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server: use poll instead of select for MCP stdio reads #19

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Merged
pwilkin merged 1 commit into from
Jul 22, 2026
Merged

server: use poll instead of select for MCP stdio reads #19

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24 changes: 11 additions & 13 deletions tools/server/server-mcp.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -27,7 +27,7 @@
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/select.h>
#include <poll.h>
#endif

using json = nlohmann::ordered_json;
Expand Down Expand Up @@ -636,7 +636,7 @@ json server_mcp_instance::send_rpc(const json & request, int timeout_ms) {
return {{"error", {{"code", -32603}, {"message", "request timed out"}}}};
}

// Drain any complete lines already buffered before blocking on select/peek
// Drain any complete lines already buffered before blocking on poll/peek
if (read_message(resp)) {
if (!proc) {
return {{"error", {{"code", -32603}, {"message", "process not running"}}}};
Expand Down Expand Up @@ -695,20 +695,18 @@ json server_mcp_instance::send_rpc(const json & request, int timeout_ms) {
return {{"error", {{"code", -32603}, {"message", "process exited"}}}};
}
#else
fd_set rfds;
FD_ZERO(&rfds);
FD_SET(proc->stdout_fd, &rfds);
struct timeval tv;
// poll instead of select: stdout_fd can exceed FD_SETSIZE on servers with many open descriptors
auto remaining = std::chrono::duration_cast<std::chrono::milliseconds>(deadline - now);
if (remaining.count() <= 0) {
return {{"error", {{"code", -32603}, {"message", "request timed out"}}}};
}
long tv_usec = remaining.count() * 1000;
if (tv_usec > 50 * 1000) tv_usec = 50 * 1000;
tv.tv_sec = 0;
tv.tv_usec = tv_usec;
int sel = select(proc->stdout_fd + 1, &rfds, NULL, NULL, &tv);
if (sel > 0 && FD_ISSET(proc->stdout_fd, &rfds)) {
int slice_ms = remaining.count() > 50 ? 50 : (int) remaining.count();
struct pollfd pfd;
pfd.fd = proc->stdout_fd;
pfd.events = POLLIN;
pfd.revents = 0;
int sel = poll(&pfd, 1, slice_ms);
if (sel > 0 && (pfd.revents & (POLLIN | POLLHUP | POLLERR))) {
if (read_message(resp)) {
if (!proc) {
return {{"error", {{"code", -32603}, {"message", "process not running"}}}};
Expand Down Expand Up @@ -737,7 +735,7 @@ json server_mcp_instance::send_rpc(const json & request, int timeout_ms) {
}
continue;
}
return {{"error", {{"code", -32603}, {"message", "select failed"}}}};
return {{"error", {{"code", -32603}, {"message", "poll failed"}}}};
}
#endif
// Deadline is checked at the top of the next iteration
Expand Down
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