Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

[ghazel]

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
     at ServerResponse.setHeader (_http_outgoing.js:482:11)
     at vary (/myproj/node_modules/vary/index.js:147:9)
     at send (/myproj/node_modules/webp-middleware/index.js:24:3)
     at sendAndSave (/myproj/node_modules/webp-middleware/index.js:30:3)
     at /myproj/node_modules/webp-middleware/index.js:84:3
     at suppressedCallback (fs.js:205:5)
     at FSReqCallback.args [as oncomplete] (fs.js:146:20)

Looking at the headers around error time, Vary has already been set:

{
'x-powered-by': 'Express',
vary: 'Accept',
'accept-ranges': 'bytes',
'cache-control': 'public, max-age=0',
'last-modified': 'Tue, 05 Feb 2019 03:48:14 GMT',
etag: 'W/"1319a-168bbc5443a"',
'content-type': 'image/webp',
'content-length': 78234
}

[ghazel]

This appears to happen after sendFile is attempted but fails: https://github.com/garthenweb/webp-middleware/blob/master/index.js#L73 and is retried, because Express sends the headers before the error occurs. (They specifically mention the caller should check res.headersSent: https://github.com/expressjs/express/blob/3d10279826f59bf68e28995ce423f7bc4d2f11cf/lib/response.js#L518 )

[garthenweb]

Thank you for the report and investigation. I will in the near future probably not have the time to fix this, but I would highly appreciate a PR!

[ghazel]

The only solution I found was to disable the "try lookup cache for fast access" section entirely.

[garthenweb]

We might want to consider not to retry in case sent file failed... In the end, this will probably not solve your issue as it will fail anyhow.
Do you have an idea what can cause an error while sending the file? Are you able to provide a reproducible example?