Catch panics in the server and transmit them as errors to the client.

Currently, when the server panics, the client never gets a response, and
the compilation gets stuck. The only indication of something going wrong
is in the logs if they are enabled.

With this change, the error is transmitted to the client, which then
fails and prints it. For instance, one of the out-of-bound accesses
fixed in mozilla#1980 now prints this on the client side before returning a
non-zero exit code:

sccache: encountered fatal error
sccache: error: thread 'tokio-runtime-worker' panicked at src/compiler/c.rs:612:25: range end index 7 out of range for slice of length 1
sccache: caused by: thread 'tokio-runtime-worker' panicked at src/compiler/c.rs:612:25: range end index 7 out of range for slice of length 1

Fixes mozilla#756

src/server.rs

@@ -36,6 +36,7 @@ use futures::future::FutureExt;
 use futures::{future, stream, Sink, SinkExt, Stream, StreamExt, TryFutureExt};
 use number_prefix::NumberPrefix;
 use serde::{Deserialize, Serialize};
+use std::cell::Cell;
 use std::collections::HashMap;
 use std::env;
 use std::ffi::{OsStr, OsString};
@@ -392,12 +393,27 @@ impl DistClientContainer {
     }
 }
 
+thread_local! {
+    /// catch_unwind doesn't provide panic location, so we store that
+    /// information via a panic hook to be used when catch_unwind
+    /// catches a panic.
+    static PANIC_LOCATION: Cell<Option<(String, u32, u32)>> = Cell::new(None);
+}
+
 /// Start an sccache server, listening on `port`.
 ///
 /// Spins an event loop handling client connections until a client
 /// requests a shutdown.
 pub fn start_server(config: &Config, port: u16) -> Result<()> {
     info!("start_server: port: {}", port);
+    let panic_hook = std::panic::take_hook();
+    std::panic::set_hook(Box::new(move |info| {
+        PANIC_LOCATION.set(
+            info.location()
+                .map(|loc| (loc.file().to_string(), loc.line(), loc.column())),
+        );
+        panic_hook(info)
+    }));
     let client = unsafe { Client::new() };
     let runtime = tokio::runtime::Builder::new_multi_thread()
         .enable_all()
@@ -1182,20 +1198,35 @@ where
         let task = async move {
             let dist_client = me.dist_client.get_client().await;
             let result = match dist_client {
-                Ok(client) => {
-                    hasher
-                        .get_cached_or_compile(
-                            client,
-                            creator,
-                            storage,
-                            arguments,
-                            cwd,
-                            env_vars,
-                            cache_control,
-                            pool,
+                Ok(client) => std::panic::AssertUnwindSafe(hasher.get_cached_or_compile(
+                    client,
+                    creator,
+                    storage,
+                    arguments,
+                    cwd,
+                    env_vars,
+                    cache_control,
+                    pool,
+                ))
+                .catch_unwind()
+                .await
+                .map_err(|e| {
+                    let panic = e
+                        .downcast_ref::<&str>()
+                        .map(|s| &**s)
+                        .or_else(|| e.downcast_ref::<String>().map(|s| &**s))
+                        .unwrap_or_else(|| "An unknown panic was caught.");
+                    let thread = std::thread::current();
+                    let thread_name = thread.name().unwrap_or("unnamed");
+                    if let Some((file, line, column)) = PANIC_LOCATION.take() {
+                        anyhow!(
+                            "thread '{thread_name}' panicked at {file}:{line}:{column}: {panic}"
                         )
-                        .await
-                }
+                    } else {
+                        anyhow!("thread '{thread_name}' panicked: {panic}")
+                    }
+                })
+                .and_then(std::convert::identity),
                 Err(e) => Err(e),
             };
             let mut cache_write = None;