bug#37884: 27.0.50; Cannot write to a file in VirtualBox shared directory

[Robert Pluim]

>>>>> On Mon, 28 Oct 2019 20:23:22 +1100, Bernardo <bernardo.bacic@pobox.com> 
>>>>> said:
    Bernardo> $ git diff src/filelock.c
    Bernardo> diff --git a/src/filelock.c b/src/filelock.c
    Bernardo> index ff25d6475d..79eb8fa91e 100644
    Bernardo> --- a/src/filelock.c
    Bernardo> +++ b/src/filelock.c
    Bernardo> @@ -403,7 +403,7 @@ create_lock_file (char *lfname, char 
*lock_info_str, bool force)
    Bernardo>           lock_info_len = strlen (lock_info_str);
    Bernardo>           err = 0;
    Bernardo>           if (emacs_write (fd, lock_info_str, lock_info_len) != 
lock_info_len
    Bernardo> -             || fchmod (fd, S_IRUSR | S_IRGRP | S_IROTH) != 0)
    Bernardo> +             || fchmod (fd, S_IRUSR | S_IWUSR | S_IRGRP | 
S_IROTH) != 0)
    Bernardo>             err = errno;
    Bernardo>           /* There is no need to call fsync here, as the contents 
of
    Bernardo>              the lock file need not survive system crashes.  */

    Bernardo> and Emacs is happy again, no problems with writing to files in
    Bernardo> VirtualBox shared directory.

    Bernardo> Please let me know if you want me to test anything else.

I think Eli's suggestion was more like the below, which only calls
fchmod if the unlink fails.

diff --git a/src/filelock.c b/src/filelock.c
index ff25d6475d..7fb14774b0 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -732,6 +732,15 @@ unlock_file (Lisp_Object fn)
   int err = current_lock_owner (0, lfname);
   if (err == -2 && unlink (lfname) != 0 && errno != ENOENT)
     err = errno;
+  /* On certain filesystems the file must be writable for unlink to
+     succeed (Bug#37784).  */
+  if (errno == EPERM)
+    {
+      fchmod (fd, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
+      errno = 0;
+      if (unlink (lfname) != 0 && errno != ENOENT)
+        err = errno;
+    }
   if (0 < err)
     report_file_errno ("Unlocking file", filename, err);