vim-patch:8.1.1231: asking about existing swap file unnecessarily

Problem:    Asking about existing swap file unnecessarily.
Solution:   When it is safe, delete the swap file.  Remove
            HAS_SWAP_EXISTS_ACTION, it is always defined. (closes vim/vim#1237)
https://github.com/vim/vim/commit/67cf86bfff5fd5224d557d81cb146f46e33b831c

N/A:
vim-patch:8.1.1232
vim-patch:8.1.1233
vim-patch:8.1.1236

6 files changed, 140 insertions, 24 deletions

diff --git a/runtime/doc/usr_11.txt b/runtime/doc/usr_11.txt
index e5591ac1d1..42b564e962 100644
--- a/runtime/doc/usr_11.txt
+++ b/runtime/doc/usr_11.txt
@@ -205,6 +205,13 @@ something wrong.  It may be one of these two situations.
       NEWER than swap file! ~
 
 
+NOTE that in the following situation Vim knows the swap file is not useful and
+will automatically delete it:
+- The file is a valid swap file (Magic number is correct).
+- The flag that the file was modified is not set.
+- The process is not running.
+
+
 UNREADABLE SWAP FILE
 
 Sometimes the line
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index b2840c9402..7abc75916c 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -633,7 +633,7 @@ readfile (
 #endif
   }
 
-  /* If "Quit" selected at ATTENTION dialog, don't load the file */
+  // If "Quit" selected at ATTENTION dialog, don't load the file.
   if (swap_exists_action == SEA_QUIT) {
     if (!read_buffer && !read_stdin)
       close(fd);
diff --git a/src/nvim/main.c b/src/nvim/main.c
index 27e2abe1ad..4e1c7dff57 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -1520,10 +1520,11 @@ static void create_windows(mparm_T *parmp)
       dorewind = FALSE;
       curbuf = curwin->w_buffer;
       if (curbuf->b_ml.ml_mfp == NULL) {
-        /* Set 'foldlevel' to 'foldlevelstart' if it's not negative. */
-        if (p_fdls >= 0)
+        // Set 'foldlevel' to 'foldlevelstart' if it's not negative..
+        if (p_fdls >= 0) {
           curwin->w_p_fdl = p_fdls;
-        /* When getting the ATTENTION prompt here, use a dialog */
+        }
+        // When getting the ATTENTION prompt here, use a dialog.
         swap_exists_action = SEA_DIALOG;
         set_buflisted(TRUE);
 
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index 7e15627292..23f223fcac 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -71,6 +71,7 @@
 #include "nvim/undo.h"
 #include "nvim/window.h"
 #include "nvim/os/os.h"
+#include "nvim/os/process.h"
 #include "nvim/os/input.h"
 
 #ifndef UNIX            /* it's in os/unix_defs.h for Unix */
@@ -1453,9 +1454,7 @@ static char *make_percent_swname(const char *dir, char *name)
   return d;
 }
 
-#ifdef UNIX
 static bool process_still_running;
-#endif
 
 /// Return information found in swapfile "fname" in dictionary "d".
 /// This is used by the swapinfo() function.
@@ -1566,12 +1565,10 @@ static time_t swapfile_info(char_u *fname)
         if (char_to_long(b0.b0_pid) != 0L) {
           MSG_PUTS(_("\n        process ID: "));
           msg_outnum(char_to_long(b0.b0_pid));
-#if defined(UNIX)
-          if (kill((pid_t)char_to_long(b0.b0_pid), 0) == 0) {
+          if (os_proc_running((int)char_to_long(b0.b0_pid))) {
             MSG_PUTS(_(" (STILL RUNNING)"));
             process_still_running = true;
           }
-#endif
         }
 
         if (b0_magic_wrong(&b0)) {
@@ -1588,6 +1585,51 @@ static time_t swapfile_info(char_u *fname)
   return x;
 }
 
+/// Returns TRUE if the swap file looks OK and there are no changes, thus it
+/// can be safely deleted.
+static time_t swapfile_unchanged(char *fname)
+{
+  struct block0 b0;
+  int ret = true;
+
+  // Swap file must exist.
+  if (!os_path_exists((char_u *)fname)) {
+    return false;
+  }
+
+  // must be able to read the first block
+  int fd = os_open(fname, O_RDONLY, 0);
+  if (fd < 0) {
+    return false;
+  }
+  if (read_eintr(fd, &b0, sizeof(b0)) != sizeof(b0)) {
+    close(fd);
+    return false;
+  }
+
+  // the ID and magic number must be correct
+  if (ml_check_b0_id(&b0) == FAIL|| b0_magic_wrong(&b0)) {
+    ret = false;
+  }
+
+  // must be unchanged
+  if (b0.b0_dirty) {
+    ret = false;
+  }
+
+  // process must be known and not running.
+  long pid = char_to_long(b0.b0_pid);
+  if (pid == 0L || os_proc_running((int)pid)) {
+    ret = false;
+  }
+
+  // TODO: Should we check if the swap file was created on the current
+  // system?  And the current user?
+
+  close(fd);
+  return ret;
+}
+
 static int recov_file_names(char_u **names, char_u *path, int prepend_dot)
   FUNC_ATTR_NONNULL_ALL
 {
@@ -3389,17 +3431,24 @@ static char *findswapname(buf_T *buf, char **dirp, char *old_fname,
             && vim_strchr(p_shm, SHM_ATTENTION) == NULL) {
           int choice = 0;
 
