Filter empty wasm hash from snapshot lookups

[leighmcculloch]

What

Wrap the snapshot source used by the test environment so that read-throughs for the ContractCode entry of the empty WASM hash (the SHA256 of zero bytes that the host uses as the synthetic wasm hash for native test contracts) are short circuited and return None without ever reaching the underlying source.

Why

When registering a native test contract with Env::register(), the host reads a ContractCode entry for the empty WASM hash, and because the test environment uses recording-footprint storage that read falls through to the configured SnapshotSource. For custom SnapshotSource implementations that fetch ledger state from remote or RPC endpoints, this forces a request for an entry that can never exist on any real network, adding latency and consuming rate-limited lookups during test setup for what is purely an internal implementation detail of native test contracts.

Known limitations

The empty WASM ContractCode entry is intentionally left in saved ledger snapshots, so to_ledger_snapshot() output is unchanged. A persisted native contract instance still references that hash, so removing the entry would break reloading the snapshot, and keeping it matches how every other contract carries a corresponding code entry. The fix only suppresses the redundant SnapshotSource lookup, which is safe because native contracts dispatch through their registered functions rather than the wasm code, so the empty code entry is never executed on reload.

Close #1635

---

Generated by Claude Code

[Copilot]

Pull request overview

This PR optimizes the Soroban SDK test environment’s snapshot read-through behavior by intercepting SnapshotSource lookups for the synthetic “empty WASM” ContractCode hash used by native test contracts, returning None immediately instead of forwarding the request to potentially remote snapshot sources.

Changes:

• Added a FilteringSnapshotSource wrapper in the testutils Env construction path to short-circuit ContractCode lookups for the empty WASM hash.
• Added a regression test that asserts the empty WASM hash is never requested from a custom SnapshotSource, while other expected lookups still pass through.
• Added a new recorded snapshot output for the new test.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
soroban-sdk/src/env.rs	Wraps the configured snapshot source with a filter that suppresses empty-WASM ContractCode lookups.
soroban-sdk/src/tests/snapshot_source_empty_wasm_hash.rs	Adds a unit test verifying the empty-WASM lookup is not forwarded to the underlying snapshot source.
soroban-sdk/src/tests.rs	Registers the new test module in the crate’s test suite.
soroban-sdk/test_snapshots/tests/snapshot_source_empty_wasm_hash/test_empty_wasm_hash_not_requested_from_snapshot_source.1.json	Adds the golden snapshot output associated with the new test.

[leighmcculloch]

Solving this problem will become more important once we had capabilities like:

• Add a way to install a native contract for later deployment #1945

Nothing needs changing today to support that, but this filtering snapshot source added in here will probably as part of that issue expand to filter out all natively installed contracts.