Warn when saving a file which has been changed outside the editor

editor.c

@@ -293,6 +293,7 @@ static File *file_new_text(Editor *ed, Text *text) {
 	if (!file)
 		return NULL;
 	file->text = text;
+	file->stat = text_stat(text);
 	file->refcount++;
 	if (ed->files)
 		ed->files->prev = file;

editor.h

@@ -195,6 +195,7 @@ struct File {
 	Text *text;
 	const char *name;
 	bool stdin;
+	struct stat stat;
 	int refcount;
 	Mark marks[MARK_LAST];
 	File *next, *prev;

text.c

@@ -802,7 +802,7 @@ static bool preserve_selinux_context(int src, int dest) {
  *   - SELinux security context can not be preserved (if enabled)
  */
 static bool text_range_save_atomic(Text *txt, Filerange *range, const char *filename) {
-	struct stat meta;
+	struct stat meta = { 0 };
 	int fd = -1, oldfd = -1, saved_errno;
 	char *tmpname = NULL;
 	size_t size = text_range_size(range);
@@ -877,6 +877,8 @@ static bool text_range_save_atomic(Text *txt, Filerange *range, const char *file
 	if (rename(tmpname, filename) == -1)
 		goto err;
 
+	if (meta.st_mtime)
+		txt->info = meta;
 	free(tmpname);
 	return true;
 err:
@@ -955,6 +957,7 @@ bool text_range_save(Text *txt, Filerange *range, const char *filename) {
 		goto err;
 	if (close(fd) == -1)
 		return false;
+	txt->info = meta;
 ok:
 	txt->saved_action = txt->history;
 	text_snapshot(txt);
@@ -1043,6 +1046,10 @@ Text *text_load(const char *filename) {
 	return NULL;
 }
 
+struct stat text_stat(Text *txt) {
+	return txt->info;
+}
+
 /* A delete operation can either start/stop midway through a piece or at
  * a boundry. In the former case a new piece is created to represent the
  * remaining text before/after the modification point.

text.h

@@ -3,7 +3,9 @@
 
 #include <stdbool.h>
 #include <time.h>
+#include <unistd.h>
 #include <sys/types.h>
+#include <sys/stat.h>
 
 #define EPOS ((size_t)-1)         /* invalid position */
 
@@ -35,6 +37,8 @@ typedef struct {
 	     text_iterator_next(&it))
 
 Text *text_load(const char *file);
+/* file information at time of load or last save */
+struct stat text_stat(Text*);
 bool text_insert(Text*, size_t pos, const char *data, size_t len);
 bool text_delete(Text*, size_t pos, size_t len);
 void text_snapshot(Text*);

vis.c

@@ -1683,19 +1683,28 @@ static bool cmd_write(Filerange *range, enum CmdOpt opt, const char *argv[]) {
 		return false;
 	}
 	for (const char **name = &argv[1]; *name; name++) {
+		struct stat meta;
+		if (!(opt & CMD_OPT_FORCE) && file->stat.st_mtime && stat(*name, &meta) == 0 &&
+		    file->stat.st_mtime < meta.st_mtime) {
+			editor_info_show(vis, "WARNING: file has been changed since reading it");
+			return false;
+		}
 		if (!text_range_save(text, range, *name)) {
 			editor_info_show(vis, "Can't write `%s'", *name);
 			return false;
 		}
-		if (!file->name)
+		if (!file->name) {
 			editor_window_name(vis->win, *name);
+			file->stat = text_stat(text);
+		}
 	}
 	return true;
 }
 
 static bool cmd_saveas(Filerange *range, enum CmdOpt opt, const char *argv[]) {
 	if (cmd_write(range, opt, argv)) {
 		editor_window_name(vis->win, argv[1]);
+		vis->win->file->stat = text_stat(vis->win->file->text);
 		return true;
 	}
 	return false;