-#ifdef UNIX
           process_still_running = false;
-#endif
-          /*
-           * If there is a SwapExists autocommand and we can handle
-           * the response, trigger it.  It may return 0 to ask the
-           * user anyway.
-           */
-          if (swap_exists_action != SEA_NONE
-              && has_autocmd(EVENT_SWAPEXISTS, (char_u *) buf_fname, buf))
+          // It's safe to delete the swap file if all these are true:
+          // - the edited file exists
+          // - the swap file has no changes and looks OK
+          if (os_path_exists(buf->b_fname) && swapfile_unchanged(fname)) {
+            choice = 4;
+            if (p_verbose > 0) {
+              verb_msg(_("Found a swap file that is not useful, deleting it"));
+            }
+          }
+
+          // If there is a SwapExists autocommand and we can handle the
+          // response, trigger it.  It may return 0 to ask the user anyway.
+          if (choice == 0
+              && swap_exists_action != SEA_NONE
+              && has_autocmd(EVENT_SWAPEXISTS, (char_u *) buf_fname, buf)) {
             choice = do_swapexists(buf, (char_u *) fname);
+          }
 
           if (choice == 0) {
             // Show info about the existing swap file.
@@ -3431,21 +3480,18 @@ static char *findswapname(buf_T *buf, char **dirp, char *old_fname,
             xstrlcat(name, sw_msg_2, name_len);
             choice = do_dialog(VIM_WARNING, (char_u *)_("VIM - ATTENTION"),
                                (char_u *)name,
-# if defined(UNIX)
                                process_still_running
                                ? (char_u *)_(
                                    "&Open Read-Only\n&Edit anyway\n&Recover"
                                    "\n&Quit\n&Abort") :
-# endif
                                (char_u *)_(
                                    "&Open Read-Only\n&Edit anyway\n&Recover"
                                    "\n&Delete it\n&Quit\n&Abort"),
                                1, NULL, false);
 
-# if defined(UNIX)
-            if (process_still_running && choice >= 4)
-              choice++;                 /* Skip missing "Delete it" button */
-# endif
+            if (process_still_running && choice >= 4) {
+              choice++;                 // Skip missing "Delete it" button.
+            }
             xfree(name);
 
             // pretend screen didn't scroll, need redraw anyway
diff --git a/src/nvim/os/process.c b/src/nvim/os/process.c
index a67e7487eb..a1020be215 100644
--- a/src/nvim/os/process.c
+++ b/src/nvim/os/process.c
@@ -265,3 +265,9 @@ Dictionary os_proc_info(int pid)
   return pinfo;
 }
 #endif
+
+/// Return true if process `pid` is running.
+bool os_proc_running(int pid)
+{
+  return uv_kill(pid, 0) == 0;
+}
diff --git a/src/nvim/testdir/test_swap.vim b/src/nvim/testdir/test_swap.vim
index 1bd73707a8..1000422813 100644
--- a/src/nvim/testdir/test_swap.vim
+++ b/src/nvim/testdir/test_swap.vim
@@ -125,3 +125,59 @@ func Test_swapname()
   call delete('Xtest2')
   call delete('Xtest3')
 endfunc
+
+func Test_swapfile_delete()
+  autocmd! SwapExists
+  function s:swap_exists()
+    let v:swapchoice = s:swap_choice
+    let s:swapname = v:swapname
+    let s:filename = expand('<afile>')
+  endfunc
+  augroup test_swapfile_delete
+    autocmd!
+    autocmd SwapExists * call s:swap_exists()
+  augroup END
+
+
+  " Create a valid swapfile by editing a file.
+  split XswapfileText
+  call setline(1, ['one', 'two', 'three'])
+  write  " file is written, not modified
+  " read the swapfile as a Blob
+  let swapfile_name = swapname('%')
+  let swapfile_bytes = readfile(swapfile_name, 'B')
+
+  " Close the file and recreate the swap file.
+  " Now editing the file will run into the process still existing
+  quit
+  call writefile(swapfile_bytes, swapfile_name)
+  let s:swap_choice = 'e'
+  let s:swapname = ''
+  split XswapfileText
+  quit
+  call assert_equal(swapfile_name, s:swapname)
+
+  " Write the swapfile with a modified PID, now it will be automatically
+  " deleted. Process one should never be Vim.
+  let swapfile_bytes[24:27] = 0z01000000
+  call writefile(swapfile_bytes, swapfile_name)
+  let s:swapname = ''
+  split XswapfileText
+  quit
+  call assert_equal('', s:swapname)
+
+  " Now set the modified flag, the swap file will not be deleted
+  let swapfile_bytes[28 + 80 + 899] = 0x55
+  call writefile(swapfile_bytes, swapfile_name)
+  let s:swapname = ''
+  split XswapfileText
+  quit
+  call assert_equal(swapfile_name, s:swapname)
+
+  call delete('XswapfileText')
+  call delete(swapfile_name)
+  augroup test_swapfile_delete
+    autocmd!
+  augroup END
+  augroup! test_swapfile_delete
+endfunc