Make tests much easier to debug

- Don't panic on the mutex in all tests if a single test fails
- Give a more helpful message if a test handler wasn't registered
- Enable env_logger for tests

src/test/mod.rs

@@ -24,8 +24,14 @@ pub(crate) fn set_handler<H>(path: &str, handler: H)
 where
     H: Fn(&Unit) -> Result<Stream, Error> + Send + 'static,
 {
-    let mut handlers = TEST_HANDLERS.lock().unwrap();
-    handlers.insert(path.to_string(), Box::new(handler));
+    let path = path.to_string();
+    let handler = Box::new(handler);
+    // See `resolve_handler` below for why poisoning isn't necessary.
+    let mut handlers = match TEST_HANDLERS.lock() {
+        Ok(h) => h,
+        Err(poison) => poison.into_inner(),
+    };
+    handlers.insert(path, handler);
 }
 
 #[allow(clippy::write_with_newline)]
@@ -46,8 +52,17 @@ pub(crate) fn make_response(
 }
 
 pub(crate) fn resolve_handler(unit: &Unit) -> Result<Stream, Error> {
-    let mut handlers = TEST_HANDLERS.lock().unwrap();
     let path = unit.url.path();
-    let handler = handlers.remove(path).unwrap();
+    // The only way this can panic is if
+    // 1. `remove(path).unwrap()` panics, in which case the HANDLERS haven't been modified.
+    // 2. `make_hash` for `handlers.insert` panics (in `set_handler`), in which case the HANDLERS haven't been modified.
+    // In all cases, another test will fail as a result, so it's ok to continue other tests in parallel.
+    let mut handlers = match TEST_HANDLERS.lock() {
+        Ok(h) => h,
+        Err(poison) => poison.into_inner(),
+    };
+    let handler = handlers.remove(path)
+        .unwrap_or_else(|| panic!("call make_response(\"{}\") before fetching it in tests (or if you did make it, avoid fetching it more than once)", path));
+    drop(handlers);
     handler(unit)
 }

src/testserver.rs

@@ -13,6 +13,9 @@ use crate::{Agent, AgentBuilder};
 // An agent to be installed by default for tests and doctests, such
 // that all hostnames resolve to a TestServer on localhost.
 pub(crate) fn test_agent() -> Agent {
+    #[cfg(test)]
+    let _ = env_logger::try_init();
+
     let testserver = TestServer::new(|mut stream: TcpStream| -> io::Result<()> {
         let headers = read_request(&stream);
         if headers.0.is_empty() {

src/unit.rs

@@ -170,8 +170,6 @@ pub(crate) fn connect(
     body: SizedReader,
     previous: Option<Arc<Response>>,
 ) -> Result<Response, Error> {
-    //
-
     let host = unit
         .url
         .host_str()