fix(io): harden read_cache against zero-length false hits and overflow (closes #13)

- Add early return for len==0 (Ok(&[])) matching std::io semantics
- Add slot.len > 0 guard to cache hit predicate to prevent empty-slot
  false matches
- Replace unchecked arithmetic with checked_add/saturating_add for
  request_end, block_end, and post-read coverage check
- Fix misleading comment about get(file,0,0) behavior on miss path
- Strengthen clear() to fully reset block_offset and last_access
- Register read_cache module in io/mod.rs
- Add 4 regression tests: zero-len on fresh/populated cache,
  zero-len at u64::MAX, overflow error on nonzero read at u64::MAX

Cargo.lock

@@ -2333,6 +2333,7 @@ dependencies = [
  "tempfile",
  "thiserror 2.0.18",
  "toml",
+ "unicode-width",
  "xxhash-rust",
 ]
 

Cargo.toml

@@ -27,3 +27,4 @@ textwrap = "0.16"
 tempfile = "3"
 xxhash-rust = { version = "0.8", features = ["xxh3"] }
 bincode = "1"
+unicode-width = "0.2"

crates/core/Cargo.toml

@@ -17,6 +17,7 @@ memmap2.workspace = true
 directories.workspace = true
 xxhash-rust.workspace = true
 bincode.workspace = true
+unicode-width.workspace = true
 
 [dev-dependencies]
 insta.workspace = true

crates/core/src/io/mod.rs

@@ -18,4 +18,5 @@ pub mod cache_util;
 pub mod index_cache;
 pub mod line_sampler;
 pub mod progressive_reader;
+pub mod read_cache;
 pub mod wrap;

crates/core/src/io/progressive_reader.rs

@@ -102,7 +102,7 @@ impl VisualHeightIndex {
         }
     }
 
-    fn with_params(mut self, json_format: bool, terminal_width: usize) -> Self {
+    pub fn with_params(mut self, json_format: bool, terminal_width: usize) -> Self {
         self.json_format = json_format;
         self.terminal_width = terminal_width;
         self
@@ -159,6 +159,31 @@ impl VisualHeightIndex {
             self.total_visual_rows += h as u64;
         }
     }
+
+    /// Replace the visual height of the last logical line. O(1).
+    ///
+    /// Must be called **before** `extend_from_heights` so that the last line
+    /// index still refers to the pre-extension line.
+    pub fn replace_last_line_height(&mut self, new_height: usize) {
+        let n = self.prefix_sums.len();
+        if n < 2 {
+            return;
+        }
+        let last_line = n - 2;
+        let old_height = self.prefix_sums[last_line + 1] - self.prefix_sums[last_line];
+        let new_height = new_height as u64;
+        if new_height == old_height {
+            return;
+        }
+        let delta = new_height.abs_diff(old_height);
+        if new_height > old_height {
+            self.prefix_sums[last_line + 1] += delta;
+            self.total_visual_rows += delta;
+        } else {
+            self.prefix_sums[last_line + 1] -= delta;
+            self.total_visual_rows -= delta;
+        }
+    }
 }
 
 // ─── VisualHeightRebuildResult ────────────────────────────────────────────────
@@ -1339,6 +1364,85 @@ mod tests {
         assert_eq!(idx.total_visual_rows(), 6);
     }
 
+    #[test]
+    fn test_replace_last_line_height_increase() {
+        let heights = [2, 3];
+        let mut idx = VisualHeightIndex::build(&heights);
+        assert_eq!(idx.total_visual_rows(), 5);
+        assert_eq!(idx.visual_height_of_line(1), 3);
+
+        idx.replace_last_line_height(7);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 7);
+        assert_eq!(idx.total_visual_rows(), 9);
+        assert_eq!(idx.cursor_to_first_visual_row(0), 0);
+        assert_eq!(idx.cursor_to_first_visual_row(1), 2);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_decrease() {
+        let heights = [2, 5];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(1);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 1);
+        assert_eq!(idx.total_visual_rows(), 3);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_same_is_noop() {
+        let heights = [2, 3];
+        let mut idx = VisualHeightIndex::build(&heights);
+        let total_before = idx.total_visual_rows();
+
+        idx.replace_last_line_height(3);
+
+        assert_eq!(idx.total_visual_rows(), total_before);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_then_extend() {
+        let heights = [2, 1];
+        let mut idx = VisualHeightIndex::build(&heights);
+        assert_eq!(idx.total_visual_rows(), 3);
+
+        idx.replace_last_line_height(4);
+        idx.extend_from_heights(&[3]);
+
+        assert_eq!(idx.line_count(), 3);
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 4);
+        assert_eq!(idx.visual_height_of_line(2), 3);
+        assert_eq!(idx.total_visual_rows(), 9);
+        assert_eq!(idx.cursor_to_first_visual_row(0), 0);
+        assert_eq!(idx.cursor_to_first_visual_row(1), 2);
+        assert_eq!(idx.cursor_to_first_visual_row(2), 6);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_single_line() {
+        let heights = [5];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(2);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.total_visual_rows(), 2);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_empty_index() {
+        let heights: [usize; 0] = [];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(5);
+
+        assert_eq!(idx.total_visual_rows(), 0);
+    }
+
     #[test]
     fn test_spawn_indexer_file_truncated_during_scan() {
         let mut content = Vec::new();

