metamask-extension/app/scripts/controllers/permissions/methodMiddleware.js

import createAsyncMiddleware from 'json-rpc-engine/src/createAsyncMiddleware'
import { ethErrors } from 'eth-json-rpc-errors'

/**
 * Create middleware for handling certain methods and preprocessing permissions requests.
 */
export default function createMethodMiddleware ({
  addDomainMetadata,
  getAccounts,
  getUnlockPromise,
  hasPermission,
  requestAccountsPermission,
}) {

  let isProcessingRequestAccounts = false

  return createAsyncMiddleware(async (req, res, next) => {

    switch (req.method) {

      // Intercepting eth_accounts requests for backwards compatibility:
      // The getAccounts call below wraps the rpc-cap middleware, and returns
      // an empty array in case of errors (such as 4100:unauthorized)
      case 'eth_accounts':

        res.result = await getAccounts()
        return

      case 'eth_requestAccounts':

        if (isProcessingRequestAccounts) {
          res.error = ethErrors.rpc.resourceUnavailable(
            'Already processing eth_requestAccounts. Please wait.'
          )
          return
        }

        if (hasPermission('eth_accounts')) {
          isProcessingRequestAccounts = true
          await getUnlockPromise()
          isProcessingRequestAccounts = false
        }

        // first, just try to get accounts
        let accounts = await getAccounts()
        if (accounts.length > 0) {
          res.result = accounts
          return
        }

        // if no accounts, request the accounts permission
        try {
          await requestAccountsPermission()
        } catch (err) {
          res.error = err
          return
        }

        // get the accounts again
        accounts = await getAccounts()
        /* istanbul ignore else: too hard to induce, see below comment */
        if (accounts.length > 0) {
          res.result = accounts
        } else {
          // this should never happen, because it should be caught in the
          // above catch clause
          res.error = ethErrors.rpc.internal(
            'Accounts unexpectedly unavailable. Please report this bug.'
          )
        }

        return

      // custom method for getting metadata from the requesting domain,
      // sent automatically by the inpage provider when it's initialized
      case 'wallet_sendDomainMetadata':

        if (typeof req.domainMetadata?.name === 'string') {
          addDomainMetadata(req.origin, req.domainMetadata)
        }
        res.result = true
        return

      default:
        break
    }

    next()
  })
}