Poll the checker server's socket before a reply read blocks

The native compiler sends most checker requests one at a time and blocks
in recv for each reply. The server answers most of them in a few
microseconds, and a reader the reply wakes takes tens of microseconds more
per round trip, over about 130,000 round trips on a large program.

scriptc_ts7_read now polls the socket with MSG_DONTWAIT for up to 200 us
before it blocks. Windows keeps its blocking pipe read.

packages/compiler/native/ts7-process-child.test.c

@@ -13,6 +13,7 @@
 #include <windows.h>
 #else
 #include <poll.h>
+#include <time.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <unistd.h>
@@ -102,7 +103,9 @@ static int child_main(int argc, char **argv) {
 #endif
     }
   }
-  if (strcmp(mode, "echo") != 0) return 73;
+  // "slow-echo" answers each read 20 ms later: after the client's poll.
+  int slow = strcmp(mode, "slow-echo") == 0;
+  if (!slow && strcmp(mode, "echo") != 0) return 73;
   // Unbuffered reads let short writes make progress without waiting for a
   // complete stdio buffer. Fragment each echoed reply independently.
   uint8_t bytes[17];
@@ -113,6 +116,14 @@ static int child_main(int argc, char **argv) {
     ssize_t count = read(STDIN_FILENO, bytes, sizeof(bytes));
 #endif
     if (count <= 0) return count < 0 ? 74 : 0;
+    if (slow) {
+#ifdef _WIN32
+      Sleep(20);
+#else
+      struct timespec delay = {0, 20000000L};
+      nanosleep(&delay, NULL);
+#endif
+    }
     if (fwrite(bytes, 1, (size_t)count, stdout) != (size_t)count || fflush(stdout) != 0) return 75;
   }
 }

packages/compiler/native/ts7-process.c

@@ -552,6 +552,41 @@ static int valid_range(const uint8_t *bytes, size_t size, uint32_t offset, uint3
   return 1;
 }
 
+#ifndef _WIN32
+/* How long a read polls the socket before it blocks. The server answers
+ * most requests within a few microseconds; a blocked reader that the reply
+ * wakes takes tens of microseconds more per round trip. */
+#define TS7_REPLY_POLL_NS 200000L
+
+static long monotonic_ns(void) {
+  struct timespec now;
+  clock_gettime(CLOCK_MONOTONIC, &now);
+  return now.tv_sec * 1000000000L + now.tv_nsec;
+}
+
+/* recv on the server's socket: polls it for TS7_REPLY_POLL_NS, then waits. */
+static ssize_t ts7_recv(int socket, uint8_t *bytes, size_t length) {
+  long started = -1;
+  for (;;) {
+    ssize_t count = recv(socket, bytes, length, MSG_DONTWAIT);
+    if (count >= 0) return count;
+    if (errno == EINTR) continue;
+    if (errno != EAGAIN && errno != EWOULDBLOCK) return -1;
+    long now = monotonic_ns();
+    if (started < 0) started = now;
+    else if (now - started >= TS7_REPLY_POLL_NS) break;
+#if defined(__x86_64__) || defined(__i386__)
+    __builtin_ia32_pause();
+#elif defined(__aarch64__)
+    __asm__ volatile("yield");
+#endif
+  }
+  ssize_t count;
+  do { count = recv(socket, bytes, length, 0); } while (count < 0 && errno == EINTR);
+  return count;
+}
+#endif
+
 int32_t scriptc_ts7_read(uint32_t handle, uint8_t *bytes, size_t size,
                         uint32_t offset, uint32_t length) {
   Ts7Process *p = find_process(handle);
@@ -565,8 +600,7 @@ int32_t scriptc_ts7_read(uint32_t handle, uint8_t *bytes, size_t size,
   if (code == ERROR_BROKEN_PIPE) return 0;
   system_error(code); return -1;
 #else
-  ssize_t count;
-  do { count = recv(p->socket, bytes + offset, length, 0); } while (count < 0 && errno == EINTR);
+  ssize_t count = ts7_recv(p->socket, bytes + offset, length);
   if (count < 0) { system_error(errno); return -1; }
   return (int32_t)count;
 #endif

packages/compiler/native/ts7-process.test.c

@@ -149,6 +149,20 @@ static void exchange_cases(void) {
   scriptc_ts7_close(newer);
 }
 
+/* A reply that arrives after the read's poll window: the read waits for it. */
+static void slow_reply_cases(void) {
+  uint32_t id = start("slow-echo", 0);
+  assert(id != 0);
+  const uint8_t bytes[3] = {1, 2, 3};
+  uint8_t result[3] = {0};
+  for (unsigned round = 0; round < 3; round++) {
+    write_exact(id, bytes, sizeof(bytes), 0, sizeof(bytes));
+    read_exact(id, result, sizeof(result));
+    assert(memcmp(bytes, result, sizeof(bytes)) == 0);
+  }
+  scriptc_ts7_close(id);
+}
+
 static void failure_cases(void) {
   uint8_t bytes[1] = {42};
   const char *saved = executable;
@@ -289,6 +303,7 @@ static int process_main(int argc, char **argv) {
   unsigned before = resource_count();
   argument_cases();
   exchange_cases();
+  slow_reply_cases();
   failure_cases();
 #ifndef _WIN32
   attach_cases();