datafusion-proto: column qualifiers containing . are silently corrupted on round-trip #24776

Describe the bug

datafusion-proto serializes a column's qualifier as a single unquoted string and re-parses it on the way back:

• encode — proto-common/src/to_proto/mod.rs:

  relation: c.relation.map(|relation| protobuf::ColumnRelation {
      relation: relation.to_string(),
  }),

• decode — proto-common/src/from_proto/mod.rs:

  impl From<protobuf::ColumnRelation> for TableReference {
      fn from(rel: protobuf::ColumnRelation) -> Self {
          Self::parse_str_normalized(rel.relation.as_str(), true)
      }
  }

TableReference's Display writes the parts joined by . without quoting, while parse_str_normalized splits on unquoted .. So any TableReference whose segments contain a . does not round-trip. The same applies to DfField.qualifier, which is encoded the same way.

This is silent: no error is raised, the reference is just re-partitioned into different segments. A Partial { schema: "my.schema", table: "t" } comes back as Full { catalog: "my", schema: "schema", table: "t" }, which then resolves against a different (or non-existent) table.

Identifiers containing dots are legal — any dialect that allows quoted identifiers can produce them, and they show up in practice when an external catalog's naming is mapped into DataFusion.

To Reproduce

Through the actual proto conversions (datafusion-proto-common, main @ 8332cfa):

use datafusion_common::{Column, TableReference};
use datafusion_proto_common as protobuf;

fn roundtrip(col: Column) {
    let encoded: protobuf::Column = (&col).into();
    let decoded: Column = (&encoded).into();
    let ok = if decoded == col { "OK  " } else { "LOSS" };
    println!("{ok} {col:?}\n       -> proto relation {:?}\n       -> {decoded:?}\n",
        encoded.relation.as_ref().map(|r| &r.relation));
}

fn main() {
    roundtrip(Column::new(Some(TableReference::full("c", "s", "t")), "x"));
    roundtrip(Column::new(Some(TableReference::partial("my.schema", "t")), "x"));
    roundtrip(Column::new(Some(TableReference::bare("has.dot")), "x"));
}

Output:

OK   Column { relation: Some(Full { catalog: "c", schema: "s", table: "t" }), name: "x" }
       -> proto relation Some("c.s.t")
       -> Column { relation: Some(Full { catalog: "c", schema: "s", table: "t" }), name: "x" }

LOSS Column { relation: Some(Partial { schema: "my.schema", table: "t" }), name: "x" }
       -> proto relation Some("my.schema.t")
       -> Column { relation: Some(Full { catalog: "my", schema: "schema", table: "t" }), name: "x" }

LOSS Column { relation: Some(Bare { table: "has.dot" }), name: "x" }
       -> proto relation Some("has.dot")
       -> Column { relation: Some(Partial { schema: "has", table: "dot" }), name: "x" }

The underlying TableReference round-trip (Display -> parse_str_normalized), which is what the proto layer relies on, fails the same way and also has a case that collapses to a single bare segment:

LOSS Full { catalog: "cat", schema: "my.schema", table: "t" }
       -> "cat.my.schema.t"
       -> Bare { table: "cat.my.schema.t" }

LOSS Partial { schema: "s", table: "tbl.with.dots" }
       -> "s.tbl.with.dots"
       -> Bare { table: "s.tbl.with.dots" }

Expected behavior

A Column/DfField qualifier round-trips through datafusion-proto unchanged, whatever characters its segments contain.

Additional context

Two possible fixes:

1. Quote on the way out. Encode with TableReference::to_quoted_string() and decode with a parser that honours the quoting. This is a wire-compatible change in the sense that the proto schema is untouched, but blobs written by older versions still decode by the old rules, so it is only correct if both ends move together.

2. Make ColumnRelation structured, e.g. adding repeated string parts = 2; alongside the existing string relation = 1;. New writers populate both, readers prefer parts when present. This is properly backward and forward compatible and removes the parse round-trip entirely.

I'd lean towards (2), since it also removes the ambiguity for readers of blobs written by older versions. Happy to put up a PR for whichever the maintainers prefer.