Fix #1972: bypass body-reading for HEAD requests in io_client

[amanmaurya92]

Fixes #1972

This PR resolves an issue where HEAD requests unexpectedly throw a ClientException due to a mismatch between the Content-Length header and the actual response body length (which is naturally empty for HEAD requests).

Because dart:io's HttpClient will sometimes fail when verifying the Content-Length of an empty HEAD response body, we now intercept HEAD requests in io_client.dart. Instead of attempting to read the body stream, we asynchronously drain the underlying connection (safely swallowing any spurious dart:io length-mismatch exceptions) and immediately return the headers with a safe, empty stream.

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[amanmaurya92]

While making this change, I also noticed a TODO in io_client.dart regarding dart-lang/sdk#53005 (removing the .trimRight() call when parsing headers).

Since that SDK issue appears to be resolved, we could probably clean this up now. Should I make a separate PR to refactor and remove the .trimRight() workaround, or is someone already planning to tackle that alongside a minimum SDK version bump?

[Keetz]

I have no experience with the Dart Lang codebase, but out of curiosity I checked your code (I created the issue).
Why not find the place where the content length and response length check is made, and omit it from HEAD calls, instead of this approach of "swallowing" the exception. What if it is another exception? That doesn't seem to be handled correctly, it would just ignore any exceptions here right?

[brianquinlan]

I added a comment to #1972 and #1979 attempts to reproduce the problem without success.

Do you have a better reproduction?

[amanmaurya92]

Hi @brianquinlan,

Thanks for looking into this and creating #1979.

I do not have a reproduction either. I tested this against master (both with your conformance test from #1979 and with a local test server returning Content-Length: 100 with an empty body on a HEAD request), and IOClient handles it without throwing any ClientException (since dart:io's HTTP parser already sets isHead = true and _noMessageBody = true).

This PR was opened under the assumption from #1972 that IOClient was failing on HEAD responses with a Content-Length header, but it appears IOClient is not the culprit here. The issue reporter (@Keetz) might be using a different client (e.g. browser/Fetch, a proxy, or a third-party wrapper), or experiencing an edge case with a specific server behavior.

Given that, I'm happy to close this PR for now until @Keetz provides a minimal reproduction and specifies their client and environment on #1972. If it turns out to affect a different client in this repo, we can address it there.

[brianquinlan]

Thanks @amanmaurya92 !