accept RENAME_NOREPLACE in rename

sshfs.c

@@ -61,6 +61,10 @@
 #  define MAP_ANONYMOUS MAP_ANON
 #endif
 
+#ifndef RENAME_NOREPLACE
+#  define RENAME_NOREPLACE (1 << 0)
+#endif
+
 
 #define SSH_FXP_INIT                1
 #define SSH_FXP_VERSION             2
@@ -2708,15 +2712,35 @@ static int sshfs_rename(const char *from, const char *to, unsigned int flags)
 {
 	int err;
 	struct conntab_entry *ce;
+	int noreplace = flags & RENAME_NOREPLACE;
 
-	if(flags != 0)
+	if (flags & ~RENAME_NOREPLACE)
 		return -EINVAL;
 
-	if (sshfs.ext_posix_rename)
+	if (noreplace && sshfs.follow_symlinks) {
+		struct buffer buf;
+		struct buffer outbuf;
+
+		/* getattr follows symlinks, so the kernel misses a dangling target */
+		buf_init(&buf, 0);
+		buf_add_path(&buf, to);
+		err = sftp_request(get_conn(NULL, NULL), SSH_FXP_LSTAT, &buf,
+				   SSH_FXP_ATTRS, &outbuf);
+		buf_free(&buf);
+		if (!err) {
+			buf_free(&outbuf);
+			return -EEXIST;
+		}
+		if (err != -ENOENT)
+			return err;
+	}
+
+	/* posix-rename and the rename workaround both replace the target */
+	if (sshfs.ext_posix_rename && !noreplace)
 		err = sshfs_ext_posix_rename(from, to);
 	else
 		err = sshfs_do_rename(from, to);
-	if (err == -EPERM && sshfs.rename_workaround) {
+	if (err == -EPERM && sshfs.rename_workaround && !noreplace) {
 		size_t tolen = strlen(to);
 		if (tolen + RENAME_TEMP_CHARS < PATH_MAX) {
 			int tmperr;
@@ -2733,7 +2757,8 @@ static int sshfs_rename(const char *from, const char *to, unsigned int flags)
 			}
 		}
 	}
-	if (err == -EPERM && sshfs.renamexdev_workaround)
+	/* EXDEV would make mv fall back to copying over the target */
+	if (err == -EPERM && sshfs.renamexdev_workaround && !noreplace)
 		err = -EXDEV;
 
 	if (!err && sshfs.max_conns > 1) {

test/test_sshfs.py

@@ -7,6 +7,7 @@
     sys.exit(pytest.main([__file__] + sys.argv[1:]))
 
 import subprocess
+import ctypes
 import os
 import sys
 import pytest
@@ -151,6 +152,7 @@ def test_sshfs(
         tst_rmdir(src_dir, mnt_dir, cache_timeout)
         tst_rename(mnt_dir)
         tst_rename_over(mnt_dir)
+        tst_rename_noreplace(mnt_dir)
         tst_chmod(mnt_dir)
         tst_fsync(src_dir, mnt_dir)
         tst_unlink(src_dir, mnt_dir, cache_timeout)
@@ -264,6 +266,22 @@ def tst_rename_over(mnt_dir):
     os.unlink(dst_name)
 
 
+def tst_rename_noreplace(mnt_dir):
+    # macFUSE's FSKit backend sets RENAME_NOREPLACE on every rename to a new
+    # name. The kernel rejects a target it can see, so that is not tested here.
+    libc = ctypes.CDLL(None, use_errno=True)
+    src, dst = pjoin(mnt_dir, name_generator()), pjoin(mnt_dir, name_generator())
+    with open(src, "wb") as fh:
+        fh.write(b"old\n")
+    # AT_FDCWD, RENAME_NOREPLACE
+    if libc.renameat2(-100, os.fsencode(src), -100, os.fsencode(dst), 1):
+        raise OSError(ctypes.get_errno(), "renameat2")
+    assert not os.path.exists(src)
+    with open(dst, "rb") as fh:
+        assert fh.read() == b"old\n"
+    os.unlink(dst)
+
+
 def tst_chmod(mnt_dir):
     filename = pjoin(mnt_dir, name_generator())
     with open(filename, "wb") as fh:
@@ -866,6 +884,36 @@ def test_follow_symlinks(tmpdir, capfd):
         with open(mnt_link, "rb") as fh:
             assert fh.read() == b"symlink target data"
 
+        # A dangling symlink looks absent through follow_symlinks, so the
+        # kernel lets RENAME_NOREPLACE through and sshfs must refuse it.
+        libc = ctypes.CDLL(None, use_errno=True)
+        dangling = name_generator()
+        os.symlink("missing", pjoin(src_dir, dangling))
+        with pytest.raises(FileNotFoundError):
+            os.lstat(pjoin(mnt_dir, dangling))
+        # AT_FDCWD, RENAME_NOREPLACE
+        ret = libc.renameat2(
+            -100, os.fsencode(mnt_link), -100, os.fsencode(pjoin(mnt_dir, dangling)), 1
+        )
+        assert ret == -1 and ctypes.get_errno() == errno.EEXIST
+        assert os.readlink(pjoin(src_dir, dangling)) == "missing"
+        assert os.readlink(link) == target_name
+        os.unlink(pjoin(src_dir, dangling))
+
+        # A genuinely absent target is still renamed
+        fresh, moved = name_generator(), name_generator()
+        os.symlink(target_name, pjoin(src_dir, fresh))
+        ret = libc.renameat2(
+            -100,
+            os.fsencode(pjoin(mnt_dir, fresh)),
+            -100,
+            os.fsencode(pjoin(mnt_dir, moved)),
+            1,
+        )
+        assert ret == 0
+        assert os.readlink(pjoin(src_dir, moved)) == target_name
+        os.unlink(pjoin(src_dir, moved))
+
         os.unlink(link)
         os.unlink(target)
     except Exception: