Use a file lock as well as a mutex to isolate tests

tooling/nargo_cli/Cargo.toml

@@ -78,6 +78,7 @@ dirs.workspace = true
 assert_cmd = "2.0.8"
 assert_fs = "1.0.10"
 predicates = "2.1.5"
+file-lock = "2.1.11"
 fm.workspace = true
 criterion.workspace = true
 pprof.workspace = true

tooling/nargo_cli/build.rs

@@ -168,7 +168,10 @@ fn generate_test_cases(
     }
     let test_cases = test_cases.join("\n");
 
-    // Use a common mutex for all test cases.
+    // Locking the Nargo.toml file in the test directory so that we can run tests exclusively.
+    // NB a Mutex is not enough on CI because we use `cargo nextest --partition` which can run
+    // run test cases in the group in different processes. For some reason the file lock alone
+    // doesn't seem to do the trick locally.
     let mutex_name = format! {"TEST_MUTEX_{}", test_name.to_uppercase()};
     write!(
         test_file,
@@ -180,10 +183,16 @@ lazy_static::lazy_static! {{
 
 {test_cases}
 fn test_{test_name}(force_brillig: bool, inliner_aggressiveness: i64) {{
+    let test_program_dir = PathBuf::from("{test_dir}");
+
     // Ignore poisoning errors if some of the matrix cases failed.
-    let _guard = {mutex_name}.lock().unwrap_or_else(|e| e.into_inner()); 
+    let mutex_guard = {mutex_name}.lock().unwrap_or_else(|e| e.into_inner());
 
-    let test_program_dir = PathBuf::from("{test_dir}");
+    let file_guard = file_lock::FileLock::lock(
+        test_program_dir.join("Nargo.toml"),
+        true,
+        file_lock::FileOptions::new().read(true).write(true).append(true)
+    ).expect("failed to lock Nargo.toml");
 
     let mut nargo = Command::cargo_bin("nargo").unwrap();
     nargo.arg("--program-dir").arg(test_program_dir);
@@ -194,6 +203,9 @@ fn test_{test_name}(force_brillig: bool, inliner_aggressiveness: i64) {{
     }}
 
     {test_content}
+
+    drop(file_guard);
+    drop(mutex_guard);
 }}
 "#
     )
@@ -363,7 +375,7 @@ fn generate_compile_success_empty_tests(test_file: &mut File, test_data_dir: &Pa
             "info",
             &format!(
                 r#"
-                nargo.arg("--json");                
+                nargo.arg("--json");
                 {assert_zero_opcodes}
             "#,
             ),