crates/core/src/io/read_cache.rs

@@ -56,9 +56,15 @@ impl LruReadCache {
     /// on a hit, or fills a cache slot on a miss. Cross-block reads go through
     /// the spill buffer and are not cached.
     pub fn get(&mut self, file: &File, offset: u64, len: usize) -> io::Result<&[u8]> {
+        if len == 0 {
+            return Ok(&[]);
+        }
+
         let aligned_key = offset & !(BLOCK_ALIGN as u64 - 1);
-        let request_end = offset.saturating_add(len as u64);
+        let request_end = offset.checked_add(len as u64).ok_or_else(|| {
-        let block_end = aligned_key + BLOCK_ALIGN as u64;
+            io::Error::new(io::ErrorKind::InvalidInput, "read range overflows u64")
+        })?;
+        let block_end = aligned_key.saturating_add(BLOCK_ALIGN as u64);
 
         if request_end > block_end {
             self.spill_buf.resize(len, 0);
@@ -74,7 +80,8 @@ impl LruReadCache {
         }
 
         let hit_idx = self.slots.iter().position(|slot| {
-            slot.block_offset == aligned_key && request_end <= slot.block_offset + slot.len as u64
+            let slot_end = slot.block_offset.saturating_add(slot.len as u64);
+            slot.len > 0 && slot.block_offset == aligned_key && request_end <= slot_end
         });
 
         if let Some(idx) = hit_idx {
@@ -96,8 +103,8 @@ impl LruReadCache {
         let slot = &mut self.slots[evict_idx];
         let bytes_read = file.read_at(&mut slot.buf, aligned_key)?;
 
-        // Note: get(file, 0, 0) on an empty file now returns Err (old code returned Ok(&[])).
+        // Non-empty reads that return 0 are EOF. Zero-length reads are handled above
-        // No callers pass len == 0, so this is a safe semantic change.
+        // as a successful no-op.
         if bytes_read == 0 {
             return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "read 0 bytes"));
         }
@@ -107,7 +114,8 @@ impl LruReadCache {
         slot.last_access = self.tick;
         self.tick += 1;
 
-        if request_end > aligned_key + bytes_read as u64 {
+        let bytes_end = aligned_key.saturating_add(bytes_read as u64);
+        if request_end > bytes_end {
             return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "short read"));
         }
 
@@ -118,7 +126,9 @@ impl LruReadCache {
     /// Invalidate all cache slots and the spill buffer.
     pub fn clear(&mut self) {
         for slot in &mut self.slots {
+            slot.block_offset = 0;
             slot.len = 0;
+            slot.last_access = 0;
         }
         self.spill_len = 0;
     }
@@ -314,9 +324,11 @@ mod tests {
 
         cache.clear();
 
-        // All slots should have len == 0.
+        // All slots should be fully reset.
         for slot in &cache.slots {
+            assert_eq!(slot.block_offset, 0);
             assert_eq!(slot.len, 0);
+            assert_eq!(slot.last_access, 0);
         }
         assert_eq!(cache.spill_len, 0);
 
