patch 9.0.1583: get E304 when using 'cryptmethod' "xchacha20v2"

Problem:    Get E304 when using 'cryptmethod' "xchacha20v2". (Steve Mynott)
Solution:   Add 4th crypt method to block zero ID check.  Avoid syncing a swap
            file before reading the file. (closes #12433)

src/fileio.c

@@ -125,6 +125,7 @@ readfile(
     exarg_T	*eap,			// can be NULL!
     int		flags)
 {
+    int		retval = FAIL;	// jump to "theend" instead of returning
     int		fd = 0;
     int		newfile = (flags & READ_NEW);
     int		check_readonly;
@@ -239,7 +240,7 @@ readfile(
 	    && !(flags & READ_DUMMY))
     {
 	if (set_rw_fname(fname, sfname) == FAIL)
-	    return FAIL;
+	    goto theend;
     }
 
     // Remember the initial values of curbuf, curbuf->b_ffname and
@@ -289,35 +290,41 @@ readfile(
 	    if (apply_autocmds_exarg(EVENT_BUFREADCMD, NULL, sfname,
 							  FALSE, curbuf, eap))
 	    {
-		int status = OK;
+		retval = OK;
 #ifdef FEAT_EVAL
 		if (aborting())
-		    status = FAIL;
+		    retval = FAIL;
 #endif
 		// The BufReadCmd code usually uses ":read" to get the text and
 		// perhaps ":file" to change the buffer name. But we should
 		// consider this to work like ":edit", thus reset the
 		// BF_NOTEDITED flag.  Then ":write" will work to overwrite the
 		// same file.
-		if (status == OK)
+		if (retval == OK)
 		    curbuf->b_flags &= ~BF_NOTEDITED;
-		return status;
+		goto theend;
 	    }
 	}
 	else if (apply_autocmds_exarg(EVENT_FILEREADCMD, sfname, sfname,
 							    FALSE, NULL, eap))
+	{
 #ifdef FEAT_EVAL
-	    return aborting() ? FAIL : OK;
+	    retval = aborting() ? FAIL : OK;
 #else
-	    return OK;
+	    retval = OK;
 #endif
+	    goto theend;
+	}
 
 	curbuf->b_op_start = orig_start;
 
 	if (flags & READ_NOFILE)
+	{
 	    // Return NOTDONE instead of FAIL so that BufEnter can be triggered
 	    // and other operations don't fail.
-	    return NOTDONE;
+	    retval = NOTDONE;
+	    goto theend;
+	}
     }
 
     if ((shortmess(SHM_OVER) || curbuf->b_help) && p_verbose == 0)
@@ -335,7 +342,7 @@ readfile(
 	    filemess(curbuf, fname, (char_u *)_("Illegal file name"), 0);
 	    msg_end();
 	    msg_scroll = msg_save;
-	    return FAIL;
+	    goto theend;
 	}
 
 	// If the name ends in a path separator, we can't open it.  Check here,
@@ -346,7 +353,8 @@ readfile(
 	    filemess(curbuf, fname, (char_u *)_(msg_is_a_directory), 0);
 	    msg_end();
 	    msg_scroll = msg_save;
-	    return NOTDONE;
+	    retval = NOTDONE;
+	    goto theend;
 	}
     }
 
