vim-patch:8.2.3510: changes are only detected with one second accuracy

Problem:    Changes are only detected with one second accuracy.
Solution:   Use the nanosecond time if possible.  (Leah Neukirchen,
            closes vim/vim#8873, closes vim/vim#8875)
https://github.com/vim/vim/commit/0a7984af5601323fae7b3398f05a48087db7b767

In Nvim Test_checktime_fast() is also flaky. Add a delay to avoid that.

5 files changed, 58 insertions, 12 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 7b17c5b506..7ae5df164f 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -587,7 +587,9 @@ struct file_buffer {
                                 // where invoked
 
   long b_mtime;                 // last change time of original file
+  long b_mtime_ns;              // nanoseconds of last change time
   long b_mtime_read;            // last change time when reading
+  long b_mtime_read_ns;         // nanoseconds of last read time
   uint64_t b_orig_size;         // size of original file in bytes
   int b_orig_mode;              // mode of original file
   time_t b_last_used;           // time when the buffer was last used; used
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 7772d9ffc2..c6baa105b0 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -4539,6 +4539,7 @@ static void f_has(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     "mouse",
     "multi_byte",
     "multi_lang",
+    "nanotime",
     "num64",
     "packages",
     "path_extra",
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 8e1be3bbf7..475b44bc49 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -405,6 +405,7 @@ int readfile(char_u *fname, char_u *sfname, linenr_T from, linenr_T lines_to_ski
     if (os_fileinfo((char *)fname, &file_info)) {
       buf_store_file_info(curbuf, &file_info);
       curbuf->b_mtime_read = curbuf->b_mtime;
+      curbuf->b_mtime_read_ns = curbuf->b_mtime_ns;
 #ifdef UNIX
       /*
        * Use the protection bits of the original file for the swap file.
@@ -421,7 +422,9 @@ int readfile(char_u *fname, char_u *sfname, linenr_T from, linenr_T lines_to_ski
 #endif
     } else {
       curbuf->b_mtime = 0;
+      curbuf->b_mtime_ns = 0;
       curbuf->b_mtime_read = 0;
+      curbuf->b_mtime_read_ns = 0;
       curbuf->b_orig_size = 0;
       curbuf->b_orig_mode = 0;
     }
@@ -3695,11 +3698,12 @@ nofail:
       msg_puts_attr(_("don't quit the editor until the file is successfully written!"),
                     attr | MSG_HIST);
 
-      /* Update the timestamp to avoid an "overwrite changed file"
-       * prompt when writing again. */
+      // Update the timestamp to avoid an "overwrite changed file"
+      // prompt when writing again.
       if (os_fileinfo((char *)fname, &file_info_old)) {
         buf_store_file_info(buf, &file_info_old);
         buf->b_mtime_read = buf->b_mtime;
+        buf->b_mtime_read_ns = buf->b_mtime_ns;
       }
     }
   }
@@ -3893,8 +3897,7 @@ static void msg_add_eol(void)
 static int check_mtime(buf_T *buf, FileInfo *file_info)
 {
   if (buf->b_mtime_read != 0
-      && time_differs(file_info->stat.st_mtim.tv_sec,
-                      buf->b_mtime_read)) {
+      && time_differs(file_info, buf->b_mtime_read, buf->b_mtime_read_ns)) {
     msg_scroll = true;  // Don't overwrite messages here.
     msg_silent = 0;     // Must give this prompt.
     // Don't use emsg() here, don't want to flush the buffers.
@@ -3908,19 +3911,17 @@ static int check_mtime(buf_T *buf, FileInfo *file_info)
   return OK;
 }
 
-/// Return true if the times differ
-///
-/// @param t1 first time
-/// @param t2 second time
-static bool time_differs(long t1, long t2) FUNC_ATTR_CONST
+static bool time_differs(const FileInfo *file_info, long mtime, long mtime_ns) FUNC_ATTR_CONST
 {
 #if defined(__linux__) || defined(MSWIN)
   // On a FAT filesystem, esp. under Linux, there are only 5 bits to store
   // the seconds.  Since the roundoff is done when flushing the inode, the
   // time may change unexpectedly by one second!!!
-  return t1 - t2 > 1 || t2 - t1 > 1;
+  return (long)file_info->stat.st_mtim.tv_sec - mtime > 1
+         || mtime - (long)file_info->stat.st_mtim.tv_sec > 1
+         || (long)file_info->stat.st_mtim.tv_nsec != mtime_ns;
 #else
-  return t1 != t2;
+  return (long)file_info->stat.st_mtim.tv_sec != mtime;
 #endif
 }
 
