Support dirfd relative iops on Windows

We always favor calling functions like openat(), fstatat(), etc. because
Linux, XNU, FreeBSD, and OpenBSD all elected to support them, while some
systems like Android love them so much, that they stopped supporting the
old interfaces.

This change ensures that when dirfd is actually a dirfd and not AT_FDCWD
we'll do the right thing on Windows NT. We use an API that's been around
since Vista to accomplish that.

This change also adds exponential backoff to chdir() on Windows since it
seems almost as flaky on Windows 7 as the rmdir() function.

Author: jart

libc/calls/access.c

@@ -17,10 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
-#include "libc/calls/internal.h"
-#include "libc/dce.h"
 #include "libc/sysv/consts/at.h"
-#include "libc/sysv/errfuns.h"
 
 /**
  * Checks if effective user can access path in particular ways.
@@ -31,12 +28,5 @@
  * @asyncsignalsafe
  */
 int access(const char *path, int mode) {
-  char16_t path16[PATH_MAX];
-  if (!path) return efault();
-  if (!IsWindows()) {
-    return faccessat$sysv(AT_FDCWD, path, mode, 0);
-  } else {
-    if (__mkntpath(path, path16) == -1) return -1;
-    return ntaccesscheck(path16, mode);
-  }
+  return faccessat(AT_FDCWD, path, mode, 0);
 }

libc/calls/calls.h

@@ -230,6 +230,7 @@ long ptrace(int, int, void *, void *);
 int chroot(const char *);
 int prctl();
 int sysctl(const int *, unsigned, void *, size_t *, void *, size_t);
+int fchdir(int);
 
 #define getcwd(BUF, SIZE)                                                    \
   (__builtin_constant_p(BUF) && (&(BUF)[0] == NULL) ? get_current_dir_name() \

libc/calls/chdir-nt.c

@@ -17,22 +17,39 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
+#include "libc/nt/errors.h"
 #include "libc/nt/files.h"
 #include "libc/nt/runtime.h"
+#include "libc/nt/synchronization.h"
 #include "libc/sysv/errfuns.h"
 
 textwindows int chdir$nt(const char *path) {
-  int len;
+  int e, ms, len;
   char16_t path16[PATH_MAX];
   if ((len = __mkntpath(path, path16)) == -1) return -1;
   if (path16[len - 1] != u'\\') {
     if (len + 1 + 1 > PATH_MAX) return enametoolong();
     path16[len + 0] = u'\\';
     path16[len + 1] = u'\0';
   }
-  if (SetCurrentDirectory(path16)) {
-    return 0;
-  } else {
-    return __winerr();
+  /*
+   * chdir() seems flaky on windows 7
+   * in a similar way to rmdir() sigh
+   */
+  for (ms = 1;; ms *= 2) {
+    if (SetCurrentDirectory(path16)) {
+      return 0;
+    } else {
+      e = GetLastError();
+      if (ms <= 512 &&
+          (e == kNtErrorFileNotFound || e == kNtErrorAccessDenied)) {
+        Sleep(ms);
+        continue;
+      } else {
+        break;
+      }
+    }
   }
+  errno = e;
+  return -1;
 }

libc/calls/chdir.c

@@ -23,11 +23,11 @@
 
 /**
  * Sets current directory.
+ *
  * @asyncsignalsafe
- * @syscall
+ * @see fchdir()
  */
 int chdir(const char *path) {
-  if (!path) return efault();
   if (!IsWindows()) {
     return chdir$sysv(path);
   } else {

libc/calls/chown.c

@@ -32,9 +32,7 @@
  * @see /etc/passwd for user ids
  * @see /etc/group for group ids
  * @asyncsignalsafe
- * @syscall
  */
 int chown(const char *pathname, uint32_t uid, uint32_t gid) {
-  if (!pathname) return efault();
   return fchownat$sysv(AT_FDCWD, pathname, uid, gid, 0);
 }

libc/calls/faccessat-nt.c

@@ -22,7 +22,6 @@
 
 int faccessat$nt(int dirfd, const char *path, int mode, uint32_t flags) {
   char16_t path16[PATH_MAX];
-  if (dirfd != AT_FDCWD || flags) return einval();
-  if (__mkntpath(path, path16) == -1) return -1;
+  if (__mkntpathat(dirfd, path, 0, path16) == -1) return -1;
   return ntaccesscheck(path16, mode);
 }

libc/calls/faccessat.c

@@ -25,12 +25,13 @@
 /**
  * Checks if effective user can access path in particular ways.
  *
- * @param dirfd is usually AT_FDCWD
+ * @param dirfd is normally AT_FDCWD but if it's an open directory and
+ *     file is a relative path, then file is opened relative to dirfd
  * @param path is a filename or directory
- * @param flags can be R_OK, W_OK, X_OK, F_OK
+ * @param mode can be R_OK, W_OK, X_OK, F_OK
+ * @param flags should be 0
  * @return 0 if ok, or -1 and sets errno
  * @asyncsignalsafe
- * @syscall
  */
 int faccessat(int dirfd, const char *path, int mode, uint32_t flags) {
   if (!path) return efault();

libc/calls/fchdir-nt.c

@@ -0,0 +1,41 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 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 "libc/calls/calls.h"
+#include "libc/calls/internal.h"
+#include "libc/dce.h"
+#include "libc/nt/files.h"
+#include "libc/sysv/errfuns.h"
+
+textwindows int fchdir$nt(int dirfd) {
+  uint32_t len;
+  char16_t dir[PATH_MAX];
+  if (!__isfdkind(dirfd, kFdFile)) return ebadf();
+  len = GetFinalPathNameByHandle(g_fds.p[dirfd].handle, dir, ARRAYLEN(dir),
+                                 kNtFileNameNormalized | kNtVolumeNameDos);
+  if (len + 1 + 1 > ARRAYLEN(dir)) return enametoolong();
+  if (dir[len - 1] != u'\\') {
+    dir[len + 0] = u'\\';
+    dir[len + 1] = u'\0';
+  }
+  if (SetCurrentDirectory(dir)) {
+    return 0;
+  } else {
+    return __winerr();
+  }
+}

libc/calls/lstat-nt.c renamed to libc/calls/fchdir.c

@@ -18,7 +18,18 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
+#include "libc/dce.h"
 
-textwindows int lstat$nt(const char *pathname, struct stat *st) {
-  return stat$nt(pathname, st); /* todo(jart) */
+/**
+ * Sets current directory based on file descriptor.
+ *
+ * @see open(path, O_DIRECTORY)
+ * @asyncsignalsafe
+ */
+int fchdir(int dirfd) {
+  if (!IsWindows()) {
+    return fchdir$sysv(dirfd);
+  } else {
+    return fchdir$nt(dirfd);
+  }
 }

libc/calls/fchownat.c

@@ -31,10 +31,8 @@
  * @see /etc/passwd for user ids
  * @see /etc/group for group ids
  * @asyncsignalsafe
- * @syscall
  */
 int fchownat(int dirfd, const char *pathname, uint32_t uid, uint32_t gid,
              uint32_t flags) {
-  /* TODO(jart): Windows? */
   return fchownat$sysv(dirfd, pathname, uid, gid, flags);
 }