Cannot calculate proper length in synchronous way. (node-fetch + form-data)

[Evgeny]

Error: Cannot calculate proper length in synchronous way.
      at FormData.getLengthSync (node_modules/form-data/lib/form_data.js:245:17)
      at node_modules/node-fetch/index.js:96:48
      at new Fetch (node_modules/node-fetch/index.js:49:9)
      at Fetch (node_modules/node-fetch/index.js:37:10)

This line causes the error: https://github.com/bitinn/node-fetch/blob/master/index.js#L96

[David Frank]

This is a hack to support form-data module, so you don't have to set content-length header manually. You might want to figure out why form-data throws this error in the first place (probably related to your body data type).

[Evgeny]

@bitinn

I've tried to send following body:

form.append('my_file', fs.createReadStream('/foo/bar.jpg'));

[David Frank]

Due to the nature of callback, we don't currently use getLength to determine form-data's content-length, but you can calculate the content-length yourself (either with FormData.getLength or count the stream byte itself) then pass it onto node-fetch's headers option manually, this will skip the hack as content-length is provided already.

[Fred Cox]

I have worked around this bug for the moment by removing the getLengthSync method before passing it to node-fetch

form.getLengthSync = null;

Some servers don't like it not having a content-length header, but its certainly not required by the spec and many servers will work fine without it.

[David Frank]

@mcfedr please don't do this, why can't you call getLength to figure out content-length?

UPDATE: see my reply here #106 (comment)

[David Frank]

Alternative, don't use form-data, because you are just sending files right? Only use form-data to manage your custom headers, let node-fetch handle the body as native stream instead of form-data instance.

[Fred Cox]

For local files its possible to use fs.stat and set knownLength when adding to form-data

I am sending another stream, and I dont know the length of the stream. Here are example of how things are broken, these are bugs in both node-fetch and form-data

https://gist.github.com/mcfedr/0a6fbaeaf0cfaddbeadfe145c002c8e5

I think in the 'http' example node-fetch should ignore the correctly thrown error and be ok with not setting a content-length

In the 'fetch' example form-data needs to throw the error because it doesnt know its length

[David Frank]

Quick summary: unfortunately form-data doesn't have an API for checking whether we can call getLengthSync yet, so we have 3 options:

1. to do it correctly and manually, end user can do the calculation with form.getLength callback and overwrite our default behavior with custom content-length
2. to not add content-length at all, use the form.getLengthSync = null hack mcfedr mentioned to prevent node-fetch from calling getLengthSync
3. we might add a formdata._lengthRetrievers.length > 0 check to skip calling getLengthSync automagically, as discussed in form-data/form-data#196

[David Frank]

No response from form-data dev, I am going with option 3.

[Uladzimir Havenchyk]

@bitinn @mcfedr do you have working workaround right now?

[David Frank]

@havenchyk I will update this issue when release, but you should read my reply above #102 (comment)

[Uladzimir Havenchyk]

@bitinn thanks for the quick response. I actually read it and understand that I need to skip the check somehow.

My problem is that everything worked fine for a year and now it's broken. I'm trying to figure out, is it a problem of FormData(maybe from last releases) or of node-fetch.

[Fred Cox]

@havenchyk my work around works, #102 (comment)

[David Frank]

@havenchyk I need to see the error to conclusively say you are facing the same problem. But form-data has been doing this for quite some time, so unless you changed your data source to a stream, I don't see why it would suddenly break (unless you upgrade from a very old node-fetch which doesn't use hack to handle form-data previously)

[Uladzimir Havenchyk]

@bitinn it's definitely the same, workaround from @mcfedr solved my issue. Can I help you to make release faster?

[David Frank]

workaround has been added and released as v1.5.3