net/http: finishRequest() fails to clean up files generated after modifying request using context

[10196]

Go version

go1.20.6

Output of go env in your module/workspace:

Regardless of env

What did you do?

Before executing net/http Request.ParseMultipartForm(), execute request = request.WithContext(ctx). The temporary file /tmp/multipart- generated afterwards will not be deleted in response.finishRequest().

What did you see happen?

The temporary file /tmp/multipart- will remain and will not be deleted.

What did you expect to see?

No matter what the order of executing Request.ParseMultipartForm and request = request.WithContext(ctx), finishRequest() can always delete the temporary file: /tmp/multipart-.

[10196]

Of course, I know this problem can be solved, for example:
ctx := r.Context()
req := r.WithContext(ctx)
*r = *req

I am more concerned about the fact that there is no description of this problem in the context comments (maybe there is, but I didn't see it?), and why WithContext can't do this automatically?
The previous example combined with the WithContext source code is
r2 := new(Request)
*r2 = *r
r2.ctx = ctx
req = r2
*r = *req

Why not simplify to
r.ctx = ctx

[mknyszek]

CC @neild

[seankhliao]

Duplicate of #58809