story-kit: merge 192_bug_code_fences_lose_newlines_when_pasted_from_agent_output

server/src/matrix/bot.rs

@@ -499,18 +499,51 @@ pub fn markdown_to_html(markdown: &str) -> String {
 // Paragraph buffering helper
 // ---------------------------------------------------------------------------
 
+/// Returns `true` when `text` ends while inside an open fenced code block.
+///
+/// A fenced code block opens and closes on lines that start with ` ``` `
+/// (three or more backticks).  We count the fence markers and return `true`
+/// when the count is odd (a fence was opened but not yet closed).
+fn is_inside_code_fence(text: &str) -> bool {
+    let mut in_fence = false;
+    for line in text.lines() {
+        if line.trim_start().starts_with("```") {
+            in_fence = !in_fence;
+        }
+    }
+    in_fence
+}
+
 /// Drain all complete paragraphs from `buffer` and return them.
 ///
 /// A paragraph boundary is a double newline (`\n\n`). Each drained paragraph
 /// is trimmed of surrounding whitespace; empty paragraphs are discarded.
 /// The buffer is left with only the remaining incomplete text.
+///
+/// **Code-fence awareness:** a `\n\n` that occurs *inside* a fenced code
+/// block (delimited by ` ``` ` lines) is **not** treated as a paragraph
+/// boundary.  This prevents a blank line inside a code block from splitting
+/// the fence across multiple Matrix messages, which would corrupt the
+/// rendering of the second half.
 pub fn drain_complete_paragraphs(buffer: &mut String) -> Vec<String> {
     let mut paragraphs = Vec::new();
-    while let Some(pos) = buffer.find("\n\n") {
-        let chunk = buffer[..pos].trim().to_string();
-        *buffer = buffer[pos + 2..].to_string();
-        if !chunk.is_empty() {
-            paragraphs.push(chunk);
+    let mut search_from = 0;
+    loop {
+        let Some(pos) = buffer[search_from..].find("\n\n") else {
+            break;
+        };
+        let abs_pos = search_from + pos;
+        // Only split at this boundary when we are NOT inside a code fence.
+        if is_inside_code_fence(&buffer[..abs_pos]) {
+            // Skip past this \n\n and keep looking for the next boundary.
+            search_from = abs_pos + 2;
+        } else {
+            let chunk = buffer[..abs_pos].trim().to_string();
+            *buffer = buffer[abs_pos + 2..].to_string();
+            search_from = 0;
+            if !chunk.is_empty() {
+                paragraphs.push(chunk);
+            }
         }
     }
     paragraphs
@@ -733,6 +766,55 @@ mod tests {
         assert_eq!(buf, "  World  ");
     }
 
+    // -- drain_complete_paragraphs: code-fence awareness -------------------
+
+    #[test]
+    fn drain_complete_paragraphs_code_fence_blank_line_not_split() {
+        // A blank line inside a fenced code block must NOT trigger a split.
+        // Before the fix the function would split at the blank line and the
+        // second half would be sent without the opening fence, breaking rendering.
+        let mut buf = "```rust\nfn foo() {\n    let x = 1;\n\n    let y = 2;\n}\n```\n\nNext paragraph."
+            .to_string();
+        let paras = drain_complete_paragraphs(&mut buf);
+        assert_eq!(
+            paras.len(),
+            1,
+            "code fence with blank line should not be split into multiple messages: {paras:?}"
+        );
+        assert!(
+            paras[0].starts_with("```rust"),
+            "first paragraph should be the code fence: {:?}",
+            paras[0]
+        );
+        assert!(
+            paras[0].contains("let y = 2;"),
+            "code fence should contain content from both sides of the blank line: {:?}",
+            paras[0]
+        );
+        assert_eq!(buf, "Next paragraph.");
+    }
+
+    #[test]
+    fn drain_complete_paragraphs_text_before_and_after_fenced_block() {
+        // Text paragraph, then a code block with an internal blank line, then more text.
+        let mut buf =
+            "Before\n\n```\ncode\n\nmore code\n```\n\nAfter".to_string();
+        let paras = drain_complete_paragraphs(&mut buf);
+        assert_eq!(paras.len(), 2, "expected two paragraphs: {paras:?}");
+        assert_eq!(paras[0], "Before");
+        assert!(
+            paras[1].starts_with("```"),
+            "second paragraph should be the code fence: {:?}",
+            paras[1]
+        );
+        assert!(
+            paras[1].contains("more code"),
+            "code fence content must include the part after the blank line: {:?}",
+            paras[1]
+        );
+        assert_eq!(buf, "After");
+    }
+
     #[test]
     fn drain_complete_paragraphs_incremental_simulation() {
         // Simulate tokens arriving one character at a time.