bazelbuild/bazel

Loaded wasm modules are not shareable between module extension and instantiated repository rules

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#26974 opened on Sep 12, 2025

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P2help wantedteam-ExternalDepstype: bug

Description

Description of the bug:

I have a wasm module that converts toml to json. It's used in rules_rs in the hub-and-spokes pattern ("hub" @cargo repo provides aliases to targets in "spoke" repos, 1 repo for each crate). The module extension needs to parse the Cargo.lock toml file, and each spoke repo needs to parse the Cargo.toml in the downloaded crate. Since the download is triggered in the spoke repo via rctx.download_and_extract, the parsing must also occur in the spoke repo.

repository_ctx.load_wasm is intended to allow loading the module once and executing it repeatedly, but it's not clear how to pass the reference to the loaded module from the module extension to the spoke repos, and It seems wasteful to load the module repeatedly. (Not sure how to get the timing here, but loading this module into wazero, a different wasm runtime, takes around 70ms).

This hub-and-spoke model is a common pattern for rulesets that mirror package-manager dependency trees into Bazel (cargo, pnpm/rules_js, pypi/rules_python, etc.)

Which category does this issue belong to?

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What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

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Which operating system are you running Bazel on?

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What is the output of bazel info release?

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If bazel info release returns development version or (@non-git), tell us how you built Bazel.

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What's the output of git remote get-url origin; git rev-parse HEAD ?

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

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Have you found anything relevant by searching the web?

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Any other information, logs, or outputs that you want to share?

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Contributor guide

Tech stack
javawasm
Domain
build systemperformance
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Knowledge of Bazel's module extension systemUnderstanding of WebAssembly (WASM) runtimeFamiliarity with Bazel's repository rules
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate Bazel's source code for module extension and repository rule implementations, particularly how wasm modules are loaded via repository ctx.load wasm. Propose a mechanism to share loaded wasm modules across contexts, potentially by caching or exposing an API. Examine the wazero runtime performance and look at hub and spoke patterns in rules rl to understand the requirements.