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.

Author: jart

libc/calls/close-nt.c

@@ -17,13 +17,14 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/weaken.h"
 #include "libc/nt/enum/filetype.h"
 #include "libc/nt/files.h"
 #include "libc/nt/runtime.h"
+#include "libc/runtime/runtime.h"
 #include "libc/runtime/zipos.internal.h"
 #include "libc/sock/syscall_fd.internal.h"
 #include "libc/sysv/consts/o.h"
@@ -64,5 +65,7 @@ textwindows int sys_close_nt(int fd, int fildes) {
     default:
       break;
   }
+  if (f->shared && !f->isdup)
+    munmap(f->shared, sizeof(struct Cursor));
   return CloseHandle(f->handle) ? 0 : __winerr();
 }

libc/calls/dup-nt.c

@@ -82,6 +82,8 @@ static textwindows int sys_dup_nt_impl(int oldfd, int newfd, int flags,
 
   g_fds.p[newfd] = g_fds.p[oldfd];
   g_fds.p[newfd].handle = handle;
+  g_fds.p[newfd].isdup = true;
+  g_fds.p[oldfd].isdup = true;  // TODO(jart): is it possible to avoid leak?
   if (flags & _O_CLOEXEC) {
     g_fds.p[newfd].flags |= _O_CLOEXEC;
   } else {

libc/calls/fcntl-nt.c

@@ -20,13 +20,13 @@
 #include "libc/calls/calls.h"
 #include "libc/calls/createfileflags.internal.h"
 #include "libc/calls/internal.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/struct/flock.h"
 #include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/calls/wincrash.internal.h"
 #include "libc/errno.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/kprintf.h"
 #include "libc/intrin/weaken.h"
 #include "libc/limits.h"
@@ -151,7 +151,7 @@ static textwindows int sys_fcntl_nt_lock(struct Fd *f, int fd, int cmd,
     case SEEK_SET:
       break;
     case SEEK_CUR:
-      off = f->pointer + off;
+      off = f->shared->pointer + off;
       break;
     case SEEK_END: {
       int64_t size;
@@ -351,9 +351,14 @@ textwindows int sys_fcntl_nt(int fd, int cmd, uintptr_t arg) {
       }
       rc = 0;
     } else if (cmd == F_SETLK || cmd == F_SETLKW || cmd == F_GETLK) {
-      pthread_mutex_lock(&g_locks.mu);
-      rc = sys_fcntl_nt_lock(g_fds.p + fd, fd, cmd, arg);
-      pthread_mutex_unlock(&g_locks.mu);
+      struct Fd *f = g_fds.p + fd;
+      if (f->shared) {
+        pthread_mutex_lock(&g_locks.mu);
+        rc = sys_fcntl_nt_lock(f, fd, cmd, arg);
+        pthread_mutex_unlock(&g_locks.mu);
+      } else {
+        rc = ebadf();
+      }
     } else if (cmd == F_DUPFD || cmd == F_DUPFD_CLOEXEC) {
       rc = sys_fcntl_nt_dupfd(fd, cmd, arg);
     } else {

libc/calls/lseek-nt.c

@@ -31,7 +31,7 @@ static textwindows int64_t GetPosition(struct Fd *f, int whence) {
     case SEEK_SET:
       return 0;
     case SEEK_CUR:
-      return f->pointer;
+      return f->shared->pointer;
     case SEEK_END: {
       struct NtByHandleFileInformation wst;
       if (!GetFileInformationByHandle(f->handle, &wst)) {
@@ -67,11 +67,14 @@ textwindows int64_t sys_lseek_nt(int fd, int64_t offset, int whence) {
   } else if (__isfdkind(fd, kFdFile)) {
     struct Fd *f = g_fds.p + fd;
     int filetype = GetFileType(f->handle);
-    if (filetype != kNtFileTypePipe && filetype != kNtFileTypeChar) {
+    if (filetype != kNtFileTypePipe &&  //
+        filetype != kNtFileTypeChar &&  //
+        f->shared) {
       int64_t res;
-      if ((res = Seek(f, offset, whence)) != -1) {
-        f->pointer = res;
-      }
+      __fd_lock(f);
+      if ((res = Seek(f, offset, whence)) != -1)
+        f->shared->pointer = res;
+      __fd_unlock(f);
       return res;
     } else {
       return espipe();

libc/calls/open-nt.c

@@ -138,6 +138,7 @@ static textwindows int sys_open_nt_file(int dirfd, const char *file,
   int64_t handle;
   if ((handle = sys_open_nt_impl(dirfd, file, flags, mode,
                                  kNtFileFlagOverlapped)) != -1) {
+    g_fds.p[fd].shared = __cursor_new();
     g_fds.p[fd].handle = handle;
     g_fds.p[fd].kind = kFdFile;
     g_fds.p[fd].flags = flags;
@@ -170,14 +171,14 @@ static textwindows int sys_open_nt_no_handle(int fd, int flags, int mode,
 
