databendlabs/databend · error

internal error: entered unreachable code

Error message

internal error: entered unreachable code

What it means

In the Delta Lake table source, async_process advances the read state machine and panics with unreachable! when it encounters an unexpected state variant. The state machine expects only the prepared-partition-file state at that point; anything else means internal state got out of sync.

Solutions

1. Re-run the query; transient scheduling issues may not reproduce
2. Simplify the query plan (remove filters/partition pruning) to avoid the code path and confirm it's a state-machine bug
3. Collect the query profile/trace and file a Databend bug with the plan
4. Upgrade Databend — pipeline state-machine fixes land regularly

// catch pipeline panic around Delta scan and retry once
match execute_delta_scan(query).await {
    Err(e) if is_panic_unreachable(&e) => retry_with_simplified_plan(query).await,
    other => other,
}

When it happens

Trigger: Reading a Delta table when the processor's internal state enum is in a variant other than the expected one when async_process runs — typically due to a poll/reschedule ordering bug or a previous failed transition.

Common situations: Delta table scans under async backpressure or processor rescheduling; concurrency bugs in the pipeline state machine; exceptions interleaved with state transitions.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• internal error: entered unreachable code
• internal error: entered unreachable code
• internal error: entered unreachable code
• internal error: entered unreachable code
• internal error: entered unreachable code

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/9eb31cbf01a81cbe. Report an issue: GitHub.

Appendix: source

Thrown at src/query/storages/delta/src/table_source.rs:198

            match &part.data {
                ParquetPart::File(f) => {
                    let partition_fields = self
                        .partition_fields
                        .iter()
                        .cloned()
                        .zip(part.partition_values.iter().cloned())
                        .collect::<Vec<_>>();
                    self.partition_block_scalars = partition_fields
                        .iter()
                        .map(|(f, v)| (f.data_type().into(), v.clone()))
                        .collect::<Vec<_>>();
                    let stream = self
                        .parquet_reader
                        .prepare_data_stream(&f.file, f.compressed_size, Some(&partition_fields))
                        .await?;
                    self.stream = Some(stream);
                }
                _ => unreachable!(),
            }
        } else {
            self.is_finished = true;
        }

        Ok(())
    }
}

fn check_block_schema(schema: &DataSchema, mut block: DataBlock) -> Result<DataBlock> {
    // Check if the schema of the data block is matched with the schema of the table.
    if block.num_columns() != schema.num_fields() {
        return Err(ErrorCode::TableSchemaMismatch(format!(
            "Data schema mismatched. Data columns length: {}, schema fields length: {}",
            block.num_columns(),
            schema.num_fields()
        )));
    }

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