Fix CreateInstance failure when Windows hosts file exceeds message size cap

[benhillis]

Summary

On Windows 10 where DNS tunneling is unavailable, the Windows hosts file content is embedded in the configuration message sent to the guest init. If the parsed hosts file exceeds 4MB (e.g. ad-blocker hosts lists), the guest rejects it with the message size safety check added in 2.7.x, causing Wsl/Service/CreateInstance/E_UNEXPECTED.

Changes

• Skip oversized hosts file: If the parsed hosts content exceeds 1MB, return empty and emit a user warning instead of embedding it in the message
• Bump message size cap: Increase the receive-side safety cap from 4MB to 16MB (in both socketshared.h shared path and HandleIO.cpp Windows path) as additional headroom

Why this only affects Win10

On Win11, DNS tunneling is enabled by default so the hosts file is never embedded in the config message. On Win10 19044, the DNS tunneling APIs don't exist (ERROR_PROC_NOT_FOUND), so the code falls back to embedding the full hosts file.

Validation

• Built successfully
• The fix is a graceful degradation: users with oversized hosts files get a warning and lose host file reflection in WSL, but the distro starts successfully

Fixes #40696
Fixes #40700
Fixes #40699

[Copilot]

Pull request overview

This PR addresses WSL CreateInstance failures on Windows 10 when DNS tunneling is unavailable and the Windows hosts file content is embedded into the init configuration message. It avoids sending oversized hosts content and increases the socket receive-side message size cap to reduce unexpected failures.

Changes:

• Skip embedding the Windows hosts file content when the parsed payload exceeds 1 MiB, emitting a user warning instead.
• Increase the socket message size safety cap from 4 MiB to 16 MiB in both the shared and Windows receive paths.
• Add a new localized user-warning string describing the oversized-hosts behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/windows/common/HandleIO.cpp	Raises the Windows socket receive message-size limit to 16 MiB.
src/windows/common/filesystem.cpp	Adds a 1 MiB guard for parsed Windows hosts content and emits a user warning when exceeded.
src/shared/inc/socketshared.h	Raises the shared receive message-size limit to 16 MiB for host/guest communication.
localization/strings/en-US/Resources.resw	Introduces a new user-facing warning message about oversized hosts files.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/shared/inc/socketshared.h:74

• The protocol max message size (16 MiB) is now hard-coded here and separately in src/windows/common/HandleIO.cpp. Since both are enforcing the same safety cap for the same message header field, keeping the limit as a shared constant would reduce the risk of future drift (e.g., one side updated but not the other).

    if (MessageSize > 16 * 1024 * 1024) // 16 MiB
    {
#if defined(_MSC_VER)
        THROW_HR_MSG(E_UNEXPECTED, "Message size too large: %llu", MessageSize);
#elif defined(__GNUC__)
        THROW_UNEXPECTED();
#endif
    }