Fix out-of-bounds access on TextView

Frequent updates to `TextContent` while the screen is being refreshed can
trigger an out-of-bounds access: an update with a smaller string being
performed between calls to `compute_rows()` and `draw()` on TextView will
cause an out-of-bounds access when slicing the string at `Segment::resolve`.

This change fixes the issue by caching the content value when performing
size calculations and using this cached content when drawing the view.

Some additional changes were also made to reuse `TextContent` invalidation
code on `TextView` and wrap content data with `RefCell<Arc<..>>` to enable
sharing the inner string between `content_value` and `content_cache`.

src/views/text_view.rs

@@ -1,4 +1,6 @@
+use std::cell::RefCell;
 use std::ops::Deref;
+use std::ptr;
 use std::sync::Arc;
 use std::sync::{Mutex, MutexGuard};
 
@@ -12,6 +14,9 @@ use crate::utils::markup::StyledString;
 use crate::view::{SizeCache, View};
 use crate::{Printer, Vec2, With, XY};
 
+// Content type used internally for caching and storage
+type InnerContentType = RefCell<Arc<StyledString>>;
+
 /// Provides access to the content of a [`TextView`].
 ///
 /// Cloning this object will still point to the same content.
@@ -42,11 +47,12 @@ impl TextContent {
     where
         S: Into<StyledString>,
     {
-        let content = content.into();
+        let content = Arc::new(content.into());
 
         TextContent {
             content: Arc::new(Mutex::new(TextContentInner {
-                content,
+                content_value: RefCell::new(content),
+                content_cache: RefCell::new(Arc::new(StyledString::default())),
                 size_cache: None,
             })),
         }
@@ -60,18 +66,22 @@ impl TextContent {
 /// [`StyledString`]: ../utils/markup/type.StyledString.html
 ///
 /// This keeps the content locked. Do not store this!
+
 pub struct TextContentRef {
-    handle: OwningHandle<
+    _handle: OwningHandle<
         ArcRef<Mutex<TextContentInner>>,
         MutexGuard<'static, TextContentInner>,
     >,
+    // We also need to keep a copy of Arc so `deref` can return
+    // a reference to the `StyledString`
+    data: Arc<StyledString>,
 }
 
 impl Deref for TextContentRef {
     type Target = StyledString;
 
     fn deref(&self) -> &StyledString {
-        &self.handle.content
+        self.data.as_ref()
     }
 }
 
@@ -81,7 +91,9 @@ impl TextContent {
     where
         S: Into<StyledString>,
     {
-        self.with_content(|c| *c = content.into());
+        self.with_content(|c| {
+            c.content_value.replace(Arc::new(content.into()))
+        });
     }
 
     /// Append `content` to the end of a `TextView`.
@@ -89,7 +101,17 @@ impl TextContent {
     where
         S: Into<StyledString>,
     {
-        self.with_content(|c| c.append(content))
+        self.with_content(|c| {
+            // This will only clone content if content_cached and content_value
+            // are sharing the same underlying Rc.
+            if c.is_content_shared() {
+                c.content_value
+                    .replace(Arc::new(c.get_value().as_ref().clone()));
+            };
+            Arc::get_mut(&mut c.content_value.borrow_mut())
+                .expect("should not have a shared content here")
+                .append(content)
+        })
     }
 
     /// Returns a reference to the content.
@@ -102,11 +124,11 @@ impl TextContent {
 
     fn with_content<F, O>(&mut self, f: F) -> O
     where
-        F: FnOnce(&mut StyledString) -> O,
+        F: FnOnce(&mut TextContentInner) -> O,
     {
         let mut lock = self.content.lock().unwrap();
 
-        let out = f(&mut lock.content);
+        let out = f(&mut lock);
 
         lock.size_cache = None;
 
@@ -121,7 +143,8 @@ impl TextContent {
 /// Can be shared (through a `Arc<Mutex>`).
 struct TextContentInner {
     // content: String,
-    content: StyledString,
+    content_value: InnerContentType,
+    content_cache: InnerContentType,
 
     // We keep the cache here so it can be busted when we change the content.
     size_cache: Option<XY<SizeCache>>,
@@ -133,11 +156,13 @@ impl TextContentInner {
         let arc_ref: ArcRef<Mutex<TextContentInner>> =
             ArcRef::new(Arc::clone(content));
 