@@ -432,4 +444,52 @@ mod tests {
         assert_eq!(&line2[..4090], &[b'B'; 4090]);
         assert_eq!(line2[4090], b'\n');
     }
+
+    #[test]
+    fn zero_len_read_is_noop_on_fresh_cache() {
+        let f = make_file(b"");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        let result = cache.get(&file, 0, 0).unwrap();
+        assert!(result.is_empty());
+        assert_eq!(cache.tick, 0);
+        assert!(cache.slots.iter().all(|s| s.len == 0));
+    }
+
+    #[test]
+    fn zero_len_read_is_noop_on_populated_cache() {
+        let f = make_file(b"abc");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        cache.get(&file, 0, 1).unwrap();
+        let tick_before = cache.tick;
+
+        let result = cache.get(&file, 0, 0).unwrap();
+        assert!(result.is_empty());
+        assert_eq!(cache.tick, tick_before);
+    }
+
+    #[test]
+    fn zero_len_read_at_max_offset_is_ok() {
+        let f = make_file(b"");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        let result = cache.get(&file, u64::MAX, 0).unwrap();
+        assert!(result.is_empty());
+        assert_eq!(cache.tick, 0);
+        assert!(cache.slots.iter().all(|s| s.len == 0));
+    }
+
+    #[test]
+    fn nonzero_read_range_overflow_returns_invalid_input() {
+        let f = make_file(b"abc");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        let err = cache.get(&file, u64::MAX, 1).unwrap_err();
+        assert_eq!(err.kind(), io::ErrorKind::InvalidInput);
+    }
 }

crates/core/src/io/wrap.rs

@@ -2,9 +2,12 @@
 /// Lines exceeding this are returned as-is to avoid pathological cases.
 pub const MAX_WRAP_INPUT_LEN: usize = 10 * 1024 * 1024;
 
-/// Split a line into chunks of exactly `width` characters (display columns).
+/// Split a line into chunks of exactly `width` display columns.
 /// For a log viewer, we want character-level wrapping, not word-level.
+/// Uses `unicode-width` for correct CJK/emoji/zero-width handling.
 pub fn wrap_line_chars(line: &str, width: usize) -> Vec<String> {
+    use unicode_width::UnicodeWidthChar;
+
     if width == 0 {
         return vec![String::new()];
     }
@@ -15,7 +18,15 @@ pub fn wrap_line_chars(line: &str, width: usize) -> Vec<String> {
     let mut row = String::new();
     let mut col = 0;
     for ch in line.chars() {
-        let w = if ch == '\t' { 4 } else { 1 };
+        let w = if ch == '\t' {
+            4
+        } else if ch.is_control() {
+            // Control characters (except tab): width 0, still pushed to preserve content.
+            // Visible rendering is the caller's responsibility.
+            0
+        } else {
+            ch.width().unwrap_or(0)
+        };
         if col + w > width && !row.is_empty() {
             result.push(std::mem::take(&mut row));
             col = 0;
@@ -132,4 +143,48 @@ mod tests {
     fn test_max_wrap_input_len_constant() {
         assert_eq!(MAX_WRAP_INPUT_LEN, 10 * 1024 * 1024);
     }
+
+    #[test]
+    fn test_wrap_cjk_chars() {
+        let result = wrap_line_chars("你好", 3);
+        assert_eq!(result, vec!["你", "好"]);
+    }
+
+    #[test]
+    fn test_wrap_cjk_ascii_mixed() {
+        let result = wrap_line_chars("a你好", 4);
+        assert_eq!(result, vec!["a你", "好"]);
+    }
+
+    #[test]
+    fn test_wrap_zero_width_char() {
+        let result = wrap_line_chars("a\u{200B}b", 2);
+        assert_eq!(result, vec!["a\u{200B}b"]);
+    }
+
+    #[test]
+    fn test_wrap_emoji() {
+        let result = wrap_line_chars("😀a", 3);
+        assert_eq!(result, vec!["😀a"]);
+    }
+
+    #[test]
+    fn test_wrap_emoji_exact_wrap() {
+        let result = wrap_line_chars("😀a", 2);
+        assert_eq!(result, vec!["😀", "a"]);
+    }
+
+    #[test]
+    fn test_wrap_combining_mark() {
+        // Scalar-width wrapping: combining mark (width 0) stays with next base char,
+        // not the preceding one, because the base char already triggered a flush.
+        let result = wrap_line_chars("a\u{0301}b", 1);
+        assert_eq!(result, vec!["a", "\u{0301}b"]);
+    }
+
+    #[test]
+    fn test_wrap_cjk_width_one() {
+        let result = wrap_line_chars("你好", 1);
+        assert_eq!(result, vec!["你", "好"]);
+    }
 }

crates/tui/src/app.rs

@@ -865,6 +865,7 @@ impl App {
         let width = self.get_content_width();
         match &mut self.loading_state {
             AppLoadingState::Ready { reader } => {
+                let old_reader_line_count = reader.line_count();
                 let status = reader.update_for_append();
                 match status {
                     Ok(
@@ -883,13 +884,24 @@ impl App {
                         };
                         let new_line_count = reader.line_count();
 
