$return http binding returns body buffers as JSON

[dhensby]

I am trying to serve an image from a function app running on node. At the moment we are doing this successfully using the context.res binding, however we are migrating our function apps to use the $return output name/binding (as is the recommendation in the docs for functions with a single output).

However, there seems to be inconsistent behaviour with named output bindings and the $return output binding. It appears that the isRaw flag is being ignored or not acted on in the same way when using the $return binding compared to a named binding.

Investigative information

Please provide the following:

• Timestamp: 10/21/2019, 2:57:25.913 PM
• Function App version (1.0 or 2.0): 2.0
• Function App name: -
• Function name(s) (as appropriate): -
• Invocation ID: f40d7cbb-c5d1-435f-ae4c-058d4f9fe647
• Region: West Europe

Repro steps

Create a function like so:

function.json:

{
  "disabled": false,
  "tracing": {
    "consoleLevel": "verbose"
  },
  "bindings": [
    {
      "authLevel": "anonymous",
      "type": "httpTrigger",
      "direction": "in",
      "route": "image",
      "methods": ["GET"],
      "name": "req"
    },
    {
      "type": "http",
      "direction": "out",
      "name": "$return"
    }
  ]
}

index.js:

module.exports = async () => {
    return {
        status: 200,
        headers: { 'content-type': 'image/jpeg' },
        body: Buffer.from([255, 216, 255, ...]), // this is the opening byte sequence for a jpg
        isRaw: true,
    };
}

Expected behavior

The response should be sent to the browser without being mangled (the isRaw flag is set to true) and the Buffer should be output as binary

$ curl [function host]/image
Date: Mon, 21 Oct 2019 15:22:45 GMT
Content-Type: image/jpeg
Content-Length: 38

���

Actual behavior

The response is a json-encoded Buffer and sent to the browser (with ; charset=utf-8 appended to the content type)

$ curl [function host]/image
Date: Mon, 21 Oct 2019 15:22:45 GMT
Content-Type: image/jpeg; charset=utf-8
Content-Length: 38

{"type":"Buffer","data":[255,216,255]}

Known workarounds

Use a named output binding:

function.json:

{
  "disabled": false,
  "tracing": {
    "consoleLevel": "verbose"
  },
  "bindings": [
    {
      "authLevel": "anonymous",
      "type": "httpTrigger",
      "direction": "in",
      "route": "image",
      "methods": ["GET"],
      "name": "req"
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    }
  ]
}

index.js:

module.exports = async (context) => {
    context.res = {
        status: 200,
        headers: { 'content-type': 'image/jpeg' },
        body: Buffer.from([255, 216, 255, ...]), // this is the opening byte sequence for a jpg
        isRaw: true,
    };
}

Related information

This is a duplicate of Azure/azure-functions-host#3892 but hopefully provides some greater detail

[pragnagopa]

cc @mhoeger

Moving this issue to nodejs worker. 'isRaw flag should be honored with $return as well

[dhensby]

Moving this issue to nodejs worker. 'isRaw flag should be honored with $return as well

@pragnagopa is there an GitHub issue you can share for that?

[pragnagopa]

@dhensby - this is is the issue tracking the fix

[mhoeger]

This is the root cause - #228

Unfortunately, a known bug, saving for v3 for the breaking fix :\

[pragnagopa]

closing as dup of #228