test(testkit): keep the JUnit report current during a run

tests/testkit/README.md

@@ -185,7 +185,9 @@ A filter selects tests whose name contains it. `run` also takes:
   skipped.
 - `--timeout SECS` (default 900): a test that runs longer gets SIGTERM, then
   SIGKILL after `--grace SECS` (default 10).
-- `--junit PATH`: writes a JUnit report with one test suite per family.
+- `--junit PATH`: writes a JUnit report with one test suite per family. The
+  report is rewritten after each test, so a run that is stopped from outside
+  still leaves the results so far.
 
 Each test runs in its own process group. When the test's process exits, `run`
 kills anything left in the group, such as CLI processes the test started.

tests/testkit/testkit/src/program/junit.rs

@@ -74,7 +74,13 @@ pub fn write(path: &Path, report: &Report) -> std::io::Result<()> {
         std::fs::create_dir_all(parent)?;
     }
     let xml = report.to_string().map_err(std::io::Error::other)?;
-    std::fs::write(path, xml)
+    // Replace the file in one step, so a run that is killed while writing
+    // leaves the previous report, not a truncated one.
+    let mut partial = path.as_os_str().to_owned();
+    partial.push(".partial");
+    let partial = std::path::PathBuf::from(partial);
+    std::fs::write(&partial, xml)?;
+    std::fs::rename(&partial, path)
 }
 
 #[cfg(test)]
@@ -125,4 +131,25 @@ mod tests {
             assert!(xml.contains(expected), "missing {expected} in\n{xml}");
         }
     }
+
+    #[test]
+    fn write_replaces_the_report_and_leaves_no_partial_file() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("results/junit.xml");
+        let one = [TestRun {
+            test: &PASSED,
+            outcome: Outcome::Passed,
+            duration: None,
+            stdout: Vec::new(),
+            stderr: Vec::new(),
+        }];
+        super::write(&path, &report("p", &one)).unwrap();
+        super::write(&path, &report("p", &[])).unwrap();
+        let xml = std::fs::read_to_string(&path).unwrap();
+        assert!(xml.contains(r#"tests="0""#), "{xml}");
+        assert_eq!(
+            std::fs::read_dir(path.parent().unwrap()).unwrap().count(),
+            1
+        );
+    }
 }

tests/testkit/testkit/src/program/supervisor.rs

@@ -132,6 +132,13 @@ pub fn run(program: &str, exe: &Path, tests: &[Test], options: &Options) -> Exit
             ),
         }
         runs.push(run);
+        // Keep the report current, so a run that is stopped from outside, such
+        // as by a run-level timeout, still leaves the results so far.
+        if let Some(path) = &options.junit
+            && let Err(error) = junit::write(path, &junit::report(program, &runs))
+        {
+            eprintln!("{program}: write {}: {error}", path.display());
+        }
     }
 
     let not_run = selected_count - runs.len();