network: keep fcntl modes across MSG_DONTWAIT and decompress room packets once

A receive with MSG_DONTWAIT restored the blocking mode from the descriptor copy taken
before the call. When fcntl(SETFL, O_NONBLOCK) ran during the receive, the fd kept
reporting O_NONBLOCK while its socket blocked again. Receives now switch the mode only
for MSG_DONTWAIT and read the current flags under file_descriptors_mutex at both ends,
and SETFL changes the mode and the stored flags in the same critical section.

OnProxyPacketReceived decompressed each room packet once per matching socket while
holding file_descriptors_mutex, so every bsd call waited for it. It now copies the
distinct sockets under the lock, decompresses once without it and delivers the result.

Also reject fd == MAX_FD in IsFileDescriptorValid and PollImpl, which read past the
table, and guard the ProxySocket blocking and receive timeout setters with
packets_mutex, matching their reader.

Assisted-by: Claude Opus 5.5 (investigation and implementation)

1 files changed, 2 insertions, 0 deletions

diff --git a/src/core/hle/service/sockets/bsd.h b/src/core/hle/service/sockets/bsd.h
index f908edc7d2..559830d7a1 100644
--- a/src/core/hle/service/sockets/bsd.h
+++ b/src/core/hle/service/sockets/bsd.h
@@ -180,6 +180,8 @@ private:
     bool IsFileDescriptorValid(s32 fd) const noexcept;
     /// Copies a descriptor under file_descriptors_mutex; nullopt if fd has no socket
     std::optional<FileDescriptor> CopyDescriptor(s32 fd);
+    /// Switches a blocking socket for one MSG_DONTWAIT call; false if fd is already nonblocking
+    bool SetDontWait(s32 fd, const FileDescriptor& descriptor, bool enable);
 
     void BuildErrnoResponse(HLERequestContext& ctx, Errno bsd_errno) const noexcept;