From b47d069bf8f5e3bbe18b142d45efd630e6af118b Mon Sep 17 00:00:00 2001
From: Akash Kumar <116457960+akashchamp@users.noreply.github.com>
Date: Wed, 28 Oct 2026 20:06:17 +0530
Subject: [PATCH] [piper] do not index past the end for an empty demuxed body

When a demuxed line has an empty body, like a blank docker-compose
line "db-1  | ", the memcpy() destination in
piper_log_format::get_subline() was &plf_cached_line[prefix_len], which
is one past the end of the vector. Builds with -D_GLIBCXX_ASSERTIONS
(Arch Linux, for one) abort on that. Use data() + prefix_len instead.

Fixes #1759
---
 src/log_format_impls.cc                                      | 5 +++--
 test/Makefile.am                                             | 1 +
 ...est_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.err | 0
 ...est_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.out | 2 ++
 test/logfile_docker_compose_empty_body.0                     | 2 ++
 test/test_demux.sh                                           | 3 +++
 6 files changed, 11 insertions(+), 2 deletions(-)
 create mode 100644 test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.err
 create mode 100644 test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.out
 create mode 100644 test/logfile_docker_compose_empty_body.0

diff --git a/src/log_format_impls.cc b/src/log_format_impls.cc
index 6b4caa99ba2..3c8371e7c4f 100644
--- a/src/log_format_impls.cc
+++ b/src/log_format_impls.cc
@@ -123,8 +123,9 @@ class piper_log_format : public log_format {
         const auto prefix_len = this->plf_cached_line.size();
         this->plf_cached_line.resize(this->plf_cached_line.size()
                                      + sbr.length());
-        memcpy(
-            &this->plf_cached_line[prefix_len], sbr.get_data(), sbr.length());
+        memcpy(this->plf_cached_line.data() + prefix_len,
+               sbr.get_data(),
+               sbr.length());
 
         sbr.share(this->plf_share_manager,
                   this->plf_cached_line.data(),
diff --git a/test/Makefile.am b/test/Makefile.am
index 2bccff7da21..ecae9447f8b 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -363,6 +363,7 @@ dist_noinst_DATA = \
 	logfile_cloudflare.1 \
 	logfile_cxx.0 \
 	logfile_docker_compose.0 \
+	logfile_docker_compose_empty_body.0 \
 	logfile_docker_compose_with_noise.0 \
 	logfile_docker_compose_with_ts.0 \
 	logfile_dst.0 \
diff --git a/test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.err b/test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.err
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.out b/test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.out
new file mode 100644
index 00000000000..8fbc490ce5a
--- /dev/null
+++ b/test/expected/test_demux.sh_83e339125bf0af442ee46a6f498cb750cda01d5f.out
@@ -0,0 +1,2 @@
+2013-06-06T19:13:20.123456+0000 
+2013-06-06T19:13:20.123456+0000 hello
diff --git a/test/logfile_docker_compose_empty_body.0 b/test/logfile_docker_compose_empty_body.0
new file mode 100644
index 00000000000..a6a8a2592e6
--- /dev/null
+++ b/test/logfile_docker_compose_empty_body.0
@@ -0,0 +1,2 @@
+db-1  | 
+db-1  | hello
diff --git a/test/test_demux.sh b/test/test_demux.sh
index 0227030a9ce..c37024ca5fe 100644
--- a/test/test_demux.sh
+++ b/test/test_demux.sh
@@ -19,6 +19,9 @@ cat ${test_dir}/logfile_docker_compose_with_ts.0 | run_cap_test env TEST_COMMENT
 
 run_cap_test ${lnav_test} -n ${test_dir}/logfile_docker_compose_with_ts.0
 
+# The first message has an empty body
+run_cap_test ${lnav_test} -n ${test_dir}/logfile_docker_compose_empty_body.0
+
 run_cap_test ${lnav_test} -n ${test_dir}/logfile_mux_zookeeper.0
 
 run_cap_test ${lnav_test} -n \