4 files changed, 101 insertions, 11 deletions

diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 58e6b2ae92..fcf15638c7 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -2650,6 +2650,7 @@ buf_write(
    */
   if (!(append && *p_pm == NUL) && !filtering && perm >= 0 && dobackup) {
     FileInfo file_info;
+    const bool no_prepend_dot = false;
 
     if ((bkc & BKC_YES) || append) {       /* "yes" */
       backup_copy = TRUE;
@@ -2737,6 +2738,7 @@ buf_write(
       int some_error = false;
       char_u      *dirp;
       char_u      *rootname;
+      char_u      *p;
 
       /*
        * Try to make the backup in each directory in the 'bdir' option.
@@ -2756,6 +2758,17 @@ buf_write(
          * Isolate one directory name, using an entry in 'bdir'.
          */
         (void)copy_option_part(&dirp, IObuff, IOSIZE, ",");
+        p = IObuff + STRLEN(IObuff);
+        if (after_pathsep((char *)IObuff, (char *)p) && p[-1] == p[-2]) {
+          // Ends with '//', Use Full path
+          if ((p = (char_u *)make_percent_swname((char *)IObuff, (char *)fname))
+              != NULL) {
+            backup = (char_u *)modname((char *)p, (char *)backup_ext,
+                                       no_prepend_dot);
+            xfree(p);
+          }
+        }
+
         rootname = get_file_in_dir(fname, IObuff);
         if (rootname == NULL) {
           some_error = TRUE;                /* out of memory */
@@ -2764,10 +2777,14 @@ buf_write(
 
         FileInfo file_info_new;
         {
-          /*
-           * Make backup file name.
-           */
-          backup = (char_u *)modname((char *)rootname, (char *)backup_ext, FALSE);
+          //
+          // Make the backup file name.
+          //
+          if (backup == NULL) {
+            backup = (char_u *)modname((char *)rootname, (char *)backup_ext,
+                                       no_prepend_dot);
+          }
+
           if (backup == NULL) {
             xfree(rootname);
             some_error = TRUE;                          /* out of memory */
@@ -2893,12 +2910,26 @@ nobackup:
          * Isolate one directory name and make the backup file name.
          */
         (void)copy_option_part(&dirp, IObuff, IOSIZE, ",");
-        rootname = get_file_in_dir(fname, IObuff);
-        if (rootname == NULL)
-          backup = NULL;
-        else {
-          backup = (char_u *)modname((char *)rootname, (char *)backup_ext, FALSE);
-          xfree(rootname);
+        p = IObuff + STRLEN(IObuff);
+        if (after_pathsep((char *)IObuff, (char *)p) && p[-1] == p[-2]) {
+          // path ends with '//', use full path
+          if ((p = (char_u *)make_percent_swname((char *)IObuff, (char *)fname))
+              != NULL) {
+            backup = (char_u *)modname((char *)p, (char *)backup_ext,
+                                       no_prepend_dot);
+            xfree(p);
+          }
+        }
+
+        if (backup == NULL) {
+          rootname = get_file_in_dir(fname, IObuff);
+          if (rootname == NULL) {
+            backup = NULL;
+          } else {
+            backup = (char_u *)modname((char *)rootname, (char *)backup_ext,
+                                       no_prepend_dot);
+            xfree(rootname);
+          }
         }
 
         if (backup != NULL) {
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index 05cc62bb33..2824d57f49 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -1437,7 +1437,7 @@ recover_names (
  * Append the full path to name with path separators made into percent
  * signs, to dir. An unnamed buffer is handled as "" (<currentdir>/"")
  */
-static char *make_percent_swname(const char *dir, char *name)
+char *make_percent_swname(const char *dir, char *name)
   FUNC_ATTR_NONNULL_ARG(1)
 {
   char *d = NULL;
diff --git a/src/nvim/testdir/test_alot.vim b/src/nvim/testdir/test_alot.vim
index f1274b01c8..5668f45dea 100644
--- a/src/nvim/testdir/test_alot.vim
+++ b/src/nvim/testdir/test_alot.vim
@@ -2,6 +2,7 @@
 " This makes testing go faster, since Vim doesn't need to restart.
 
 source test_assign.vim
+source test_backup.vim
 source test_behave.vim
 source test_cd.vim
 source test_changedtick.vim
diff --git a/src/nvim/testdir/test_backup.vim b/src/nvim/testdir/test_backup.vim
new file mode 100644
index 0000000000..fa10430613
--- /dev/null
+++ b/src/nvim/testdir/test_backup.vim
@@ -0,0 +1,58 @@
+" Tests for the backup function
+
+func Test_backup()
+  set backup backupdir=.
+  new
+  call setline(1, ['line1', 'line2'])
+  :f Xbackup.txt
+  :w! Xbackup.txt
+  " backup file is only created after
+  " writing a second time (before overwriting)
+  :w! Xbackup.txt
+  let l = readfile('Xbackup.txt~')
+  call assert_equal(['line1', 'line2'], l)
+  bw!
+  set backup&vim backupdir&vim
+  call delete('Xbackup.txt')
+  call delete('Xbackup.txt~')
+endfunc
+
+func Test_backup2()
+  set backup backupdir=.//
+  new
+  call setline(1, ['line1', 'line2', 'line3'])
+  :f Xbackup.txt
+  :w! Xbackup.txt
+  " backup file is only created after
+  " writing a second time (before overwriting)
+  :w! Xbackup.txt
+  sp *Xbackup.txt~
+  call assert_equal(['line1', 'line2', 'line3'], getline(1,'$'))
+  let f=expand('%')
+  call assert_match('src%nvim%testdir%Xbackup.txt\~', f)
+  bw!
+  bw!
+  call delete('Xbackup.txt')
+  call delete(f)
+  set backup&vim backupdir&vim
+endfunc
+
+func Test_backup2_backupcopy()
+  set backup backupdir=.// backupcopy=yes
+  new
+  call setline(1, ['line1', 'line2', 'line3'])
+  :f Xbackup.txt
+  :w! Xbackup.txt
+  " backup file is only created after
+  " writing a second time (before overwriting)
+  :w! Xbackup.txt
+  sp *Xbackup.txt~
+  call assert_equal(['line1', 'line2', 'line3'], getline(1,'$'))
+  let f=expand('%')
+  call assert_match('src%nvim%testdir%Xbackup.txt\~', f)
+  bw!
+  bw!
+  call delete('Xbackup.txt')
+  call delete(f)
+  set backup&vim backupdir&vim backupcopy&vim
+endfunc