From 69307eaf2dbc028a13be7dfc924abab9e6c395e7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dawid=20M=C4=99drek?= <dawid.medrek@scylladb.com>
Date: Fri, 25 Jul 2025 16:33:59 +0200
Subject: [PATCH] db/commitlog: Extend error messages for corrupted data

We're providing additional information in error messages when throwing
an exception related to data corruption: when a segment is truncated
and when it's content is invalid. That might prove helpful when debugging.

Closes scylladb/scylladb#25190

(cherry picked from commit 408b45fa7e7c3847b0cb26cf4c3ae33e69b9a1bb)

Closes scylladb/scylladb#25459
---
 db/commitlog/commitlog.cc | 23 ++++++++++++++++-------
 db/commitlog/commitlog.hh |  6 ++----
 2 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/db/commitlog/commitlog.cc b/db/commitlog/commitlog.cc
index 02e3af861d..de46b5d037 100644
--- a/db/commitlog/commitlog.cc
+++ b/db/commitlog/commitlog.cc
@@ -3252,9 +3252,13 @@ const db::commitlog::config& db::commitlog::active_config() const {
     return _segment_manager->cfg;
 }
 
+db::commitlog::segment_data_corruption_error::segment_data_corruption_error(std::string_view msg, uint64_t s)
+    : _msg(fmt::format("Segment data corruption: {}", msg))
+    , _bytes(s)
+{}
 
-db::commitlog::segment_truncation::segment_truncation(uint64_t pos) 
-    : _msg(fmt::format("Segment truncation at {}", pos))
+db::commitlog::segment_truncation::segment_truncation(std::string_view reason, uint64_t pos)
+    : _msg(fmt::format("Segment truncation at {}. Reason: {}", pos, reason))
     , _pos(pos)
 {}
 
@@ -3444,7 +3448,8 @@ db::commitlog::read_log_file(const replay_state& state, sstring filename, sstrin
 
             while (rem < size) {
                 if (eof) {
-                    throw segment_truncation(block_boundry);
+                    auto reason = fmt::format("unexpected EOF, rem={}, size={}", rem, size);
+                    throw segment_truncation(std::move(reason), block_boundry);
                 }
 
                 auto block_size = alignment - initial.size_bytes();
@@ -3455,7 +3460,8 @@ db::commitlog::read_log_file(const replay_state& state, sstring filename, sstrin
 
                 if (tmp.size_bytes() == 0) {
                     eof = true;
-                    throw segment_truncation(block_boundry);
+                    auto reason = fmt::format("read 0 bytes, while tried to read {}", block_size);
+                    throw segment_truncation(std::move(reason), block_boundry);
                 }
 
                 crc32_nbo crc;
@@ -3490,10 +3496,12 @@ db::commitlog::read_log_file(const replay_state& state, sstring filename, sstrin
                     auto checksum = crc.checksum();
 
                     if (check != checksum) {
-                        throw segment_data_corruption_error("Data corruption", alignment);
+                        auto reason = fmt::format("checksums do not match: {:x} vs. {:x}", check, checksum);
+                        throw segment_data_corruption_error(std::move(reason), alignment);
                     }
                     if (id != this->id) {
-                        throw segment_truncation(pos + rem);
+                        auto reason = fmt::format("IDs do not match: {} vs. {}", id, this->id);
+                        throw segment_truncation(std::move(reason), pos + rem);
                     }
                 }
                 tmp.remove_suffix(detail::sector_overhead_size);
@@ -3768,7 +3776,8 @@ db::commitlog::read_log_file(const replay_state& state, sstring filename, sstrin
                     co_await read_chunk();
                 }
                 if (corrupt_size > 0) {
-                    throw segment_data_corruption_error("Data corruption", corrupt_size);
+                    auto reason = fmt::format("corrupted size while reading file: {}", corrupt_size);
+                    throw segment_data_corruption_error(std::move(reason), corrupt_size);
                 }
             } catch (...) {
                 p = std::current_exception();
diff --git a/db/commitlog/commitlog.hh b/db/commitlog/commitlog.hh
index df79c6ae6b..b2e54338ff 100644
--- a/db/commitlog/commitlog.hh
+++ b/db/commitlog/commitlog.hh
@@ -392,9 +392,7 @@ public:
     class segment_data_corruption_error: public segment_error {
         std::string _msg;
     public:
-        segment_data_corruption_error(std::string msg, uint64_t s)
-                : _msg(std::move(msg)), _bytes(s) {
-        }
+        segment_data_corruption_error(std::string_view msg, uint64_t s);
         uint64_t bytes() const {
             return _bytes;
         }
@@ -425,7 +423,7 @@ public:
         std::string _msg;
         uint64_t _pos;
     public:
-        segment_truncation(uint64_t);
+        segment_truncation(std::string_view reason, uint64_t position);
 
         uint64_t position() const;
         const char* what() const noexcept override;