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

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Tue, 29 Oct 2019 10:04:01 +0100, Andreas Schwab <schwab@suse.de> said:
    Andreas> You need to move this check under the condition above, otherwise you may
    Andreas> get a false positive if unlink didn't fail or wasn't called at all.

    >> +    {
    >> +      chmod (lfname, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
    >> +      err = 0;
    >> +      errno = 0;

Indeed. Next time maybe Iʼll try actually compiling my patches before
I send them. Take 3

diff --git a/src/filelock.c b/src/filelock.c
index ff25d6475d..bd1e8d9b2d 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -731,9 +731,20 @@ unlock_file (Lisp_Object fn)
 
   int err = current_lock_owner (0, lfname);
   if (err == -2 && unlink (lfname) != 0 && errno != ENOENT)
+    {
     err = errno;
-  if (0 < err)
-    report_file_errno ("Unlocking file", filename, err);
+  /* On certain filesystems the file must be writable for unlink to
+     succeed, so make it writable and retry (Bug#37784).  */
+    if (errno == EPERM)
+      {
+        chmod (lfname, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
+        err = 0;
+        if (unlink (lfname) != 0 && errno != ENOENT)
+          err = errno;
+      }
+    }
+    if (0 < err)
+      report_file_errno ("Unlocking file", filename, err);
 
   SAFE_FREE ();
 }