Allow to read iceberg table data from any location

[zvonand]

Supersedes #1092, #1163, #1212

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Support Iceberg tables that have files outside table location or on different storage.

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All with Aarch64
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [82c6f88]

[vzakaznikov]

AI audit note: This review comment was generated by AI (gpt-5.3-codex).

Audit update for PR #1461 (Iceberg external location support):

Confirmed defects:

• Medium: Inconsistent same-storage key normalization after prefetch exhaustion

  • Impact: Iceberg reads can fail when absolute-path files exceed prefetch window (max_threads), because later tasks may keep non-normalized path while still using base storage.
  • Anchor: src/Storages/ObjectStorage/StorageObjectStorageSource.cpp / ReadTaskIterator::next() vs constructor prefetch branch.
  • Trigger: absolute same-storage paths with file count > prefetch size.
  • Why defect: path normalization behavior differs by iterator branch, so correctness depends on task position.
  • Fix direction (short): always apply resolved key rewrite when absolute path is present; keep storage-switch decision separate.
  • Regression test direction (short): integration case with >max_threads files using absolute same-storage paths and validation that all tasks resolve key consistently.

• Low: Secondary storage creation is performed under shared cache mutex

  • Impact: high contention and elevated deadlock-risk surface under concurrent cold-cache resolutions.
  • Anchor: src/Storages/ObjectStorage/Utils.cpp / getOrCreateStorageAndKey.
  • Trigger: concurrent scans that resolve many external locations simultaneously.
  • Why defect: potentially heavy ObjectStorageFactory::create(...) is executed inside locked section.
  • Fix direction (short): use double-checked creation (check under lock, create outside lock, emplace under lock).
  • Regression test direction (short): concurrent resolver stress test over many distinct cache keys; assert no deadlock and bounded latency.

Coverage summary:

• Scope reviewed: resolver/iterator/position-delete/protocol paths touched by this PR.
• Categories failed: path normalization parity; cache-lock concurrency.
• Categories passed: protocol compatibility and unsupported-scheme error handling.
• Assumptions/limits: static audit only; no runtime test execution in this pass.