-        TextContentRef {
-            handle: OwningHandle::new_with_fn(arc_ref, |mutex| unsafe {
-                (*mutex).lock().unwrap()
-            }),
-        }
+        let _handle = OwningHandle::new_with_fn(arc_ref, |mutex| unsafe {
+            (*mutex).lock().unwrap()
+        });
+
+        let data = _handle.get_value();
+
+        TextContentRef { _handle, data }
     }
 
     fn is_cache_valid(&self, size: Vec2) -> bool {
@@ -146,6 +171,18 @@ impl TextContentInner {
             Some(ref last) => last.x.accept(size.x) && last.y.accept(size.y),
         }
     }
+
+    fn get_value(&self) -> Arc<StyledString> {
+        Arc::clone(&*self.content_value.borrow())
+    }
+
+    fn get_cache(&self) -> Arc<StyledString> {
+        Arc::clone(&*self.content_cache.borrow())
+    }
+
+    fn is_content_shared(&self) -> bool {
+        ptr::eq(self.get_value().as_ref(), self.get_cache().as_ref())
+    }
 }
 
 /// A simple view showing a fixed text.
@@ -161,7 +198,7 @@ impl TextContentInner {
 /// ```
 pub struct TextView {
     // content: String,
-    content: Arc<Mutex<TextContentInner>>,
+    content: TextContent,
     rows: Vec<Row>,
 
     align: Align,
@@ -202,7 +239,7 @@ impl TextView {
     /// ```
     pub fn new_with_content(content: TextContent) -> Self {
         TextView {
-            content: content.content,
+            content: content,
             effect: Effect::Simple,
             rows: Vec::new(),
             wrap: true,
@@ -285,8 +322,7 @@ impl TextView {
     where
         S: Into<StyledString>,
     {
-        self.content.lock().unwrap().content = content.into();
-        self.invalidate();
+        self.content.set_content(content);
     }
 
     /// Append `content` to the end of a `TextView`.
@@ -294,22 +330,20 @@ impl TextView {
     where
         S: Into<StyledString>,
     {
-        self.content.lock().unwrap().content.append(content.into());
-        self.invalidate();
+        self.content.append(content);
     }
 
     /// Returns the current text in this view.
     pub fn get_content(&self) -> TextContentRef {
-        TextContentInner::get_content(&self.content)
+        TextContentInner::get_content(&self.content.content)
     }
 
     /// Returns a shared reference to the content, allowing content mutation.
     pub fn get_shared_content(&mut self) -> TextContent {
         // We take &mut here without really needing it,
         // because it sort of "makes sense".
-
         TextContent {
-            content: Arc::clone(&self.content),
+            content: Arc::clone(&self.content.content),
         }
     }
 
@@ -318,7 +352,7 @@ impl TextView {
     fn compute_rows(&mut self, size: Vec2) {
         let size = if self.wrap { size } else { Vec2::max_value() };
 
-        let mut content = self.content.lock().unwrap();
+        let mut content = self.content.content.lock().unwrap();
         if content.is_cache_valid(size) {
             return;
         }
@@ -326,23 +360,21 @@ impl TextView {
         // Completely bust the cache
         // Just in case we fail, we don't want to leave a bad cache.
         content.size_cache = None;
+        content
+            .content_cache
+            .replace(content.content_value.borrow().clone());
 
         if size.x == 0 {
             // Nothing we can do at this point.
             return;
         }
 
-        self.rows = LinesIterator::new(&content.content, size.x).collect();
+        self.rows =
+            LinesIterator::new(content.get_cache().as_ref(), size.x).collect();
 
         // Desired width
         self.width = self.rows.iter().map(|row| row.width).max();
     }
-
-    // Invalidates the cache, so next call will recompute everything.
-    fn invalidate(&mut self) {
-        let mut content = self.content.lock().unwrap();
-        content.size_cache = None;
-    }
 }
 
 impl View for TextView {
@@ -352,14 +384,14 @@ impl View for TextView {
         let offset = self.align.v.get_offset(h, printer.size.y);
         let printer = &printer.offset((0, offset));
 
-        let content = self.content.lock().unwrap();
+        let content = self.content.content.lock().unwrap();
 
         printer.with_effect(self.effect, |printer| {
             for (y, row) in self.rows.iter().enumerate() {
                 let l = row.width;
                 let mut x = self.align.h.get_offset(l, printer.size.x);
 
-                for span in row.resolve(&content.content) {
+                for span in row.resolve(content.get_cache().as_ref()) {
                     printer.with_style(*span.attr, |printer| {
                         printer.print((x, y), span.content);
                         x += span.content.width();
@@ -370,7 +402,7 @@ impl View for TextView {
     }
 
     fn needs_relayout(&self) -> bool {
-        let content = self.content.lock().unwrap();
+        let content = self.content.content.lock().unwrap();
         content.size_cache.is_none()
     }
 
@@ -389,7 +421,7 @@ impl View for TextView {
         let my_size = Vec2::new(self.width.unwrap_or(0), self.rows.len());
 
         // Build a fresh cache.
-        let mut content = self.content.lock().unwrap();
+        let mut content = self.content.content.lock().unwrap();
         content.size_cache = Some(SizeCache::build(my_size, size));
     }
 }