[6.2] Use default libcurl receive buffer size - #5522

What changed

This backports #5448 to release/6.2.

Foundation Networking no longer overrides libcurl's receive buffer size when configuring web and file-transfer requests. Both protocols now use libcurl's default receive buffer size.

Why

Swift Package Manager can issue more concurrent registry downloads than the per-host connection limit. Registry archives are served through redirects, so Foundation Networking reuses an existing libcurl handle for the redirected request.

With Swift 6.2 on Linux, that combination can terminate swift package resolve with libcurl error 43. The fix is already present on main, but Swift 6.2.4 was released before it merged.

Root cause

The Swift 6.2 implementation reapplies CURLOPT_BUFFERSIZE every time _HTTPURLProtocol or _FTPURLProtocol configures a transfer. curl 8.5.0 can reject that operation with CURLE_BAD_FUNCTION_ARGUMENT when the reused handle still owns its receive buffer.

Foundation Networking treats the returned error as a programming error through try!, so the package-resolution process traps instead of reporting a recoverable download failure.

Approach

Remove the explicit buffer-size assignments and let libcurl retain its default. This matches the accepted fix in #5448.

The removed value was capped to CURL_MAX_WRITE_SIZE, which is also libcurl's default receive buffer size, so this avoids the unsafe reconfiguration without changing the effective buffer size.

Impact

Swift Package Manager registry resolution no longer crashes when concurrent package downloads cross redirects on affected Linux distributions. File-transfer requests also stop applying the same unsafe override.

Reproduction

The failure can be reproduced with Swift Package Manager and the Swift 6.2 container:

git clone --depth 1 \
  --branch backport/local-package-test-targets-4.197.x \
  https://github.com/tuist/tuist.git
cd tuist

docker run --rm \
  -v "$PWD:/workspace" \
  -w /workspace \
  swift:6.2 \
  swift package resolve --replace-scm-with-registry

Before this backport, the command terminates in _HTTPURLProtocol.configureEasyHandle during redirect handling:

FoundationNetworking/EasyHandle.swift:293: Fatal error: 'try!' expression unexpectedly raised an error: Error Domain=libcurl.Easy Code=43

Validation

• Reproduced the crash locally with the command above using the stock Swift 6.2.4 Foundation Networking library.
• Built the patched release/6.2 source with the coordinated Swift 6.2.4 dependency revisions.
• Ran swift test --filter TestURLSession/test_httpRedirectionWithCode303. The targeted redirect test passed.