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Localhost Relay: non-blocking IO on Linux side, IOCP with fully async reads and writes on Windows side #40171

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Localhost Relay: non-blocking IO on Linux side, IOCP with fully async reads and writes on Windows side #40171

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Localhost Relay: non-blocking IO on Linux side, IOCP with fully async…
Apr 13, 2026
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Apr 13, 2026
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181 changes: 163 additions & 18 deletions src/linux/init/localhost.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
#include <vector>
#include <iostream>

#include <fcntl.h>
#include <libgen.h>
#include <sys/epoll.h>
#include <sys/socket.h>
Expand All @@ -30,6 +31,45 @@

namespace {

// Per-direction relay buffer for the non-blocking socket relay. The read limit
// for each direction is reduced by the amount of data pending from an incomplete
// write, establishing back-pressure through SOCK_STREAM flow control to throttle
// an abusive peer and bound memory usage.
struct RelayDirection
{
int srcFd;
int dstFd;
std::vector<gsl::byte> buf;
size_t head;
size_t tail;
bool srcEof;
bool done;

size_t Pending() const
{
return tail - head;
}
size_t Available() const
{
return buf.size() - tail;
}

void Compact()
{
if (head > 0)
{
auto pending = Pending();
if (pending > 0)
{
memmove(buf.data(), buf.data() + head, pending);
}

head = 0;
tail = pending;
}
}
};

void ListenThread(sockaddr_vm hvSocketAddress, int listenSocket)
{
pollfd pollDescriptors[] = {{listenSocket, POLLIN}};
Expand Down Expand Up @@ -103,40 +143,145 @@ void ListenThread(sockaddr_vm hvSocketAddress, int listenSocket)
return;
}

// Resize the buffer to be the requested size.
buffer.resize(message->BufferSize);
// Switch both sockets to non-blocking for the relay loop.
for (int fd : {tcpSocket.get(), relaySocket.get()})
{
int flags = fcntl(fd, F_GETFL, 0);
THROW_LAST_ERROR_IF(flags < 0);
THROW_LAST_ERROR_IF(fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0);
}

const auto bufferSize = message->BufferSize;
buffer.resize(bufferSize);
RelayDirection dirs[2] = {
{relaySocket.get(), tcpSocket.get(), std::move(buffer), 0, 0, false, false},
{tcpSocket.get(), relaySocket.get(), std::vector<gsl::byte>(bufferSize), 0, 0, false, false},
};

// Begin relaying data.
int outFd[2] = {tcpSocket.get(), relaySocket.get()};
pollfd pollDescriptors[] = {{relaySocket.get(), POLLIN}, {tcpSocket.get(), POLLIN}};
pollfd pfds[4] = {};
int pollDirIndex[4] = {};

for (;;)
{
if ((pollDescriptors[0].fd == -1) || (pollDescriptors[1].fd == -1))
// Complete directions where the source hit EOF and all
// pending data has been flushed to the destination.
for (auto& d : dirs)
{
if (!d.done && d.srcEof && d.Pending() == 0)
{
shutdown(d.dstFd, SHUT_WR);
d.done = true;
}
}

if (dirs[0].done && dirs[1].done)
{
return;
}

// Build the poll set based on current state.
int nfds = 0;

for (int i = 0; i < 2; i++)
{
auto& d = dirs[i];
if (d.done)
{
continue;
}

// Poll for read when the source is open and the buffer has space.
if (!d.srcEof && d.Available() > 0)
{
pfds[nfds] = {d.srcFd, POLLIN, 0};
pollDirIndex[nfds] = i;
nfds++;
}

// Poll for write when there is data waiting to go out.
if (d.Pending() > 0)
{
pfds[nfds] = {d.dstFd, POLLOUT, 0};
pollDirIndex[nfds] = i;
nfds++;
}
}

if (nfds == 0)
{
return;
}

THROW_LAST_ERROR_IF(poll(pollDescriptors, COUNT_OF(pollDescriptors), -1) < 0);
THROW_LAST_ERROR_IF(poll(pfds, nfds, -1) < 0);

bytesRead = 0;
for (int Index = 0; Index < COUNT_OF(pollDescriptors); Index += 1)
for (int j = 0; j < nfds; j++)
{
if (pollDescriptors[Index].revents & POLLIN)
auto& d = dirs[pollDirIndex[j]];

if (pfds[j].events & POLLOUT)
{
bytesRead = UtilReadBuffer(pollDescriptors[Index].fd, buffer);
if (bytesRead == 0)
// can't write to dstFd any more
if (pfds[j].revents & (POLLERR | POLLHUP))
{
pollDescriptors[Index].fd = -1;
shutdown(outFd[Index], SHUT_WR);
d.done = true;
continue;
}
else if (bytesRead < 0)

if (!(pfds[j].revents & POLLOUT))
{
return;
continue;
}

auto written = TEMP_FAILURE_RETRY(write(d.dstFd, d.buf.data() + d.head, d.Pending()));
if (written < 0)
{
if (errno == EAGAIN || errno == EWOULDBLOCK)
{
continue;
}

d.done = true;
continue;
}

d.head += written;
if (d.Pending() == 0)
{
d.head = d.tail = 0;
}
}
else
{
if (!(pfds[j].revents & POLLIN))
{
// No data to read; if the source is gone, mark EOF.
if (pfds[j].revents & (POLLERR | POLLHUP))
{
d.srcEof = true;
}

continue;
}

d.Compact();
auto nread = TEMP_FAILURE_RETRY(read(d.srcFd, d.buf.data() + d.tail, d.Available()));
if (nread == 0)
{
d.srcEof = true;
}
else if (nread < 0)
{
if (errno == EAGAIN || errno == EWOULDBLOCK)
{
continue;
}

d.srcEof = true;
continue;
}
else if (UtilWriteBuffer(outFd[Index], buffer.data(), bytesRead) < 0)
else
{
return;
d.tail += nread;
}
}
}
Expand Down
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