Wasm-git

(Wasm should be pronounced like awesome starting with a W ).

GIT for nodejs and the browser using libgit2 compiled to WebAssembly with Emscripten.

The main purpose of bringing git to the browser, is to enable storage of web application data locally in the users web browser, with the option to synchronize with a remote server.

Compatibility

• libgit2: v1.7.1
• Emscripten: Pinned to 4.0.23
• Node.js: v18+
• Browsers: Modern browsers with WebAssembly support

Demo in the browser

A simple demo in the browser can be found at:

https://wasm-git.petersalomonsen.com/

Please do not abuse, this is open for you to test and see the proof of concept

The sources for the demo can be found in the githttpserver project. It shows basic operations like cloning, edit files, add and commit, push and pull.

Demo videos

Videos showing example applications using wasm-git can bee seen in this playlist. Wasm-git is used for local and offline storage of web application data, and for syncing with a remote server.

Examples

Wasm-git packages are built in three variants: Synchronous, Asynchronous, and OPFS. To run the sync version in the browser, a webworker is needed. This is because of the use of synchronous http requests and long running operations that would block if running on the main thread. The sync version has the smallest binary, but need extra client code to communicate with the web worker. When using the sync version in nodejs worker_threads are used, with Atomics to exchange data between threads.

The async version use Emscripten Asyncify, which allows calling the Wasm-git functions with async / await. It can also run from the main thread in the browser. Asyncify increase the binary size because of instrumentation to unwind and rewind WebAssembly state, but makes it possible to have simple client code without exchanging data with worker threads like in the sync version.

The OPFS version uses Emscripten's WASMFS with the OPFS (Origin Private File System) backend. Unlike the async version, it does not use Asyncify — instead it runs synchronously inside a Web Worker (using pthreads internally for the WASMFS OPFS backend). OPFS provides better performance and quota management compared to IDBFS, making it ideal for modern browser-based applications that need persistent storage.

Examples of using Wasm-git can be found in the tests:

• test for NodeJS
• test-browser for the sync version in the browser with a web worker
• test-browser-async for the async version in the browser
• test-browser-opfs for the OPFS version in the browser

The examples shows importing the lg2.js / lg2_async.js / lg2_opfs.js modules from the local build, but you may also access these from releases available at public CDNs.

OPFS Usage Example

The OPFS version must run in a Web Worker because it requires SharedArrayBuffer (for pthreads), which in turn requires Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp (or credentialless) HTTP headers.

worker.js (Web Worker):

// Import the OPFS-enabled wasm-git module
const lgMod = await import('./lg2_opfs.js');
const lg = await lgMod.default();
const FS = lg.FS;

// WASMFS doesn't pre-create /home/web_user like MEMFS
try { FS.mkdir('/home'); } catch(e) {}
try { FS.mkdir('/home/web_user'); } catch(e) {}
FS.writeFile('/home/web_user/.gitconfig',
  '[user]\n  name = Your Name\n  email = your.email@example.com');

// Create an OPFS-backed directory using the WASMFS OPFS backend
const backend = lg._lg2_create_opfs_backend();
const workingDir = '/opfs';
// Use ccall to marshal the JS string to a C pointer
lg.ccall('lg2_create_directory', 'number', ['string', 'number', 'number'],
         [workingDir, 0o777, backend]);

// Clone using absolute path to avoid CWD ambiguity with WASMFS
const repoDir = workingDir + '/myrepo';
lg.callMain(['clone', 'https://github.com/user/repo.git', repoDir]);

// Work around a WASMFS getcwd() bug: create a root symlink so the path
// returned by getcwd() resolves correctly for libgit2's repo discovery.
FS.symlink(repoDir, '/myrepo');
FS.chdir(repoDir);

// Re-set CWD before each callMain since WASMFS may reset it
FS.chdir(repoDir);
FS.writeFile('newfile.txt', 'Hello OPFS!');
FS.chdir(repoDir);
lg.callMain(['add', 'newfile.txt']);
FS.chdir(repoDir);
lg.callMain(['commit', '-m', 'Add new file']);
FS.chdir(repoDir);
lg.callMain(['push']);

postMessage({ done: true });

See test-browser-opfs/worker.js for a complete working example.

