Adding Git Support to a Solid Server

This guide explains how to add Git HTTP backend support to a Solid server, enabling git clone and git push operations on pod containers.

Overview

The Git HTTP protocol allows clients to clone and push to repositories over HTTP. This is implemented using Git's built-in git http-backend CGI program - the same one used by Apache and Nginx.

How It Works

┌─────────────┐     HTTP      ┌──────────────┐     CGI      ┌─────────────────┐
│  Git Client │ ─────────────▶│ Solid Server │ ────────────▶│ git http-backend│
│             │◀───────────── │              │◀──────────── │                 │
└─────────────┘               └──────────────┘              └─────────────────┘

Clone flow:

GET /repo/info/refs?service=git-upload-pack - Discovery
POST /repo/git-upload-pack - Fetch objects

Push flow:

GET /repo/info/refs?service=git-receive-pack - Discovery
POST /repo/git-receive-pack - Send objects

Implementation

1. Detect Git Requests

Git protocol requests are identified by URL patterns:

function isGitRequest(urlPath) {
  return urlPath.includes('/info/refs') ||
    urlPath.includes('/git-upload-pack') ||
    urlPath.includes('/git-receive-pack');
}

function isGitWriteOperation(urlPath) {
  return urlPath.includes('/git-receive-pack');
}

2. Security: Block Direct .git Access

Important: Git protocol requests should be allowed, but direct file access to .git/ contents must be blocked:

// BLOCK: Direct access to .git contents (security risk)
GET /.git/config         → 403 Forbidden
GET /.git/objects/abc123 → 403 Forbidden

// ALLOW: Git protocol (handled by git http-backend)
GET /repo/info/refs?service=git-upload-pack → 200 OK
POST /repo/git-upload-pack                   → 200 OK

3. Authorization with WAC

Check permissions before allowing git operations:

// Clone/fetch requires Read access
// Push requires Write access

const needsWrite = isGitWriteOperation(request.url);
const requiredMode = needsWrite ? 'write' : 'read';

const { allowed } = await checkAccess({
  resourceUrl,
  resourcePath,
  agentWebId: request.webId,
  requiredMode
});

if (!allowed) {
  return reply.code(needsWrite ? 403 : 401).send({
    error: needsWrite ? 'Write access required' : 'Read access required'
  });
}

4. Git HTTP Backend Handler

The core handler spawns git http-backend with CGI environment variables:

import { spawn } from 'child_process';

async function handleGit(request, reply) {
  const urlPath = decodeURIComponent(request.url.split('?')[0]);
  const queryString = request.url.split('?')[1] || '';

  // Build CGI environment
  const env = {
    ...process.env,
    GIT_PROJECT_ROOT: dataRoot,           // Where repos are stored
    GIT_HTTP_EXPORT_ALL: '',              // Allow read access
    GIT_HTTP_RECEIVE_PACK: 'true',        // Enable push
    PATH_INFO: urlPath,
    REQUEST_METHOD: request.method,
    CONTENT_TYPE: request.headers['content-type'] || '',
    QUERY_STRING: queryString,
    CONTENT_LENGTH: request.headers['content-length'] || '0',
  };

  // For non-bare repos, set GIT_DIR to .git subdirectory
  if (isRegularRepo) {
    env.GIT_DIR = path.join(repoPath, '.git');
  }

  // Spawn git http-backend
  const child = spawn('git', ['http-backend'], { env });

  // Send request body (for POST requests)
  if (request.body && request.body.length > 0) {
    child.stdin.write(request.body);
  }
  child.stdin.end();

  // Parse CGI response and send to client
  // ... (see full implementation below)
}

5. CGI Response Parsing

Git http-backend outputs CGI format (headers + body). Parse and forward:

let buffer = Buffer.alloc(0);
let headersSent = false;

child.stdout.on('data', (data) => {
  buffer = Buffer.concat([buffer, data]);

  if (!headersSent) {
    // Find header/body separator (try both \r\n\r\n and \n\n)
    let headerEnd = buffer.indexOf('\r\n\r\n');
    let sep = '\r\n';
    let sepLen = 4;

    if (headerEnd === -1) {
      headerEnd = buffer.indexOf('\n\n');
      sep = '\n';
      sepLen = 2;
    }

    if (headerEnd !== -1) {
      const headerSection = buffer.subarray(0, headerEnd).toString();
      const bodySection = buffer.subarray(headerEnd + sepLen);

      // Parse CGI headers
      for (const line of headerSection.split(sep)) {
        const colonIdx = line.indexOf(':');
        if (colonIdx > 0) {
          const key = line.substring(0, colonIdx).trim();
          const value = line.substring(colonIdx + 1).trim();

          if (key.toLowerCase() === 'status') {
            statusCode = parseInt(value.split(' ')[0], 10);
          } else {
            reply.raw.setHeader(key, value);
          }
        }
      }

      reply.raw.writeHead(statusCode);
      reply.raw.write(bodySection);
      headersSent = true;
    }
  } else {
    reply.raw.write(buffer);
  }
  buffer = Buffer.alloc(0);
});

child.stdout.on('end', () => {
  reply.raw.end();
});

Repository Setup

Regular Repository (with working directory)

cd /path/to/pod/myrepo
git init
echo "# My Project" > README.md
git add .
git commit -m "Initial commit"

Bare Repository (server-only, more efficient)

cd /path/to/pod
git init --bare myrepo.git

ACL for Public Clone

Create /path/to/pod/myrepo/.acl:

@prefix acl: <http://www.w3.org/ns/auth/acl#>.
@prefix foaf: <http://xmlns.com/foaf/0.1/>.

<#public>
    a acl:Authorization;
    acl:agentClass foaf:Agent;
    acl:accessTo <./>;
    acl:default <./>;
    acl:mode acl:Read.

ACL for Authenticated Push

@prefix acl: <http://www.w3.org/ns/auth/acl#>.
@prefix foaf: <http://xmlns.com/foaf/0.1/>.

<#owner>
    a acl:Authorization;
    acl:agent <https://alice.example.com/#me>;
    acl:accessTo <./>;
    acl:default <./>;
    acl:mode acl:Read, acl:Write, acl:Control.

<#public>
    a acl:Authorization;
    acl:agentClass foaf:Agent;
    acl:accessTo <./>;
    acl:default <./>;
    acl:mode acl:Read.

Usage

Server

# Start server with git support enabled
jss start --git

# Or via environment variable
JSS_GIT=true jss start

Client

# Clone
git clone http://localhost:3000/myrepo

# Clone with authentication (if required)
git clone http://localhost:3000/myrepo
# Git will prompt for credentials

# Push (requires write access)
cd myrepo
echo "New content" >> README.md
git add .
git commit -m "Update readme"
git push

Complete Handler Code

See src/handlers/git.js in the JSS repository for the full implementation.

References

Prior Art

nosdav/server - Git support implementation
QuitStore - Git + RDF versioning

melvincarvalho/git-support.md

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