Native Iceberg scan and write build a new FileIO, and a new storage client, for every task #6105

What is the problem the feature request solves?

The native Iceberg operators build a fresh FileIO for every Spark task.

IcebergScanExec::execute_with_tasks calls load_file_io on each partition (native/core/src/execution/operators/iceberg_scan.rs:181), and the native write path does the same per task (iceberg_write.rs:484). Each call runs storage_factory_for, parses the storage configuration, and for S3 with a custom access provider constructs a CometS3CredentialBridge, which calls ensureInitialized over JNI and interns global refs. The instance is dropped when the task finishes, so nothing carries over to the next task on the same executor. On a production workload that was 40,317 constructions for one query.

What a shared FileIO would additionally reuse depends on the backend, because clones share the lazily created Storage behind Arc<OnceLock<Arc<dyn Storage>>>:

• HDFS through hdfs-native (feat(iceberg): support HDFS storage via iceberg-rust hdfs-native backend [NOT FOR MERGE: pending apache/iceberg-rust#3111] #5898). The fork's OpenDalStorage::HdfsNative caches operators per NameNode inside Storage, so a per-task FileIO means a new hdfs-native client and NameNode session per task. Sharing the FileIO shares that client.
• S3 family, GCS, OSS at the pinned iceberg-rust rev. Storage holds config and the access loader only; create_operator builds a new opendal Operator, and with it a new Signer, on every file open, and HTTP pooling is already process-wide. Sharing the FileIO reuses the factory, config and bridge, not the client. Making it share the client too needs an operator cache inside OpenDalStorage::S3, the pattern the HDFS backend already uses; that is an iceberg-rust change, tracked in Iceberg S3-family storage rebuilds the opendal Operator and signer on every file open #6109.

Spark's own Hadoop layer shares one FileSystem per (scheme, authority, user) for the lifetime of the JVM.

This is independent of #6091, though both were found on the same workload.

Describe the potential solution

Cache FileIO instances per executor in iceberg_common.rs and hand tasks clones. Correctness rests on the cache key, because a FileIO carries the access configuration it was built with. Key on every input that shapes the client:

• Access mode. Read and write can be granted different access by the S3 bridge.
• Catalog name. The dispatch key used when vending access.
• The full reference path. The S3 access bridge is constructed with the bucket and url.path() of the reference location and the JVM provider is called with exactly that pair, so a bridge built for one table must not serve another table in the same bucket. All tasks of one scan or write share the path.
• The full catalog property bag. A REST catalog vends short-lived access that rotates, and endpoints can be reconfigured, so a changed property produces a new entry rather than reusing a client built with the previous one.

Constraints:

• Never cache memory:///. The write path assembles manifest bytes in that in-process namespace (iceberg_write.rs:1156); sharing it would let concurrent tasks observe each other's state.
• Bound the cache with LRU eviction, one entry at a time, and drop the evicted FileIO outside the lock: the last clone of a bridge releases JNI global refs. Per-query rotation makes the key space unbounded, and clearing everything would also discard the entry a running query is using.

Clear the cache from release_runtime so the clients are released together with the Tokio runtime.

Additional context

Implementation in #6106. The S3-family follow-up, an operator cache inside OpenDalStorage::S3 in iceberg-storage-opendal, is tracked separately in #6109.