AddFiles: read side of the schema pre-pass - #39933
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…emas.canonical, CollectDistinctSchemas) New, not yet wired code that turns a PCollection of file paths into the list of distinct schemas those files carry, with file counts. The pre-pass that will use it exists because manifest entries are immutable: a file registered before the table knows one of its columns never gets stats for that column, so the table schema has to be brought up to date before any file is registered, and that requires looking at every footer first. ReadFooterSchema (DoFn<String, String>) reads each Parquet footer on the BoundedAsyncTasks pool and emits the file's canonical schema as JSON. Non-Parquet paths and unknown extensions contribute nothing; a footer that cannot be read or converted is logged and counted (numFooterReadErrors) but never fails the pipeline: the per-file registration step reports such files individually later. FileSchemas.canonical sorts struct fields by name at every level and renumbers ids in deterministic order. The ids are positional and never consumed downstream: the commit side reconciles columns by name (unionByNameWith). CollectDistinctSchemas is a CombineFn over the canonical JSON strings (Map<String, Long> accumulator) producing List<KV<String, Long>> ordered most common first, ties broken by the JSON text for determinism. The most common schema goes first because the commit side uses it as the seed when the table does not exist yet.
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ReadFooterSchema, FileScemas.canonical, CollectDistinctSchemas)
New, not yet wired code that turns a PCollection of file paths into the list of distinct schemas those files carry, with file counts. The pre-pass that will use it exists because manifest entries are immutable: a file registered before the table knows one of its columns never gets stats for that column, so the table schema has to be brought up to date before any file is registered, and that requires looking at every footer first.
ReadFooterSchema (DoFn<String, String>) reads each Parquet footer on the BoundedAsyncTasks pool and emits the file's canonical schema as JSON. Non-Parquet paths and unknown extensions contribute nothing; a footer that cannot be read or converted is logged and counted (numFooterReadErrors) but never fails the pipeline: the per-file registration step reports such files individually later.
FileSchemas.canonical sorts struct fields by name at every level and renumbers ids in deterministic order.
The ids are positional and never consumed downstream: the commit side reconciles columns by name (unionByNameWith).
CollectDistinctSchemas is a CombineFn over the canonical JSON strings (Map<String, Long> accumulator) producing List<KV<String, Long>> ordered most common first, ties broken by the JSON text for determinism. The most common schema goes first because the commit side uses it as the seed when the table does not exist yet.
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