 static textwindows int sys_open_nt_dup(int fd, int flags, int mode, int oldfd) {
   int64_t handle;
-  if (!__isfdopen(oldfd)) {
+  if (!__isfdopen(oldfd))
     return enoent();
-  }
   if (DuplicateHandle(GetCurrentProcess(), g_fds.p[oldfd].handle,
                       GetCurrentProcess(), &handle, 0, true,
                       kNtDuplicateSameAccess)) {
     g_fds.p[fd] = g_fds.p[oldfd];
     g_fds.p[fd].handle = handle;
+    g_fds.p[fd].isdup = true;
     g_fds.p[fd].mode = mode;
     if (!sys_fcntl_nt_setfl(fd, flags)) {
       return fd;

libc/calls/readwrite-nt.c

@@ -19,9 +19,9 @@
 #include "libc/calls/createfileflags.internal.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/sig.internal.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/struct/sigset.h"
 #include "libc/calls/syscall_support-nt.internal.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/weaken.h"
 #include "libc/nt/enum/filetype.h"
 #include "libc/nt/errors.h"
@@ -51,29 +51,28 @@ sys_readwrite_nt(int fd, void *data, size_t size, ssize_t offset,
   uint32_t exchanged;
   struct Fd *f = g_fds.p + fd;
 
-  // win32 i/o apis generally take 32-bit values thus we implicitly
-  // truncate outrageously large sizes. linux actually does it too!
-  size = MIN(size, 0x7ffff000);
-
   // pread() and pwrite() perform an implicit lseek() operation, so
   // similar to the lseek() system call, they too raise ESPIPE when
   // operating on a non-seekable file.
   bool pwriting = offset != -1;
-  bool seekable =
-      (f->kind == kFdFile && GetFileType(handle) == kNtFileTypeDisk) ||
-      f->kind == kFdDevNull || f->kind == kFdDevRandom;
-  if (pwriting && !seekable) {
+  bool isdisk = f->kind == kFdFile && GetFileType(handle) == kNtFileTypeDisk;
+  bool seekable = isdisk || f->kind == kFdDevNull || f->kind == kFdDevRandom;
+  if (pwriting && !seekable)
     return espipe();
-  }
+
+  // determine if we need to lock a file descriptor across processes
+  bool locked = isdisk && !pwriting && f->shared;
+  if (locked)
+    __fd_lock(f);
 
   // when a file is opened in overlapped mode win32 requires that we
   // take over full responsibility for managing our own file pointer
   // which is fine, because the one win32 has was never very good in
   // the sense that it behaves so differently from linux, that using
   // win32 i/o required more compatibilty toil than doing it by hand
   if (!pwriting) {
-    if (seekable) {
-      offset = f->pointer;
+    if (seekable && f->shared) {
+      offset = f->shared->pointer;
     } else {
       offset = 0;
     }
@@ -82,8 +81,11 @@ sys_readwrite_nt(int fd, void *data, size_t size, ssize_t offset,
 RestartOperation:
   bool eagained = false;
   // check for signals and cancelation
-  if (_check_cancel() == -1)
+  if (_check_cancel() == -1) {
+    if (locked)
+      __fd_unlock(f);
     return -1;  // ECANCELED
+  }
   if (_weaken(__sig_get) && (sig = _weaken(__sig_get)(waitmask))) {
     goto HandleInterrupt;
   }
@@ -114,40 +116,49 @@ sys_readwrite_nt(int fd, void *data, size_t size, ssize_t offset,
     }
     ok = true;
   }
-  if (ok) {
+  if (ok)
     ok = GetOverlappedResult(handle, &overlap, &exchanged, true);
-  }
   CloseHandle(overlap.hEvent);
 
   // if i/o succeeded then return its result
   if (ok) {
-    if (!pwriting && seekable) {
-      f->pointer = offset + exchanged;
-    }
+    if (!pwriting && seekable && f->shared)
+      f->shared->pointer = offset + exchanged;
+    if (locked)
+      __fd_unlock(f);
     return exchanged;
   }
 
   // only raise EINTR or EAGAIN if I/O got canceled
   if (GetLastError() == kNtErrorOperationAborted) {
     // raise EAGAIN if it's due to O_NONBLOCK mmode
     if (eagained) {
+      if (locked)
+        __fd_unlock(f);
       return eagain();
     }
     // otherwise it must be due to a kill() via __sig_cancel()
     if (_weaken(__sig_relay) && (sig = _weaken(__sig_get)(waitmask))) {
     HandleInterrupt:
+      if (locked)
+        __fd_unlock(f);
       int handler_was_called = _weaken(__sig_relay)(sig, SI_KERNEL, waitmask);
       if (_check_cancel() == -1)
         return -1;  // possible if we SIGTHR'd
+      if (locked)
+        __fd_lock(f);
       // read() is @restartable unless non-SA_RESTART hands were called
-      if (!(handler_was_called & SIG_HANDLED_NO_RESTART)) {
+      if (!(handler_was_called & SIG_HANDLED_NO_RESTART))
         goto RestartOperation;
-      }
     }
+    if (locked)
+      __fd_unlock(f);
     return eintr();
   }
 
