ \"$1\" = --tinybox-probe ] && exit 0\n: > '{out}'\nfor a in \"$@\"; do printf '%s\\0' \"$a\" >> '{out}'; done\n{body}\n",
         out = out.display()
     );
     let path = dir.join("docker");
     std::fs::write(&path, script).unwrap();
     std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o755)).unwrap();
+    wait_until_executable(&path);
     (DockerCli::with_program(&path), out)
 }
 
+/// Another test thread may `fork` while our write handle is still open, and the
+/// child keeps that handle until it `exec`s; executing the script in that window
+/// fails with `ETXTBSY`. Probe (a no-op for the script) until the window closes.
+fn wait_until_executable(path: &Path) {
+    for _ in 0..200 {
+        match std::process::Command::new(path)
+            .arg("--tinybox-probe")
+            .status()
+        {
+            Err(e) if e.raw_os_error() == Some(26) => {
+                std::thread::sleep(Duration::from_millis(10));
+            }
+            _ => return,
+        }
+    }
+}
+
 fn recorded(out: &Path) -> Vec<String> {
     std::fs::read(out)
         .unwrap()