Subject: Re: bug#13149: 24.3.50; Emacs thinks file was changed outside Emacs, but it was not

From: Paul Eggert <[email-masked]>
Date: Tue, 15 Jan 2013 21:57:29 -0800

On 01/15/2013 03:44 PM, Dmitry Gutov wrote:
> Maybe it's a sign of my system slowly falling apart.

It does sound like a fairly serious issue of some sort.
I did think of a patch (attached) but I'd rather not
apply it if the system in question is merely experimental,
since it introduces a race even on non-buggy systems.

=== modified file 'src/ChangeLog'
--- src/ChangeLog	2013-01-15 21:38:58 +0000
+++ src/ChangeLog	2013-01-16 05:01:01 +0000
@@ -1,3 +1,18 @@
+2013-01-16  Paul Eggert  <[email-masked]>
+
+	Work around bug in vboxsf file system (Bug#13149).
+	The bug was observed on Ubuntu operating inside a virtual machine,
+	editing files mounted via vboxsf from the MS Windows 7 host.
+	The workaround introduces a race condition on non-buggy hosts,
+	but it's an unlikely race and anyway there's a nearly identical
+	nearby race that can't be fixed.
+	* fileio.c (valid_timestamp_file_system, timestamp_file_system):
+	New static vars.
+	(Fwrite_region): Test for file system time stamp bug.
+	(init_fileio): New function.
+	* lisp.h (init_fileio): Declare it.
+	* emacs.c (main): Call it.
+
 2013-01-15  Paul Eggert  <[email-masked]>
 
 	* alloc.c (free_save_value): Now static.

=== modified file 'src/emacs.c'
--- src/emacs.c	2013-01-13 20:03:01 +0000
+++ src/emacs.c	2013-01-16 05:01:01 +0000
@@ -1317,6 +1317,7 @@
     }
 
   init_callproc ();	/* Must follow init_cmdargs but not init_sys_modes.  */
+  init_fileio ();
   init_lread ();
 #ifdef WINDOWSNT
   /* Check to see if Emacs has been installed correctly.  */

=== modified file 'src/fileio.c'
--- src/fileio.c	2013-01-15 10:14:31 +0000
+++ src/fileio.c	2013-01-16 05:04:51 +0000
@@ -103,6 +103,11 @@
 /* Set by auto_save_1 if an error occurred during the last auto-save.  */
 static bool auto_save_error_occurred;
 
+/* If VALID_TIMESTAMP_FILE_SYSTEM, then TIMESTAMP_FILE_SYSTEM is the device
+   number of a file system where time stamps were observed to to work.  */
+static bool valid_timestamp_file_system;
+static dev_t timestamp_file_system;
+
 /* The symbol bound to coding-system-for-read when
    insert-file-contents is called for recovering a file.  This is not
    an actual coding system name, but just an indicator to tell
@@ -5020,6 +5025,42 @@
   /* Discard the unwind protect for close_file_unwind.  */
   specpdl_ptr = specpdl + count1;
 
+  /* Some file systems have a bug where st_mtime is updated merely
+     because a file was closed.  Update MODTIME to the newer st_mtime
+     if this file system appears to have the bug.  Working around this
+     bug introduces a race condition: to avoid most instances of the
+     race condition on non-buggy file systems, skip this check if the
+     most recently encountered non-buggy file system was the current
+     file system.
+
+     A race condition can occur if some other process modifies the
+     file between the fstat above and the stat below, but the race is
+     unlikely and a similar race between the last write and the fstat
+     above cannot possibly be closed anyway.  */
+
+  if (visiting && EMACS_TIME_VALID_P (modtime)
+      && ! (valid_timestamp_file_system && st.st_dev == timestamp_file_system))
+    {
+      struct stat stat_st;
+      if (stat (fn, &stat_st) != 0)
+	ok = 0, save_errno = errno;
+      else if (stat_st.st_dev == st.st_dev && stat_st.st_ino == st.st_ino)
+	{
+	  EMACS_TIME stat_modtime = get_stat_mtime (&stat_st);
+	  if (EMACS_TIME_EQ (modtime, stat_modtime)
+	      && st.st_size == stat_st.st_size)
+	    {
+	      timestamp_file_system = st.st_dev;
+	      valid_timestamp_file_system = 1;
+	    }
+	  else
+	    {
+	      st.st_size = stat_st.st_size;
+	      modtime = stat_modtime;
+	    }
+	}
+    }
+
   /* Call write-region-post-annotation-function. */
   while (CONSP (Vwrite_region_annotation_buffers))
     {
@@ -5814,8 +5855,13 @@
   args[6] = predicate;
   RETURN_UNGCPRO (Ffuncall (7, args));
 }
+
+void
+init_fileio (void)
+{
+  valid_timestamp_file_system = 0;
+}
 
-
 void
 syms_of_fileio (void)
 {

=== modified file 'src/lisp.h'
--- src/lisp.h	2013-01-15 21:38:58 +0000
+++ src/lisp.h	2013-01-16 05:01:01 +0000
@@ -3256,6 +3256,7 @@
 extern bool internal_delete_file (Lisp_Object);
 extern bool file_directory_p (const char *);
 extern bool file_accessible_directory_p (const char *);
+extern void init_fileio (void);
 extern void syms_of_fileio (void);
 extern Lisp_Object make_temp_name (Lisp_Object, bool);
 extern Lisp_Object Qdelete_file;