From 8439aeb7cf5e46b96b58e281773c33bdb45f40a4 Mon Sep 17 00:00:00 2001
From: Tamo <tamo@meilisearch.com>
Date: Wed, 29 Jan 2025 11:51:06 +0100
Subject: [PATCH] improve error message in case of unexpected panic while
 processing tasks

---
 crates/index-scheduler/src/error.rs               |  8 ++++----
 crates/index-scheduler/src/scheduler/mod.rs       | 15 ++++++++++++++-
 .../src/scheduler/process_batch.rs                | 13 +++++++++++--
 .../index_creation_failed.snap                    |  2 +-
 4 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/crates/index-scheduler/src/error.rs b/crates/index-scheduler/src/error.rs
index d3feecd73..e749a1bcb 100644
--- a/crates/index-scheduler/src/error.rs
+++ b/crates/index-scheduler/src/error.rs
@@ -127,8 +127,8 @@ pub enum Error {
         _ => format!("{error}")
     })]
     Milli { error: milli::Error, index_uid: Option<String> },
-    #[error("An unexpected crash occurred when processing the task.")]
-    ProcessBatchPanicked,
+    #[error("An unexpected crash occurred when processing the task: {0}")]
+    ProcessBatchPanicked(String),
     #[error(transparent)]
     FileStore(#[from] file_store::Error),
     #[error(transparent)]
@@ -196,7 +196,7 @@ impl Error {
             | Error::Dump(_)
             | Error::Heed(_)
             | Error::Milli { .. }
-            | Error::ProcessBatchPanicked
+            | Error::ProcessBatchPanicked(_)
             | Error::FileStore(_)
             | Error::IoError(_)
             | Error::Persist(_)
@@ -257,7 +257,7 @@ impl ErrorCode for Error {
             Error::NoSpaceLeftInTaskQueue => Code::NoSpaceLeftOnDevice,
             Error::Dump(e) => e.error_code(),
             Error::Milli { error, .. } => error.error_code(),
-            Error::ProcessBatchPanicked => Code::Internal,
+            Error::ProcessBatchPanicked(_) => Code::Internal,
             Error::Heed(e) => e.error_code(),
             Error::HeedTransaction(e) => e.error_code(),
             Error::FileStore(e) => e.error_code(),
diff --git a/crates/index-scheduler/src/scheduler/mod.rs b/crates/index-scheduler/src/scheduler/mod.rs
index eddf8fba7..90c9584f7 100644
--- a/crates/index-scheduler/src/scheduler/mod.rs
+++ b/crates/index-scheduler/src/scheduler/mod.rs
@@ -172,7 +172,20 @@ impl IndexScheduler {
                         cloned_index_scheduler.process_batch(batch, processing_batch, progress)
                     })
                     .unwrap();
-                handle.join().unwrap_or(Err(Error::ProcessBatchPanicked))
+
+                match handle.join() {
+                    Ok(ret) => ret,
+                    Err(panic) => {
+                        let msg = match panic.downcast_ref::<&'static str>() {
+                            Some(s) => *s,
+                            None => match panic.downcast_ref::<String>() {
+                                Some(s) => &s[..],
+                                None => "Box<dyn Any>",
+                            },
+                        };
+                        Err(Error::ProcessBatchPanicked(msg.to_string()))
+                    }
+                }
             })
         };
 
diff --git a/crates/index-scheduler/src/scheduler/process_batch.rs b/crates/index-scheduler/src/scheduler/process_batch.rs
index 623bdeb53..21233429c 100644
--- a/crates/index-scheduler/src/scheduler/process_batch.rs
+++ b/crates/index-scheduler/src/scheduler/process_batch.rs
@@ -326,8 +326,17 @@ impl IndexScheduler {
                 match ret {
                     Ok(Ok(())) => (),
                     Ok(Err(e)) => return Err(Error::DatabaseUpgrade(Box::new(e))),
-                    Err(_e) => {
-                        return Err(Error::DatabaseUpgrade(Box::new(Error::ProcessBatchPanicked)));
+                    Err(e) => {
+                        let msg = match e.downcast_ref::<&'static str>() {
+                            Some(s) => *s,
+                            None => match e.downcast_ref::<String>() {
+                                Some(s) => &s[..],
+                                None => "Box<dyn Any>",
+                            },
+                        };
+                        return Err(Error::DatabaseUpgrade(Box::new(Error::ProcessBatchPanicked(
+                            msg.to_string(),
+                        ))));
                     }
                 }
 
diff --git a/crates/index-scheduler/src/scheduler/snapshots/test_failure.rs/panic_in_process_batch_for_index_creation/index_creation_failed.snap b/crates/index-scheduler/src/scheduler/snapshots/test_failure.rs/panic_in_process_batch_for_index_creation/index_creation_failed.snap
index 3f3a6f769..b0c450092 100644
--- a/crates/index-scheduler/src/scheduler/snapshots/test_failure.rs/panic_in_process_batch_for_index_creation/index_creation_failed.snap
+++ b/crates/index-scheduler/src/scheduler/snapshots/test_failure.rs/panic_in_process_batch_for_index_creation/index_creation_failed.snap
@@ -7,7 +7,7 @@ snapshot_kind: text
 []
 ----------------------------------------------------------------------
 ### All Tasks:
-0 {uid: 0, batch_uid: 0, status: failed, error: ResponseError { code: 200, message: "An unexpected crash occurred when processing the task.", error_code: "internal", error_type: "internal", error_link: "https://docs.meilisearch.com/errors#internal" }, details: { primary_key: Some("mouse") }, kind: IndexCreation { index_uid: "catto", primary_key: Some("mouse") }}
+0 {uid: 0, batch_uid: 0, status: failed, error: ResponseError { code: 200, message: "An unexpected crash occurred when processing the task: simulated panic", error_code: "internal", error_type: "internal", error_link: "https://docs.meilisearch.com/errors#internal" }, details: { primary_key: Some("mouse") }, kind: IndexCreation { index_uid: "catto", primary_key: Some("mouse") }}
 ----------------------------------------------------------------------
 ### Status:
 enqueued []