test(maitake): properly capture tracing messages (#445)

The "init tracing" test helper returned a guard that the caller
is supposed to hold, but none (or not all?) of the tests that called it
actually held it.

This makes a single return type (because `rustc` was Big Mad when I used
`Box<dyn Drop>`) based on the features, and marks it as must use.

This change allows for capturing (or viewing) of tracing output during
timer wheel tests.

Closes #444

maitake/src/time/timer/tests/concurrent.rs

@@ -16,8 +16,9 @@ use loom::model;
 
 #[cfg(not(loom))]
 fn model(f: impl Fn() + Send + Sync + 'static) {
-    crate::util::test::trace_init_with_default("info,maitake::time=trace");
+    let guard = crate::util::test::trace_init_with_default("info,maitake::time=trace");
     f();
+    drop(guard);
 }
 
 #[test]

maitake/src/time/timer/tests/wheel_tests.rs

@@ -1,5 +1,6 @@
 use super::*;
 use crate::scheduler::Scheduler;
+use crate::util::test::TestGuard;
 use std::collections::BTreeMap;
 use std::sync::{
     atomic::{AtomicUsize, Ordering},
@@ -14,6 +15,9 @@ struct SleepGroupTest {
     now: Ticks,
     groups: BTreeMap<Ticks, SleepGroup>,
     next_id: usize,
+
+    // Hold on to this so the subscriber doesn't go away
+    _guard: TestGuard,
 }
 
 struct SleepGroup {
@@ -26,13 +30,14 @@ struct SleepGroup {
 
 impl SleepGroupTest {
     fn new(timer: &'static Timer) -> Self {
-        crate::util::test::trace_init_with_default("info,maitake::time=trace");
+        let _guard = crate::util::test::trace_init_with_default("info,maitake::time=trace");
         Self {
             scheduler: Scheduler::new(),
             timer,
             now: 0,
             groups: BTreeMap::new(),
             next_id: 0,
+            _guard,
         }
     }
 

maitake/src/util.rs

@@ -93,13 +93,34 @@ pub(crate) fn expect_display<T, E: core::fmt::Display>(result: Result<T, E>, msg
 
 #[cfg(test)]
 pub(crate) mod test {
+    /// A guard that represents the tracing default subscriber guard
+    ///
+    /// *should* be held until the end of the test, to ensure that tracing messages
+    /// actually make it to the fmt subscriber for the entire test.
+    ///
+    /// Exists to abstract over tracing 01/02 guard type differences.
+    #[must_use]
+    pub struct TestGuard {
+        #[cfg(not(loom))]
+        _x2: tracing_02::collect::DefaultGuard,
+        #[cfg(loom)]
+        _x1: tracing_01::subscriber::DefaultGuard,
+    }
 
-    pub(crate) fn trace_init() -> impl Drop {
+    /// Initialize tracing with a default filter directive
+    ///
+    /// Returns a [TestGuard] that must be held for the duration of test to ensure
+    /// tracing messages are correctly output
+    pub(crate) fn trace_init() -> TestGuard {
         trace_init_with_default("maitake=debug,cordyceps=debug")
     }
 
+    /// Initialize tracing with the given filter directive
+    ///
+    /// Returns a [TestGuard] that must be held for the duration of test to ensure
+    /// tracing messages are correctly output
     #[cfg(not(loom))]
-    pub(crate) fn trace_init_with_default(default: &str) -> impl Drop {
+    pub(crate) fn trace_init_with_default(default: &str) -> TestGuard {
         use tracing_subscriber::filter::{EnvFilter, LevelFilter};
         let env = std::env::var("RUST_LOG").unwrap_or_default();
         let builder = EnvFilter::builder().with_default_directive(LevelFilter::INFO.into());
@@ -115,11 +136,18 @@ pub(crate) mod test {
             .with_test_writer()
             .without_time()
             .finish();
-        tracing_02::collect::set_default(collector)
+
+        TestGuard {
+            _x2: tracing_02::collect::set_default(collector),
+        }
     }
 
+    /// Initialize tracing with the given filter directive
+    ///
+    /// Returns a [TestGuard] that must be held for the duration of test to ensure
+    /// tracing messages are correctly output
     #[cfg(loom)]
-    pub(crate) fn trace_init_with_default(default: &str) -> impl Drop {
+    pub(crate) fn trace_init_with_default(default: &str) -> TestGuard {
         use tracing_subscriber_03::filter::{EnvFilter, LevelFilter};
         let env = std::env::var("LOOM_LOG").unwrap_or_default();
         let builder = EnvFilter::builder().with_default_directive(LevelFilter::INFO.into());
@@ -138,7 +166,11 @@ pub(crate) mod test {
             .with_test_writer()
             .without_time()
             .finish();
-        tracing_01::subscriber::set_default(collector)
+
+        use tracing_subscriber_03::util::SubscriberInitExt;
+        TestGuard {
+            _x1: collector.set_default(),
+        }
     }
 
     #[allow(dead_code)]