From 0fa9885cda0778467ca5737ac888ece5ef371b3d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc=20Andr=C3=A9=20Tanner?= <mat@brain-dump.org>
Date: Fri, 17 Jul 2015 14:38:36 +0200
Subject: vis: handle file truncation more gracefully

If we use the file / virtual memory system as cache (using mmap(2))
and another process truncates the file we are editing, we have a
problem. All we can do is catch the resulting SIGBUS, close the
corresponding window and print a warning message.

To test this use:

 $ dd if=/dev/zero of=TEST bs=8M count=1
 $ vis TEST
   :! echo TRUNCATE > TEST
---
 editor.c |  2 ++
 editor.h |  4 ++++
 text.c   |  8 ++++++++
 text.h   |  3 +++
 vis.c    | 35 +++++++++++++++++++++++++++++++++++
 5 files changed, 52 insertions(+)

diff --git a/editor.c b/editor.c
index ad9bd4e..8628458 100644
--- a/editor.c
+++ b/editor.c
@@ -358,6 +358,8 @@ void editor_window_close(Win *win) {
 		ed->windows = win->next;
 	if (ed->win == win)
 		ed->win = win->next ? win->next : win->prev;
+	if (ed->prompt_window == win)
+		ed->prompt_window = NULL;
 	window_free(win);
 	if (ed->win)
 		ed->ui->window_focus(ed->win->ui);
diff --git a/editor.h b/editor.h
index 76a3136..3e8e644 100644
--- a/editor.h
+++ b/editor.h
@@ -5,6 +5,7 @@
 #include <signal.h>
 #include <stddef.h>
 #include <stdbool.h>
+#include <setjmp.h>
 
 typedef struct Editor Editor;
 typedef struct File File;
@@ -190,6 +191,7 @@ enum Mark {
 struct File {
 	Text *text;
 	const char *name;
+	volatile sig_atomic_t truncated;
 	bool is_stdin;
 	struct stat stat;
 	int refcount;
@@ -244,6 +246,8 @@ struct Editor {
 	Mode *mode_before_prompt; /* user mode which was active before entering prompt */
 	volatile bool running; /* exit main loop once this becomes false */
 	volatile sig_atomic_t cancel_filter; /* abort external command */
+	volatile sig_atomic_t sigbus;
+	sigjmp_buf sigbus_jmpbuf;
 };
 
 Editor *editor_new(Ui*);
diff --git a/text.c b/text.c
index be9a8f5..5f516bc 100644
--- a/text.c
+++ b/text.c
@@ -1185,6 +1185,14 @@ bool text_modified(Text *txt) {
 	return txt->saved_action != txt->history;
 }
 
+bool text_sigbus(Text *txt, const char *addr) {
+	for (Buffer *buf = txt->buffers; buf; buf = buf->next) {
+		if (buf->type == MMAP && buf->data <= addr && addr < buf->data + buf->size)
+			return true;
+	}
+	return false;
+}
+
 enum TextNewLine text_newline_type(Text *txt){
 	if (!txt->newlines) {
 		txt->newlines = TEXT_NEWLINE_NL; /* default to UNIX style \n new lines */
diff --git a/text.h b/text.h
index c322a10..89a5d9b 100644
--- a/text.h
+++ b/text.h
@@ -108,6 +108,9 @@ size_t text_history_get(Text*, size_t index);
 size_t text_size(Text*);
 /* query whether the text contains any unsaved modifications */
 bool text_modified(Text*);
+/* query whether `addr` is part of a memory mapped region associated with
+ * this text instance */
+bool text_sigbus(Text*, const char *addr);
 
 /* which type of new lines does the text use? */
 enum TextNewLine {
diff --git a/vis.c b/vis.c
index 8e51bce..ffe45ce 100644
--- a/vis.c
+++ b/vis.c
@@ -2312,8 +2312,23 @@ static Key getkey(void) {
 	return key;
 }
 
+static void sigbus_handler(int sig, siginfo_t *siginfo, void *context) {
+	for (File *file = vis->files; file; file = file->next) {
+		if (text_sigbus(file->text, siginfo->si_addr))
+			file->truncated = true;
+	}
+	vis->sigbus = true;
+	siglongjmp(vis->sigbus_jmpbuf, 1);
+}
+
 static void mainloop() {
 	struct timespec idle = { .tv_nsec = 0 }, *timeout = NULL;
+	struct sigaction sa_sigbus;
+	memset(&sa_sigbus, 0, sizeof sa_sigbus);
+	sa_sigbus.sa_flags = SA_SIGINFO;
+	sa_sigbus.sa_sigaction = sigbus_handler;
+	if (sigaction(SIGBUS, &sa_sigbus, NULL))
+		die("sigaction: %s", strerror(errno));
 	sigset_t emptyset, blockset;
 	sigemptyset(&emptyset);
 	sigemptyset(&blockset);
@@ -2323,11 +2338,31 @@ static void mainloop() {
 	editor_draw(vis);
 	vis->running = true;
 
+	sigsetjmp(vis->sigbus_jmpbuf, 1);
+
 	while (vis->running) {
 		fd_set fds;
 		FD_ZERO(&fds);
 		FD_SET(STDIN_FILENO, &fds);
 
+		if (vis->sigbus) {
+			char *name = NULL;
+			for (Win *next, *win = vis->windows; win; win = next) {
+				next = win->next;
+				if (win->file->truncated) {
+					free(name);
+					name = strdup(win->file->name);
+					editor_window_close(win);
+				}
+			}
+			if (!vis->windows)
+				die("WARNING: file `%s' truncated!\n", name ? name : "-");
+			else
+				editor_info_show(vis, "WARNING: file `%s' truncated!\n", name ? name : "-");
+			vis->sigbus = false;
+			free(name);
+		}
+
 		editor_update(vis);
 		idle.tv_sec = vis->mode->idle_timeout;
 		int r = pselect(1, &fds, NULL, NULL, timeout, &emptyset);
-- 
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