ERR_HTTP_HEADERS_SENT with 5.3.0 / 5.3.1 and https-proxy-agent configured

[MelkorCC]

Describe the bug

In 5.3.0 7b0a65e added custom logic that utilizes the proxyReq event to set the origin header. This throws an ERR_HTTP_HEADERS_SENT error for me when using using with https-proxy-agent (for example required in setups in which a corporate proxy is in place). I suspect the problem is the same as described in chimurai/http-proxy-middleware#957 and similar / related to the problem described in chimurai/http-proxy-middleware#472 (explanation provided in chimurai/http-proxy-middleware#472 (comment)).

When downgrading to 5.2.13 everything works again.

Reproduction

https://stackblitz.com/edit/github-sadccs?file=src%2FApp.vue,vite.config.js

Steps to reproduce

Run npm install
Replace https://example.org with an actual https proxy in vite.config.js (i don't know a public available one, sorry)
Run npm run dev

First click on the "Without proxy agent" button, you should see the expected result then.
Now click on the "With proxy agent" button, the ERR_HTTP_HEADERS_SENT should appear in the console.

System Info

Binaries:
    Node: 18.15.0
    npm: 9.5.0

Used Package Manager

npm

Logs

Click to expand!

node:_http_outgoing:663
    throw new ERR_HTTP_HEADERS_SENT('set');
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:399:5)
    at ClientRequest.setHeader (node:_http_outgoing:663:11)
    at setOriginHeader (file:///-------/node_modules/vite/dist/node/chunks/dep-BcXSligG.js:61768:16)

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
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• The provided reproduction is a minimal reproducible example of the bug.

[stackblitz]

Start a new pull request in StackBlitz Codeflow.

• github-sadccs (Open reproduction)

[pfdgithub]

Encountered the same issue.

The exception comes from a commit introduced in vite@5.3.0-beta.0 (7b0a65e).
This commit will modify the origin in the proxied request header by default, sometimes it will throw exception Cannot set headers after they are sent to the client.

In my case, I observed that the origin does not exist in the GET request header, and no exception is triggered. However, the origin exists in the POST request header, which triggers an exception. (why?)
Modify the origin in request headers is only necessary when the server-side explicitly checks for it. In most other scenarios, it is not required.

Three solutions:

1. Disable modify of origin by default
2. Check proxyReq.headersSent before modifying origin
3. Delete origin before entering the vite proxy

[bluwy]

2. Check proxyReq.headersSent before modifying origin

This seems the be the best-fix for me, but I'm not too familiar with the original change.

@johnhunter @sapphi-red do you have thoughts on this?

[johnhunter]

Option 2 sounds like a good solution. I'll have a proper look when I'm back at my desk.

[johnhunter]

I'll address this along with #17562