[Bug] Subgraph startup rehashes the WASM module once per data source (minutes for subgraphs with millions of dynamic data sources) #6731

Bug report

When a subgraph runner starts, SubgraphInstance::new_host
(core/src/subgraph/context/instance/mod.rs) computes keccak256 over the
full WASM module bytes for every data source, and only then looks the hash
up in module_cache:

let module_hash = alloy::primitives::keccak256(module_bytes.as_ref()).0;
if let Some(sender) = self.module_cache.get(&module_hash) {
    sender.clone()
} else { /* spawn_mapping … */ }

module_cache avoids recompiling the module, but not rehashing it. All data
sources created from the same template share one Arc of module bytes, so the
same bytes are hashed again for each of them. Startup time is therefore
O(number of data sources × module size), where it could be
O(number of data sources + number of distinct modules × module size).

This is paid on every runner start: node restart, subgraph restart after an
error, unassign/reassign, etc. Hashing the module for every data source dates
back to v0.19.0 (in another form since v0.17), and is still on master
(core/src/subgraph/context/instance/mod.rs:107 at 6838f4e).

Expected: each distinct module is hashed once per runner start.
Actual: each module is hashed once per data source.

Impact

On the Uniswap v3 subgraph on Base (deployment QmTEVcvbvWdW1LhNCdGpUFTxjxGmA34aRUksvX8TPjrQkZ),
with 1,770,112 data sources (all but one created from the pool template, whose
module is 66,103 bytes), startup takes about 4 minutes, nearly all of it in this
loop: graph-node logs nothing between Data source count at start and
forcing subgraph to use static filters (~236 s in the log below).

Relevant log output

2026-10-09T19:27:41.705 INFO Resolve subgraph files using IPFS, n_templates: 1, n_data_sources: 1, runner_index: 0, subgraph_id: QmTEVcvbvWdW1LhNCdGpUFTxjxGmA34aRUksvX8TPjrQkZ
2026-10-09T19:27:45.167 INFO Data source count at start: 1770112, runner_index: 0, subgraph_id: QmTEVcvbvWdW1LhNCdGpUFTxjxGmA34aRUksvX8TPjrQkZ
2026-10-09T19:31:41.628 INFO forcing subgraph to use static filters., runner_index: 0, subgraph_id: QmTEVcvbvWdW1LhNCdGpUFTxjxGmA34aRUksvX8TPjrQkZ
2026-10-09T19:31:41.688 INFO Start processing block, triggers: 66, runner_index: 0, subgraph_id: QmTEVcvbvWdW1LhNCdGpUFTxjxGmA34aRUksvX8TPjrQkZ

IPFS hash

QmTEVcvbvWdW1LhNCdGpUFTxjxGmA34aRUksvX8TPjrQkZ

Subgraph name or link to explorer

Uniswap v3 (Base)

Some information to help us out

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OS information

Linux