@@ -4943,7 +4944,7 @@ int buf_check_timestamp(buf_T *buf)
   if (!(buf->b_flags & BF_NOTEDITED)
       && buf->b_mtime != 0
       && (!(file_info_ok = os_fileinfo((char *)buf->b_ffname, &file_info))
-          || time_differs(file_info.stat.st_mtim.tv_sec, buf->b_mtime)
+          || time_differs(&file_info, buf->b_mtime, buf->b_mtime_ns)
           || (int)file_info.stat.st_mode != buf->b_orig_mode)) {
     const long prev_b_mtime = buf->b_mtime;
 
@@ -5034,6 +5035,7 @@ int buf_check_timestamp(buf_T *buf)
             // Only timestamp changed, store it to avoid a warning
             // in check_mtime() later.
             buf->b_mtime_read = buf->b_mtime;
+            buf->b_mtime_read_ns = buf->b_mtime_ns;
           }
         }
       }
@@ -5262,6 +5264,7 @@ void buf_store_file_info(buf_T *buf, FileInfo *file_info)
   FUNC_ATTR_NONNULL_ALL
 {
   buf->b_mtime = file_info->stat.st_mtim.tv_sec;
+  buf->b_mtime_ns = file_info->stat.st_mtim.tv_nsec;
   buf->b_orig_size = os_fileinfo_size(file_info);
   buf->b_orig_mode = (int)file_info->stat.st_mode;
 }
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index 9925971783..004ef36b36 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -704,11 +704,14 @@ static void set_b0_fname(ZERO_BL *b0p, buf_T *buf)
       long_to_char((long)os_fileinfo_inode(&file_info), b0p->b0_ino);
       buf_store_file_info(buf, &file_info);
       buf->b_mtime_read = buf->b_mtime;
+      buf->b_mtime_read_ns = buf->b_mtime_ns;
     } else {
       long_to_char(0L, b0p->b0_mtime);
       long_to_char(0L, b0p->b0_ino);
       buf->b_mtime = 0;
+      buf->b_mtime_ns = 0;
       buf->b_mtime_read = 0;
+      buf->b_mtime_read_ns = 0;
       buf->b_orig_size = 0;
       buf->b_orig_mode = 0;
     }
@@ -1720,6 +1723,7 @@ void ml_sync_all(int check_file, int check_char, bool do_fsync)
       FileInfo file_info;
       if (!os_fileinfo((char *)buf->b_ffname, &file_info)
           || file_info.stat.st_mtim.tv_sec != buf->b_mtime_read
+          || file_info.stat.st_mtim.tv_nsec != buf->b_mtime_read_ns
           || os_fileinfo_size(&file_info) != buf->b_orig_size) {
         ml_preserve(buf, false, do_fsync);
         did_check_timestamps = false;
diff --git a/src/nvim/testdir/test_stat.vim b/src/nvim/testdir/test_stat.vim
index 170358e023..a6fe31b85a 100644
--- a/src/nvim/testdir/test_stat.vim
+++ b/src/nvim/testdir/test_stat.vim
@@ -1,5 +1,7 @@
 " Tests for stat functions and checktime
 
+source check.vim
+
 func CheckFileTime(doSleep)
   let fnames = ['Xtest1.tmp', 'Xtest2.tmp', 'Xtest3.tmp']
   let times = []
@@ -74,6 +76,40 @@ func Test_checktime()
   call delete(fname)
 endfunc
 
+func Test_checktime_fast()
+  CheckFeature nanotime
+
+  let fname = 'Xtest.tmp'
+
+  let fl = ['Hello World!']
+  call writefile(fl, fname)
+  set autoread
+  exec 'e' fname
+  let fl = readfile(fname)
+  let fl[0] .= ' - checktime'
+  sleep 10m  " make test less flaky in Nvim
+  call writefile(fl, fname)
+  checktime
+  call assert_equal(fl[0], getline(1))
+
+  call delete(fname)
+endfunc
+
+func Test_autoread_fast()
+  CheckFeature nanotime
+
+  new Xautoread
+  set autoread
+  call setline(1, 'foo')
+
+  w!
+  silent !echo bar > Xautoread
+  checktime
+
+  call assert_equal('bar', trim(getline(1)))
+  call delete('Xautoread')
+endfunc
+
 func Test_autoread_file_deleted()
   new Xautoread
   set autoread