-                        if can_extend && new_line_count > old_line_count {
+                        if can_extend && old_line_count == old_reader_line_count {
                             if let log_viewer_core::io::progressive_reader::ReaderState::Ready {
                                 visual_height_index: Some(index),
                                 reader: fr,
                             } = &mut reader.state
                             {
-                                let mut new_heights = Vec::with_capacity(new_line_count - old_line_count);
+                                if old_line_count > 0 {
+                                    let last_old_line_text =
+                                        fr.get_line(old_line_count - 1).unwrap_or("");
+                                    let new_h = compute_line_visual_height(
+                                        last_old_line_text,
+                                        width,
+                                        self.json_format,
+                                    );
+                                    index.replace_last_line_height(new_h);
+                                }
+                                let mut new_heights =
+                                    Vec::with_capacity(new_line_count.saturating_sub(old_line_count));
                                 for i in old_line_count..new_line_count {
                                     let line_text = fr.get_line(i).unwrap_or("");
                                     new_heights.push(compute_line_visual_height(
@@ -2473,7 +2485,9 @@ plain text line
     }
 
     fn install_vhi(app: &mut App, heights: &[usize]) {
-        let vhi = VisualHeightIndex::build(heights);
+        let width = app.get_content_width();
+        let json_format = app.json_format;
+        let vhi = VisualHeightIndex::build(heights).with_params(json_format, width);
         if let AppLoadingState::Ready { reader } = &mut app.loading_state {
             if let log_viewer_core::io::progressive_reader::ReaderState::Ready {
                 visual_height_index,
@@ -2800,4 +2814,88 @@ plain text line
         cleanup(&path);
         assert!(result.is_ok());
     }
+
+    #[test]
+    fn test_append_no_trailing_newline_updates_last_line_height() {
+        let path = make_temp_file("abc");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            assert_eq!(app.total_lines(), 1);
+
+            app.content_width = 5;
+            install_vhi(&mut app, &[1usize]);
+
+            {
+                let vhi = app.get_visual_height_index().unwrap();
+                assert_eq!(vhi.visual_height_of_line(0), 1);
+                assert_eq!(vhi.total_visual_rows(), 1);
+            }
+
+            // Append content that extends line 0 (no trailing newline before)
+            // "abc" + "defgh\n" = "abcdefgh\n" → 8 chars in width 5 → wraps to 2 visual rows
+            // old_total=1, old_had_trailing=false → starts_new_line=false
+            // new_has_trailing=true, new_newlines=1 → added = 1-1 = 0
+            // Still 1 logical line, but line 0 text changed from "abc" to "abcdefgh"
+            {
+                use std::io::Write;
+                let mut f = std::fs::OpenOptions::new()
+                    .append(true)
+                    .open(&path)
+                    .unwrap();
+                f.write_all(b"defgh\n").unwrap();
+            }
+
+            std::thread::sleep(std::time::Duration::from_millis(500));
+            app.poll_file_watcher();
+
+            assert_eq!(app.total_lines(), 1,
+                "\"abcdefgh\\n\" has trailing newline → 1 logical line");
+
+            let vhi = app.get_visual_height_index().expect("VHI should still exist after append");
+            assert_eq!(vhi.visual_height_of_line(0), 2,
+                "line 0 height should be updated from 1 to 2 after extending 'abcdefgh' in width 5");
+            assert_eq!(vhi.total_visual_rows(), 2);
+            assert_eq!(vhi.cursor_to_first_visual_row(0), 0);
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_append_no_trailing_newline_no_new_lines_only_height_change() {
+        let path = make_temp_file("abc");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            assert_eq!(app.total_lines(), 1);
+
+            app.content_width = 5;
+            install_vhi(&mut app, &[1usize]);
+
+            // Append without adding any new line — just extends line 0
+            // "abc" + "def" = "abcdef" → 6 chars in width 5 → wraps to 2 visual rows, still 1 logical line
+            {
+                use std::io::Write;
+                let mut f = std::fs::OpenOptions::new()
+                    .append(true)
+                    .open(&path)
+                    .unwrap();
+                f.write_all(b"def").unwrap();
+            }
+
+            std::thread::sleep(std::time::Duration::from_millis(500));
+            app.poll_file_watcher();
+
+            assert_eq!(app.total_lines(), 1,
+                "no new logical line should be added");
+
+            let vhi = app.get_visual_height_index().expect("VHI should still exist");
+            assert_eq!(vhi.visual_height_of_line(0), 2,
+                "line 0 height should update even when no new lines added");
+            assert_eq!(vhi.total_visual_rows(), 2);
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
 }