   // read() and write() have generally different error-handling paths
+  if (locked)
+    __fd_unlock(f);
   return -2;
 }
 

libc/intrin/fds.c

@@ -16,13 +16,13 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/intrin/fds.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/state.internal.h"
 #include "libc/calls/ttydefaults.h"
 #include "libc/dce.h"
 #include "libc/intrin/atomic.h"
 #include "libc/intrin/extend.h"
-#include "libc/intrin/fds.h"
 #include "libc/intrin/kprintf.h"
 #include "libc/intrin/nomultics.h"
 #include "libc/intrin/pushpop.h"
@@ -40,6 +40,7 @@
 #include "libc/sock/sock.h"
 #include "libc/sysv/consts/map.h"
 #include "libc/sysv/consts/o.h"
+#include "libc/sysv/consts/prot.h"
 #include "libc/thread/thread.h"
 
 #define OPEN_MAX 16
@@ -156,12 +157,29 @@ textstartup void __init_fds(int argc, char **argv, char **envp) {
         f->kind = kind;
         f->flags = flags;
         f->mode = mode;
-        f->pointer = pointer;
         f->type = type;
         f->family = family;
         f->protocol = protocol;
         atomic_store_explicit(&fds->f, fd + 1, memory_order_relaxed);
+
+        //
+        // - v1 abi: This field was originally the file pointer.
+        //
+        // - v2 abi: This field is the negated shared memory address.
+        //
+        if (f->kind == kFdFile) {
+          if (pointer < 0) {
+            f->shared = (struct Cursor *)(uintptr_t)-pointer;
+          } else if ((f->shared = __cursor_new())) {
+            f->shared->pointer = pointer;
+          }
+        }
       }
     }
+    for (int i = 0; i < 3; ++i) {
+      struct Fd *f = fds->p + i;
+      if (f->kind == kFdFile && !f->shared)
+        f->shared = __cursor_new();
+    }
   }
 }

libc/intrin/fds.h

@@ -1,5 +1,7 @@
 #ifndef COSMOPOLITAN_LIBC_CALLS_STRUCT_FD_INTERNAL_H_
 #define COSMOPOLITAN_LIBC_CALLS_STRUCT_FD_INTERNAL_H_
+#include "libc/atomic.h"
+#include "libc/thread/thread.h"
 COSMOPOLITAN_C_START_
 
 #define kFdEmpty     0
@@ -13,19 +15,25 @@ COSMOPOLITAN_C_START_
 #define kFdDevNull   9
 #define kFdDevRandom 10
 
+struct Cursor {
+  pthread_mutex_t lock;
+  long pointer;
+};
+
 struct Fd {
   char kind;
+  bool isdup;
   bool isbound;
   unsigned flags;
   unsigned mode;
   long handle;
-  long pointer;
   int family;
   int type;
   int protocol;
   unsigned rcvtimeo; /* millis; 0 means wait forever */
   unsigned sndtimeo; /* millis; 0 means wait forever */
   void *connect_op;
+  struct Cursor *shared;
 };
 
 struct Fds {
@@ -34,5 +42,9 @@ struct Fds {
   struct Fd *p, *e;
 };
 
+void __fd_lock(struct Fd *);
+void __fd_unlock(struct Fd *);
+struct Cursor *__cursor_new(void);
+
 COSMOPOLITAN_C_END_
 #endif /* COSMOPOLITAN_LIBC_CALLS_STRUCT_FD_INTERNAL_H_ */

libc/intrin/fds_lock.c

@@ -17,6 +17,8 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/state.internal.h"
+#include "libc/intrin/fds.h"
+#include "libc/runtime/runtime.h"
 #include "libc/thread/thread.h"
 
 void __fds_lock(void) {
@@ -26,3 +28,23 @@ void __fds_lock(void) {
 void __fds_unlock(void) {
   pthread_mutex_unlock(&__fds_lock_obj);
 }
+
+void __fd_lock(struct Fd *f) {
+  pthread_mutex_lock(&f->shared->lock);
+}
+
+void __fd_unlock(struct Fd *f) {
+  pthread_mutex_unlock(&f->shared->lock);
+}
+
+struct Cursor *__cursor_new(void) {
+  struct Cursor *c;
+  if ((c = _mapshared(sizeof(struct Cursor)))) {
+    pthread_mutexattr_t attr;
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
+    pthread_mutex_init(&c->lock, &attr);
+    pthread_mutexattr_destroy(&attr);
+  }
+  return c;
+}