fix(web): bound dashboard action log tails

(cherry picked from commit a4188a3e24)

hermes_cli/web_server.py

@@ -3041,6 +3041,9 @@ async def run_debug_share_endpoint(body: DebugShareRequest | None = None):
 # ---------------------------------------------------------------------------
 
 _ACTION_LOG_DIR: Path = get_hermes_home() / "logs"
+_ACTION_LOG_TAIL_MAX_BYTES = 256 * 1024
+_ACTION_LOG_TAIL_INITIAL_CHUNK_BYTES = 8 * 1024
+_ACTION_LOG_TAIL_MAX_CHUNK_BYTES = 64 * 1024
 
 # Short ``name`` (from the URL) → absolute log file path.
 _ACTION_LOG_FILES: Dict[str, str] = {
@@ -3142,17 +3145,50 @@ def _spawn_hermes_action(subcommand: List[str], name: str) -> subprocess.Popen:
 
 
 def _tail_lines(path: Path, n: int) -> List[str]:
-    """Return the last ``n`` lines of ``path``.  Reads the whole file — fine
-    for our small per-action logs.  Binary-decoded with ``errors='replace'``
-    so log corruption doesn't 500 the endpoint."""
-    if not path.exists():
+    """Return the last ``n`` lines of ``path`` without loading huge logs."""
+    if n <= 0 or not path.exists():
         return []
     try:
-        text = path.read_text(encoding="utf-8", errors="replace")
+        size = path.stat().st_size
     except OSError:
         return []
-    lines = text.splitlines()
-    return lines[-n:] if n > 0 else lines
+    if size <= 0:
+        return []
+
+    min_offset = max(0, size - _ACTION_LOG_TAIL_MAX_BYTES)
+    offset = size
+    chunk_size = _ACTION_LOG_TAIL_INITIAL_CHUNK_BYTES
+    newline_count = 0
+    chunks: List[bytes] = []
+    drop_partial_first_line = False
+
+    try:
+        with path.open("rb") as handle:
+            while offset > min_offset and newline_count <= n:
+                read_size = min(chunk_size, offset - min_offset)
+                offset -= read_size
+                handle.seek(offset)
+                chunk = handle.read(read_size)
+                chunks.append(chunk)
+                newline_count += chunk.count(b"\n")
+                chunk_size = min(
+                    chunk_size * 2,
+                    _ACTION_LOG_TAIL_MAX_CHUNK_BYTES,
+                )
+            if offset > 0:
+                handle.seek(offset - 1)
+                drop_partial_first_line = handle.read(1) != b"\n"
+    except OSError:
+        return []
+
+    lines = (
+        b"".join(reversed(chunks))
+        .decode("utf-8", errors="replace")
+        .splitlines()
+    )
+    if drop_partial_first_line and lines:
+        lines = lines[1:]
+    return lines[-n:]
 
 
 def _gateway_subcommand(profile: Optional[str], verb: str) -> List[str]: