Share file offset across processes

This change ensures that if a file descriptor for an open disk file gets
shared by multiple processes within a process tree, then lseek() changes
will be visible across processes, and read() / write() are synchronized.
Note this only applies to Windows, because UNIX kernels already do this.

test/posix/file_offset_shared_test.c

@@ -0,0 +1,107 @@
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <stdatomic.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+// test that file offset is shared across multiple processes
+
+atomic_int *phase;
+
+int main() {
+
+  if ((phase = mmap(0, sizeof(atomic_int), PROT_READ | PROT_WRITE,
+                    MAP_SHARED | MAP_ANONYMOUS, -1, 0)) == MAP_FAILED)
+    return 1;
+
+  int fd;
+  char path[] = "/tmp/fd_test.XXXXXX";
+  if ((fd = mkstemp(path)) == -1)
+    return 2;
+  if (lseek(fd, 0, SEEK_CUR) != 0)
+    return 22;
+
+  if (write(fd, "0", 1) != 1)
+    return 3;
+  if (lseek(fd, 0, SEEK_CUR) != 1)
+    return 33;
+
+  int pid;
+  if ((pid = fork()) == -1)
+    return 4;
+
+  if (!pid) {
+    if (write(fd, "1", 1) != 1)
+      _Exit(100);
+    if (lseek(fd, 0, SEEK_CUR) != 2)
+      _Exit(101);
+
+    *phase = 1;
+    for (;;)
+      if (*phase == 2)
+        break;
+
+    if (write(fd, "3", 1) != 1)
+      _Exit(102);
+    if (lseek(fd, 0, SEEK_CUR) != 4)
+      _Exit(103);
+    *phase = 3;
+    _Exit(0);
+  }
+
+  for (;;)
+    if (*phase == 1)
+      break;
+
+  if (write(fd, "2", 1) != 1)
+    return 5;
+  if (lseek(fd, 0, SEEK_CUR) != 3)
+    return 55;
+
+  *phase = 2;
+  for (;;)
+    if (*phase == 3)
+      break;
+
+  if (write(fd, "4", 1) != 1)
+    return 6;
+  if (lseek(fd, 0, SEEK_CUR) != 5)
+    return 66;
+
+  int ws;
+  if (wait(&ws) == -1)
+    return 7;
+  if (!WIFEXITED(ws))
+    return 8;
+  if (WEXITSTATUS(ws))
+    return WEXITSTATUS(ws);
+
+  char buf[16] = {0};
+  if (pread(fd, buf, 15, 0) != 5)
+    return 12;
+  if (lseek(fd, 0, SEEK_CUR) != 5)
+    return 77;
+
+  if (close(fd))
+    return 13;
+
+  if (unlink(path))
+    return 14;
+
+  if (strcmp(buf, "01234"))
+    return 15;
+}