When computing load diff, binlogs in v1/legacy format have empty child_fields. In this case, the field_id itself should be used as the child_id (group_id == field_id for legacy format).

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When computing load diff, binlogs in v1/legacy format have empty child_fields. In this case, the field_id itself should be used as the child_id (group_id == field_id for legacy format).

Without this fix, legacy format binlogs are not recognized during diff computation, causing segments to fail loading and TestProxy to timeout.

Changes:

• Add fallback to use fieldid as child_id when child_fields is empty
• Add LoadDiff::ToString() for debugging
• Add logging for diff in Load/Reopen operations
• Add comprehensive unit tests for legacy format handling

Related to #46594

• Core invariant: load-diff computation must enumerate every binlog child group for a field so current vs new segment state comparisons include all column-group/binlog groups; for legacy (v1) binlogs that have empty child_fields, the code must treat group_id == field_id to preserve that mapping.
• Bug fix (resolves #46594): SegmentLoadInfo now normalizes field_binlog.child_fields() into a vector and falls back to using field_id as the single child group when child_fields is empty; the same normalization is applied for both current and new-info paths, ensuring legacy v1 binlogs are discovered and included in Load/ComputeDiff results so segments load correctly.
• Logic simplified: removed the implicit assumption that child_fields is always present by centralizing a single normalization/fallback step used symmetrically for both diff paths, avoiding ad-hoc special-casing and unifying iteration over child groups.
• No data loss / no behavior regression: the fallback only activates when child_fields is empty — non-legacy binlogs continue to use their child_fields unchanged. Add/drop semantics are preserved because the same normalization is applied to both sides of the diff. Unit tests (v1-only, v4-only, mixed cases) were added to validate correctness; LoadDiff::ToString() and extra logging are diagnostic only.

Co-authored-by: Cai Zhang cai.zhang@zilliz.com

Originally posted by @congqixia in milvus-io/milvus#46598