Building and developing

Prerequisites

• Emscripten SDK (version 4.0.23)
• Node.js (v18 or higher)
• CMake
• Make

Quick Start

1. Clone the repository

   git clone https://github.com/petersalomonsen/wasm-git.git
   cd wasm-git

2. Install and activate Emscripten

   git clone https://github.com/emscripten-core/emsdk.git
   cd emsdk
   ./emsdk install 4.0.23
   ./emsdk activate 4.0.23
   source ./emsdk_env.sh
   cd ..

3. Set up libgit2

   ./setup.sh

   This script downloads libgit2 v1.7.1 and applies necessary patches for WebAssembly compilation.

4. Build the project

   cd emscriptenbuild
   ./build.sh         # Debug build (smaller, for development)
   ./build.sh Release # Release build (optimized)

   For async versions (with Asyncify support):

   ./build.sh Debug-async   # Debug async build
   ./build.sh Release-async # Release async build

   For OPFS versions (with WASMFS and OPFS support):

   ./build.sh Debug-opfs   # Debug OPFS build
   ./build.sh Release-opfs # Release OPFS build

5. Install npm dependencies

   npm install

6. Run tests

   npm test                  # Run Node.js tests
   npm run test-browser      # Run browser tests (sync version)
   npm run test-browser-async # Run browser tests (async version)
   npm run test-browser-opfs # Run browser tests (OPFS version)

Development Options

GitHub Codespaces

The easiest way to get started is using GitHub Codespaces. The repository includes a .devcontainer configuration that automatically sets up the complete development environment with all dependencies.

Local Development

The Github actions test pipeline shows all the commands needed for CI/CD and can be used as a reference for local setup.

Build Outputs

After building, you'll find the following files in emscriptenbuild/libgit2/examples/:

• lg2.js and lg2.wasm - Synchronous version
• lg2_async.js and lg2_async.wasm - Asynchronous version with Asyncify
• lg2_opfs.js and lg2_opfs.wasm - OPFS version with WASMFS

These files are also available from npm packages and CDNs for production use.

Filesystem Backends

Wasm-git supports multiple filesystem backends for different use cases:

MEMFS (Memory File System)

• Use case: In-memory storage, not persisted
• Build target: Default (./build.sh Release)
• Browser support: All browsers

IDBFS (IndexedDB File System)

• Use case: Browser persistent storage using IndexedDB
• Build target: Default (./build.sh Release)
• Browser support: All browsers with IndexedDB

NODEFS (Node.js File System)

• Use case: Node.js native filesystem access
• Build target: Default (./build.sh Release)
• Platform: Node.js only

OPFS (Origin Private File System via WASMFS)

• Use case: Modern browser persistent storage with better performance and quota management
• Build target: OPFS builds (./build.sh Release-opfs or ./build.sh Debug-opfs)
• Browser support: Chrome 86+, Edge 86+, Firefox 111+, Safari 15.2+
• Advantages: Better performance and quota compared to IDBFS
• Requirements: Must run in a Web Worker; server must send Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp (or credentialless) headers to enable SharedArrayBuffer
• Note: Uses Emscripten's WASMFS with OPFS backend and -pthread (no Asyncify); callMain is synchronous

Troubleshooting

Common Issues

1. writeArrayToMemory errors: Make sure you're using a compatible Emscripten version (3.1.74+). The project has been updated to use Module.HEAPU8.set() instead.

2. Build errors: Ensure Emscripten environment is properly activated:

   source /path/to/emsdk/emsdk_env.sh

3. Test failures: Remove any stale test directories before running tests:

   rm -rf nodefsclonetest
   npm test

Emscripten fixes that were needed for making Wasm-git work

As part of being able to compile libgit2 to WebAssembly and run it in a Javascript environment, some fixes to Emscripten were needed.

Here are the Pull Requests that resolved the issues identified when the first version was developed:

• emscripten-core/emscripten#10095
• emscripten-core/emscripten#10526
• emscripten-core/emscripten#10782

for using with NODEFS you'll also need emscripten-core/emscripten#10669

All of these pull requests are merged to emscripten master as of 2020-03-29.