patch 8.2.1970: it is easy to make mistakes when cleaning up swap files

Problem:    It is easy to make mistakes when cleaning up swap files after the
            system crashed.
Solution:   Warn for the process still running after recovery.  Do not
            automatically delete a swap file created on another system.
            (David Fries, closes #7273)

src/memline.c

@@ -1690,7 +1690,17 @@ ml_recover(int checkext)
 	}
 	else
 	    msg(_("Recovery completed. Buffer contents equals file contents."));
-	msg_puts(_("\nYou may want to delete the .swp file now.\n\n"));
+	msg_puts(_("\nYou may want to delete the .swp file now."));
+#if defined(UNIX) || defined(MSWIN)
+	if (mch_process_running(char_to_long(b0p->b0_pid)))
+	{
+	    // Warn there could be an active Vim on the same file, the user may
+	    // want to kill it.
+	    msg_puts(_("\nNote: process STILL RUNNING: "));
+	    msg_outnum(char_to_long(b0p->b0_pid));
+	}
+#endif
+	msg_puts("\n\n");
 	cmdline_row = msg_row;
     }
 #ifdef FEAT_CRYPT
@@ -2230,14 +2240,30 @@ swapfile_unchanged(char_u *fname)
 	ret = FALSE;
 
 #if defined(UNIX) || defined(MSWIN)
+    // Host name must be known and must equal the current host name, otherwise
+    // comparing pid is meaningless.
+    if (*(b0.b0_hname) == NUL)
+    {
+	ret = FALSE;
+    }
+    else
+    {
+	char_u	    hostname[B0_HNAME_SIZE];
+
+	mch_get_host_name(hostname, B0_HNAME_SIZE);
+	hostname[B0_HNAME_SIZE - 1] = NUL;
+	if (STRICMP(b0.b0_hname, hostname) != 0)
+	    ret = FALSE;
+    }
+
     // process must be known and not be running
     pid = char_to_long(b0.b0_pid);
     if (pid == 0L || mch_process_running(pid))
 	ret = FALSE;
 #endif
 
-    // TODO: Should we check if the swap file was created on the current
-    // system?  And the current user?
+    // We do not check the user, it should be irrelevant for whether the swap
+    // file is still useful.
 
     close(fd);
     return ret;

src/testdir/test_swap.vim

@@ -403,4 +403,50 @@ func Test_swap_symlink()
   call delete('Xswapdir', 'rf')
 endfunc
 
+func Test_swap_auto_delete()
+  " Create a valid swapfile by editing a file with a special extension.
+  split Xtest.scr
+  call setline(1, ['one', 'two', 'three'])
+  write  " file is written, not modified
+  write  " write again to make sure the swapfile is created
+  " read the swapfile as a Blob
+  let swapfile_name = swapname('%')
+  let swapfile_bytes = readfile(swapfile_name, 'B')
+
+  " Forget about the file, recreate the swap file, then edit it again.  The
+  " swap file should be automatically deleted.
+  bwipe!
+  " change the process ID to avoid the "still running" warning
+  let swapfile_bytes[24] = 0x99
+  call writefile(swapfile_bytes, swapfile_name)
+  edit Xtest.scr
+  " will end up using the same swap file after deleting the existing one
+  call assert_equal(swapfile_name, swapname('%'))
+  bwipe!
+
+  " create the swap file again, but change the host name so that it won't be
+  " deleted
+  autocmd! SwapExists
+  augroup test_swap_recover_ext
+    autocmd!
+    autocmd SwapExists * let v:swapchoice = 'e'
+  augroup END
+
+  " change the host name
+  let swapfile_bytes[28 + 40] = 0x89
+  call writefile(swapfile_bytes, swapfile_name)
+  edit Xtest.scr
+  call assert_equal(1, filereadable(swapfile_name))
+  " will use another same swap file name
+  call assert_notequal(swapfile_name, swapname('%'))
+  bwipe!
+
+  call delete('Xtest.scr')
+  call delete(swapfile_name)
+  augroup test_swap_recover_ext
+    autocmd!
+  augroup END
+  augroup! test_swap_recover_ext
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1970,
 /**/
     1969,
 /**/