swarm-builder tests: write fake nix from a child process

swarm-builder/src/nix_build.rs

@@ -548,36 +548,40 @@ mod tests {
         assert!(error_tail.ends_with("error: attribute missing"));
     }
 
+    /// Write the shell script `body` to `path` as an executable, from a child
+    /// process rather than from this one: a write fd on it held here could be
+    /// inherited by a child another test thread forks, and exec'ing a file
+    /// some process still holds open for writing fails with `ETXTBSY`.
+    fn write_executable(path: &std::path::Path, body: &str) {
+        let status = std::process::Command::new("/bin/sh")
+            .args([
+                "-c",
+                r#"printf '%s' "$1" > "$2" && chmod 755 "$2""#,
+                "sh",
+                &format!("#!/bin/sh\n{body}\n"),
+            ])
+            .arg(path)
+            .status()
+            .expect("run /bin/sh");
+        assert!(status.success(), "writing {} failed", path.display());
+    }
+
     /// A `nix` stand-in: runs `body` as a shell script, ignoring its arguments.
     fn fake_nix(tag: &str, body: &str) -> PathBuf {
-        use std::os::unix::fs::PermissionsExt as _;
-
         let dir = std::env::temp_dir().join(format!(
             "swarm-builder-fake-nix-{tag}-{}",
             std::process::id()
         ));
         std::fs::create_dir_all(&dir).expect("scratch dir");
         let path = dir.join("nix");
-        std::fs::write(&path, format!("#!/bin/sh\n{body}\n")).expect("write fake nix");
-        std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o755)).expect("chmod");
+        write_executable(&path, body);
         path
     }
 
-    /// Builds with `nix`, retrying while the kernel refuses to exec the script
-    /// just written (`ETXTBSY`, when another test's fork briefly inherits its
-    /// open write handle).
+    /// Builds with `nix`.
     async fn build_with(nix: PathBuf) -> Reply {
         let builder = Builder { nix, ..builder() };
-        for _ in 0..20 {
-            let reply = builder.build(&job(), "token").await;
-            match &reply {
-                Reply::Failed { error_tail } if error_tail.contains("Text file busy") => {
-                    tokio::time::sleep(Duration::from_millis(50)).await;
-                }
-                _ => return reply,
-            }
-        }
-        panic!("the fake nix stayed busy")
+        builder.build(&job(), "token").await
     }
 
     #[tokio::test]
@@ -677,13 +681,9 @@ mod tests {
     /// A builder whose `nix` is the shell script `body`, in a directory the
     /// returned guard removes.
     fn scripted(body: &str) -> (Builder, Scratch) {
-        use std::os::unix::fs::PermissionsExt as _;
-
         let dir = Scratch::new().expect("a temp dir");
         let nix = dir.0.join("nix");
-        std::fs::write(&nix, format!("#!/bin/sh\n{body}\n")).expect("the script is written");
-        std::fs::set_permissions(&nix, std::fs::Permissions::from_mode(0o755))
-            .expect("the script is executable");
+        write_executable(&nix, body);
         (Builder { nix, ..builder() }, dir)
     }