From 921133cfa56a2e289a1cf09e87b382a6ea0885f8 Mon Sep 17 00:00:00 2001
From: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>
Date: Thu, 23 Apr 2026 00:10:05 +0530
Subject: [PATCH] fix(debug): preserve full line at truncation boundary and cap
 memory

Adapt the byte-boundary-safe truncation fix from PR #14040 by
taosiyuan163 into the new _capture_log_snapshot() code path: when
the truncation cut lands exactly on a line boundary, keep the first
retained line instead of unconditionally dropping it.

Also add a 2x max_bytes safety cap to the backward-reading loop to
prevent unbounded memory consumption when log files contain very long
lines (e.g. JSON blobs) with few newlines.

Based on #14040 by @taosiyuan163.
---
 hermes_cli/debug.py | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/hermes_cli/debug.py b/hermes_cli/debug.py
index 318a74c433..348de59a9d 100644
--- a/hermes_cli/debug.py
+++ b/hermes_cli/debug.py
@@ -383,7 +383,7 @@ def _capture_log_snapshot(
                 total = 0
                 newline_count = 0
 
-                while pos > 0 and (total < max_bytes or newline_count <= tail_lines + 1):
+                while pos > 0 and (total < max_bytes or newline_count <= tail_lines + 1) and total < max_bytes * 2:
                     read_size = min(chunk_size, pos)
                     pos -= read_size
                     f.seek(pos)
@@ -398,10 +398,14 @@ def _capture_log_snapshot(
 
         full_raw = raw
         if truncated and len(full_raw) > max_bytes:
-            full_raw = full_raw[-max_bytes:]
-            if b"\n" in full_raw:
-                # Drop the partial first line inside the truncated window so the
-                # pasted log still starts on a real log line.
+            cut = len(full_raw) - max_bytes
+            # Check whether the cut lands exactly on a line boundary.  If the
+            # byte just before the cut position is a newline the first retained
+            # byte starts a complete line and we should keep it.  Only drop a
+            # partial first line when we're genuinely mid-line.
+            on_boundary = cut > 0 and full_raw[cut - 1 : cut] == b"\n"
+            full_raw = full_raw[cut:]
+            if not on_boundary and b"\n" in full_raw:
                 full_raw = full_raw.split(b"\n", 1)[1]
 
         all_text = raw.decode("utf-8", errors="replace")