@@ -367,8 +375,6 @@ readfile(
 # endif
 						)
 	{
-	    int retval = FAIL;
-
 	    if (S_ISDIR(perm))
 	    {
 		filemess(curbuf, fname, (char_u *)_(msg_is_a_directory), 0);
@@ -378,7 +384,7 @@ readfile(
 		filemess(curbuf, fname, (char_u *)_("is not a file"), 0);
 	    msg_end();
 	    msg_scroll = msg_save;
-	    return retval;
+	    goto theend;
 	}
 #endif
 #if defined(MSWIN)
@@ -391,7 +397,7 @@ readfile(
 	    filemess(curbuf, fname, (char_u *)_("is a device (disabled with 'opendevice' option)"), 0);
 	    msg_end();
 	    msg_scroll = msg_save;
-	    return FAIL;
+	    goto theend;
 	}
 #endif
     }
@@ -534,7 +540,7 @@ readfile(
 					 && (old_b_fname != curbuf->b_fname)))
 			{
 			    emsg(_(e_autocommands_changed_buffer_or_buffer_name));
-			    return FAIL;
+			    goto theend;
 			}
 		    }
 		    if (dir_of_file_exists(fname))
@@ -557,10 +563,10 @@ readfile(
 		    save_file_ff(curbuf);
 
 #if defined(FEAT_EVAL)
-		    if (aborting())   // autocmds may abort script processing
-			return FAIL;
+		    if (!aborting())   // autocmds may abort script processing
 #endif
-		    return OK;	    // a new file is not an error
+			retval = OK;	    // a new file is not an error
+		    goto theend;
 		}
 		else
 		{
@@ -576,7 +582,7 @@ readfile(
 		}
 	    }
 
-	return FAIL;
+	goto theend;
     }
 
     /*
@@ -614,7 +620,7 @@ readfile(
 	    emsg(_(e_autocommands_changed_buffer_or_buffer_name));
 	    if (!read_buffer)
 		close(fd);
-	    return FAIL;
+	    goto theend;
 	}
 #ifdef UNIX
 	// Set swap file protection bits after creating it.
@@ -654,7 +660,7 @@ readfile(
     {
 	if (!read_buffer && !read_stdin)
 	    close(fd);
-	return FAIL;
+	goto theend;
     }
 
     ++no_wait_return;	    // don't wait for return yet
@@ -715,7 +721,7 @@ readfile(
 	    --no_wait_return;
 	    msg_scroll = msg_save;
 	    curbuf->b_p_ro = TRUE;	// must use "w!" now
-	    return FAIL;
+	    goto theend;
 	}
 #endif
 	/*
@@ -737,7 +743,7 @@ readfile(
 	    else
 		emsg(_(e_readpre_autocommands_must_not_change_current_buffer));
 	    curbuf->b_p_ro = TRUE;	// must use "w!" now
-	    return FAIL;
+	    goto theend;
 	}
     }
 
@@ -2461,7 +2467,8 @@ failed:
 #ifdef FEAT_VIMINFO
 	    check_marks_read();
 #endif
-	    return OK;		// an interrupt isn't really an error
+	    retval = OK;	// an interrupt isn't really an error
+	    goto theend;
 	}
 
 	if (!filtering && !(flags & READ_DUMMY))
@@ -2696,13 +2703,20 @@ failed:
 	    msg_scroll = m;
 # ifdef FEAT_EVAL
 	if (aborting())	    // autocmds may abort script processing
-	    return FAIL;
+	    goto theend;
 # endif
     }
 
-    if (recoverymode && error)
-	return FAIL;
-    return OK;
+    if (!(recoverymode && error))
+	retval = OK;
+
+theend:
+    if (curbuf->b_ml.ml_mfp != NULL
+		       && curbuf->b_ml.ml_mfp->mf_dirty == MF_DIRTY_YES_NOSYNC)
+	// OK to sync the swap file now
+	curbuf->b_ml.ml_mfp->mf_dirty = MF_DIRTY_YES;
+
+    return retval;
 }
 
 #if defined(OPEN_CHR_FILES) || defined(PROTO)
@@ -2941,7 +2955,10 @@ check_for_cryptkey(
 	if (cryptkey == NULL && !*did_ask)
 	{
 	    if (*curbuf->b_p_key)
+	    {
 		cryptkey = curbuf->b_p_key;
+		crypt_check_swapfile_curbuf();
+	    }
 	    else
 	    {
 		// When newfile is TRUE, store the typed key in the 'key'