Merge branch 'err-dyn-report' into 'main'

Error logging (ErrorReport, .report()) POC

See merge request tpo/core/arti!936

crates/tor-circmgr/src/lib.rs

@@ -39,6 +39,7 @@
 
 use tor_basic_utils::retry::RetryDelay;
 use tor_chanmgr::ChanMgr;
+use tor_error::ErrorReport;
 use tor_linkspec::ChanTarget;
 use tor_netdir::{DirEvent, NetDir, NetDirProvider, Timeliness};
 use tor_proto::circuit::{CircParameters, ClientCirc, UniqId};
@@ -642,7 +643,7 @@ impl<R: Runtime> CircMgr<R> {
 
                 match statemgr.try_lock() {
                     Err(e) => {
-                        error!("Problem with state lock file: {}", e);
+                        error!("Problem with state lock file: {}", e.report());
                         break;
                     }
                     Ok(NewlyAcquired) => {

crates/tor-error/src/internal.rs

@@ -291,6 +291,17 @@ mod test {
         assert!(s.contains("Couldn't wobble the wobbling device."));
         #[cfg(feature = "backtrace")]
         assert!(s.contains("internal_macro_test"));
+
+        #[derive(thiserror::Error, Debug)]
+        enum Wrap {
+            #[error("Internal error")]
+            Internal(#[from] Bug),
+        }
+
+        let w: Wrap = e.into();
+        let s = format!("Got: {}", w.report());
+        dbg!(&s);
+        assert!(s.contains("Couldn't wobble the wobbling device."));
     }
 
     #[test]

crates/tor-error/src/report.rs

@@ -1,5 +1,6 @@
 //! The Report type which reports errors nicely
 
+use std::error::Error as StdError;
 use std::fmt::{self, Debug, Display};
 
 /// Wraps any Error, providing a nicely-reporting Display impl
@@ -7,15 +8,15 @@ use std::fmt::{self, Debug, Display};
 #[allow(clippy::exhaustive_structs)] // this is a transparent wrapper
 pub struct Report<E>(pub E)
 where
-    E: AsRef<dyn std::error::Error>;
+    E: AsRef<dyn StdError>;
 
 impl<E> Display for Report<E>
 where
-    E: AsRef<dyn std::error::Error>,
+    E: AsRef<dyn StdError>,
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         /// Non-generic inner function avoids code bloat
-        fn inner(mut e: &dyn std::error::Error, f: &mut fmt::Formatter) -> fmt::Result {
+        fn inner(mut e: &dyn StdError, f: &mut fmt::Formatter) -> fmt::Result {
             write!(f, "error")?;
             let mut last = String::new();
             loop {
@@ -44,7 +45,7 @@ where
 #[allow(clippy::print_stderr)] // this is the point of this function
 pub fn report_and_exit<E, R>(e: E) -> R
 where
-    E: AsRef<dyn std::error::Error>,
+    E: AsRef<dyn StdError>,
 {
     /// Non-generic inner function avoids code bloat
     fn eprint_progname() {
@@ -58,10 +59,41 @@ where
     std::process::exit(127)
 }
 
+/// Helper type for reporting errors that are concrete implementors of `StdError`
+///
+/// This is an opaque type, only constructable via the `ErrorExt` helper trait
+/// and only useable via its `AsRef` implementation.
+//
+// We need this because Rust's trait object handling rules, and provided AsRef impls,
+// are rather anaemic.  We cannot simply put a &dyn Error into Report, because
+// &dyn Error doesn't impl AsRef<dyn Error> even though the implementation is trivial.
+// We can't provide that AsRef impl ourselves due to trait coherency rules.
+// So instead, we wrap up the &dyn Error in a newtype, for which we *can* provide the AsRef.
+pub struct ReportHelper<'e>(&'e (dyn StdError + 'static));
+impl<'e> AsRef<dyn StdError + 'static> for ReportHelper<'e> {
+    fn as_ref(&self) -> &(dyn StdError + 'static) {
+        self.0
+    }
+}
+
+/// Extension trait providing `.report()` method on concrete errors
+///
+/// This is implemented for types that directly implement [`std::error::Error`]` + 'static`.
+/// For types like `anyhow::Error` that `impl AsRef<dyn Error>`,
+/// use `tor_error::Report(err)` directly.
+pub trait ErrorReport: StdError + Sized + 'static {
+    /// Return an object that displays the error and its causes
+    //
+    // We would ideally have returned `Report<impl AsRef<...>>` but that's TAIT.
+    fn report(&self) -> Report<ReportHelper> {
+        Report(ReportHelper(self as _))
+    }
+}
+impl<E: StdError + Sized + 'static> ErrorReport for E {}
+
 #[cfg(test)]
 mod test {
     use super::*;
-    use std::error::Error as StdError;
     use std::io;
     use thiserror::Error;
 

crates/tor-proto/src/channel/reactor.rs

@@ -495,7 +495,7 @@ pub(crate) mod test {
             .build()
             .unwrap();
         let send1 = send1.sink_map_err(|e| {
-            trace!("got sink error: {}", e);
+            trace!("got sink error: {:?}", e);
             CodecError::DecCell(tor_cell::Error::ChanProto("dummy message".into()))
         });
         let (chan, reactor) = crate::channel::Channel::new(