firecracker: the handler-exit error carries the handler's log

crates/engram-host-agent/tests/blk_source.rs

@@ -95,6 +95,16 @@ async fn read_off_runtime(device: &Arc<File>, offset: u64, len: usize) -> Vec<u8
         .unwrap()
 }
 
+/// The chunks the source recorded as read cold, less the first chunk. The
+/// kernel's partition scan reads the start of the device at a time of its
+/// own (in a work item on newer kernels), and the source records that
+/// read like a guest's.
+fn guest_reads(source: &BlkSource) -> Vec<u32> {
+    let mut touched = source.touched();
+    touched.retain(|chunk| *chunk != 0);
+    touched
+}
+
 #[tokio::test]
 #[ignore = "requires root and the ublk_drv module"]
 async fn a_source_serves_the_manifest_and_records_the_chunks_read_cold() {
@@ -116,7 +126,7 @@ async fn a_source_serves_the_manifest_and_records_the_chunks_read_cold() {
         read_off_runtime(&device, at, 4096).await,
         image[at as usize..at as usize + 4096]
     );
-    assert_eq!(source.touched(), vec![3]);
+    assert_eq!(guest_reads(&source), vec![3]);
 
     // The copy's stream: the whole device, region by region, ascending.
     for at in (0..CHUNKS * CHUNK).step_by(REGION as usize) {
@@ -126,7 +136,11 @@ async fn a_source_serves_the_manifest_and_records_the_chunks_read_cold() {
             "region at {at}"
         );
     }
-    assert_eq!(source.touched(), vec![3], "the stream is not a guest read");
+    assert_eq!(
+        guest_reads(&source),
+        vec![3],
+        "the stream is not a guest read"
+    );
 
     drop(device);
     source.stop().await.unwrap();

crates/engram-sandbox-firecracker/src/lib.rs

@@ -3309,8 +3309,14 @@ impl FirecrackerBackend {
                             if let Some(status) = handler.try_wait().map_err(|e| {
                                 SandboxError::Snapshot(format!("check uffd handler: {e}"))
                             })? {
+                                // The handler's log holds the only reason.
+                                let log = read_tail(&jail_dir.join("uffd-handler.log"), 2048)
+                                    .await
+                                    .unwrap_or_default();
                                 return Err(SandboxError::Snapshot(format!(
-                                    "uffd handler exited before the load ({status})"
+                                    "uffd handler exited before the load ({status}); \
+                                     handler log: {}",
+                                    log.trim().replace('\n', " | ")
                                 )));
                             }
                             if uffd_uds.exists() && state_path.exists() {