const clone = require('clone')
const pify = require('pify')
const createAsyncMiddleware = require('json-rpc-engine/src/createAsyncMiddleware')
const blockTagParamIndex = require('./cache-utils').blockTagParamIndex
//
// RetryOnEmptyMiddleware will retry any request with an empty response that has
// a numbered block reference at or lower than the blockTracker's latest block.
// Its useful for dealing with load-balanced ethereum JSON RPC
// nodes that are not always in sync with each other.
//
module.exports = createRetryOnEmptyMiddleware
// empty values used to determine if a request should be retried
// `<nil>` comes from https://github.com/ethereum/go-ethereum/issues/16925
const emptyValues = [undefined, null, '\u003cnil\u003e']
function createRetryOnEmptyMiddleware (opts = {}) {
const { provider, blockTracker } = opts
if (!provider) throw Error('BlockRefRewriteMiddleware - mandatory "provider" option is missing.')
if (!blockTracker) throw Error('BlockRefRewriteMiddleware - mandatory "blockTracker" option is missing.')
return createAsyncMiddleware(async (req, res, next) => {
const blockRefIndex = blockTagParamIndex(req)
// skip if method does not include blockRef
if (blockRefIndex === undefined) return next()
// skip if not exact block references
let blockRef = req.params[blockRefIndex]
// omitted blockRef implies "latest"
if (blockRef === undefined) blockRef = 'latest'
// skip if non-number block reference
if (['latest', 'pending'].includes(blockRef)) return next()
// skip if block refernce is not a valid number
const blockRefNumber = Number.parseInt(blockRef.slice(2), 16)
if (Number.isNaN(blockRefNumber)) return next()
// lookup latest block
const latestBlockNumberHex = await blockTracker.getLatestBlock()
const latestBlockNumber = Number.parseInt(latestBlockNumberHex.slice(2), 16)
// skip if request block number is higher than current
if (blockRefNumber > latestBlockNumber) return next()
// create child request with specific block-ref
const childRequest = clone(req)
// attempt child request until non-empty response is received
const childResponse = await retry(10, async () => {
const attemptResponse = await pify(provider.sendAsync).call(provider, childRequest)
// verify result
if (emptyValues.includes(attemptResponse.result)) {
throw new Error(`RetryOnEmptyMiddleware - empty response "${JSON.stringify(attemptResponse)}" for request "${JSON.stringify(childRequest)}"`)
}
return attemptResponse
})
// copy child response onto original response
res.result = childResponse.result
res.error = childResponse.error
})
}
async function retry(maxRetries, asyncFn) {
for (let index = 0; index < maxRetries; index++) {
try {
return await asyncFn()
} catch (err) {
await timeout(1000)
}
}
throw new Error('BlockReEmitMiddleware - retries exhausted')
}
function timeout(duration) {
return new Promise(resolve => setTimeout(resolve, duration))
}