fix: race condition in getFormat

[matheus23]

There can be a race condition in ipld, if multiple requests to getFormat happen concurrently. While one of the requests awaits this.loadFormat, another request might finish loading it, and both end up calling this.addFormat.

But if multiple end up calling that function then this.addFormat throws an error:

js-ipld/src/index.js

Lines 65 to 76 in 021b419

	addFormat (format) {
	const codec = format.codec
	if (this.resolvers[format.codec]) {
	const codecName = multicodec.getNameFromCode(codec)
	throw new Error(`Resolver already exists for codec "${codecName}"`)
	}
	this.resolvers[codec] = format
	return this
	}

The solution is a basic software transactional memory (STM) idea: If your reads are invalidated during your transaction, retry the transaction.

In its purest form an STM solution would mean:

async getFormat (codec) {
  // TODO vmx 2019-01-24: Once all CIDs support accessing the codec code
  // instead of the name, remove this part
  if (typeof codec === 'string') {
    codec = multicodec.getCodeFromName(codec)
  }

  if (this.resolvers[codec]) {
    return this.resolvers[codec]
  }

  // If not supported, attempt to dynamically load this format
  const format = await this.loadFormat(codec)
  if (!this.resolvers[codec]) { // detect read invalidation
    return getFormat(codec) // retry
  }
  return format
}

But we can manually inline the getFormat call and slightly improve the if-condition to be more readable and we end up with the code in this PR.

[lidel]

@vmx thoughts? (how does this work in the new ipld stuff?)

[vmx]

The change looks good to me.

The new js-multiformats stuff doesn't have any similar code. Likely you would implement something like this directly in js-ipfs.

[lidel]

@vmx mind merging & releasing? (I lack permissions / know-how :^))