terminal: Allow undo and 'modifiable'.

Partial step towards #2637. Will crash if *all* lines are deleted.

Closes #2607
References #5431

src/nvim/buffer_defs.h

@@ -36,7 +36,7 @@ typedef struct {
 // for Map(K, V)
 #include "nvim/map.h"
 
-#define MODIFIABLE(buf) (!buf->terminal && buf->b_p_ma)
+#define MODIFIABLE(buf) (buf->b_p_ma)
 
 /*
  * Flags for w_valid.
@@ -91,32 +91,22 @@ typedef struct frame_S frame_T;
 
 // for struct memline (it needs memfile_T)
 #include "nvim/memline_defs.h"
-
 // for struct memfile, bhdr_T, blocknr_T... (it needs buf_T)
 #include "nvim/memfile_defs.h"
 
-/*
- * This is here because regexp_defs.h needs win_T and buf_T. regprog_T is
- * used below.
- */
+// for regprog_T. Needs win_T and buf_T.
 #include "nvim/regexp_defs.h"
-
-// for  synstate_T (needs reg_extmatch_T, win_T and buf_T)
+// for synstate_T (needs reg_extmatch_T, win_T, buf_T)
 #include "nvim/syntax_defs.h"
-
 // for signlist_T
 #include "nvim/sign_defs.h"
-
 // for bufhl_*_T
 #include "nvim/bufhl_defs.h"
 
 typedef Map(linenr_T, bufhl_vec_T) bufhl_info_T;
 
-// for FileID
-#include "nvim/os/fs_defs.h"
-
-// for Terminal
-#include "nvim/terminal.h"
+#include "nvim/os/fs_defs.h"    // for FileID
+#include "nvim/terminal.h"      // for Terminal
 
 /*
  * The taggy struct is used to store the information about a :tag command.

src/nvim/normal.c

@@ -7435,12 +7435,10 @@ static void nv_edit(cmdarg_T *cap)
 
   /* in Visual mode and after an operator "a" and "i" are for text objects */
   else if ((cap->cmdchar == 'a' || cap->cmdchar == 'i')
-           && (cap->oap->op_type != OP_NOP
-               || VIsual_active
-               )) {
+           && (cap->oap->op_type != OP_NOP || VIsual_active)) {
     nv_object(cap);
-  } else if (!curbuf->b_p_ma && !p_im) {
-    /* Only give this error when 'insertmode' is off. */
+  } else if (!curbuf->b_p_ma && !p_im && !curbuf->terminal) {
+    // Only give this error when 'insertmode' is off.
     EMSG(_(e_modifiable));
     clearop(cap->oap);
   } else if (!checkclearopq(cap->oap)) {

src/nvim/terminal.c

@@ -237,12 +237,16 @@ Terminal *terminal_open(TerminalOptions opts)
   rv->invalid_end = opts.height;
   refresh_screen(rv, curbuf);
   set_option_value((uint8_t *)"buftype", 0, (uint8_t *)"terminal", OPT_LOCAL);
+
   // some sane settings for terminal buffers
+  curbuf->b_p_ma = false;   // 'nomodifiable'
+  curbuf->b_p_ul = -1;      // disable undo
   set_option_value((uint8_t *)"wrap", false, NULL, OPT_LOCAL);
   set_option_value((uint8_t *)"number", false, NULL, OPT_LOCAL);
   set_option_value((uint8_t *)"relativenumber", false, NULL, OPT_LOCAL);
   buf_set_term_title(curbuf, (char *)curbuf->b_ffname);
   RESET_BINDING(curwin);
+
   // Apply TermOpen autocmds so the user can configure the terminal
   apply_autocmds(EVENT_TERMOPEN, NULL, NULL, false, curbuf);
 
@@ -958,7 +962,7 @@ static void refresh_terminal(Terminal *term)
   buf_T *buf = handle_get_buffer(term->buf_handle);
   bool valid = true;
   if (!buf || !(valid = buf_valid(buf))) {
-    // destroyed by `close_buffer`. Dont do anything else
+    // Destroyed by `close_buffer`. Do not do anything else.
     if (!valid) {
       term->buf_handle = 0;
     }
@@ -1039,7 +1043,7 @@ static void refresh_scrollback(Terminal *term, buf_T *buf)
   }
 }
 
-// Refresh the screen(visible part of the buffer when the terminal is
+// Refresh the screen (visible part of the buffer when the terminal is
 // focused) of a invalidated terminal
 static void refresh_screen(Terminal *term, buf_T *buf)
 {

src/nvim/undo.c

@@ -305,14 +305,9 @@ bool undo_allowed(void)
   return true;
 }
 
-/*
- * Get the undolevle value for the current buffer.
- */
+/// Get the 'undolevels' value for the current buffer.
 static long get_undolevel(void)
 {
-  if (curbuf->terminal) {
-    return -1;
-  }
   if (curbuf->b_p_ul == NO_LOCAL_UNDOLEVEL)
     return p_ul;
   return curbuf->b_p_ul;