Only reuse the old counter if the file name pattern has a counter placeholder.

If text_file_backend is set up to append to a previously written file, and
the actively written file name pattern does not include a file counter
placeholder but the target file name pattern does, we used to skip incrementing
the file counter in an attempt to generate the same file name as was last used,
so that we open the last used file for appending. While it did result in
reusing the last written file, since the counter was not incremented, the next
rotation would generate the last used target file name, which would result in
overwriting the last rotated file instead of adding a new file to the storage.

To mitigate this, only skip incrementing the counter if the file name pattern
for the actively written file actually has a counter placeholder. This way,
the counter will get incremented in the case described above, and on rotation
a new target file name will be generated.

Fixes #245.

Author: Lastique

doc/changelog.qbk

@@ -1,5 +1,5 @@
 [/
-              Copyright Andrey Semashev 2007 - 2024.
+              Copyright Andrey Semashev 2007 - 2025.
      Distributed under the Boost Software License, Version 1.0.
         (See accompanying file LICENSE_1_0.txt or copy at
               http://www.boost.org/LICENSE_1_0.txt)
@@ -13,6 +13,7 @@
 
 * Disabled usage of `std::codecvt<char16_t>` and `std::codecvt<char32_t>` locale facets in C++20 and later modes as they were deprecated in C++20. This means character code conversions to/from `char16_t` and `char32_t` is no longer available in C++20 and later.
 * Fixed building issues when using CMake and MinGW-w64. ([pull_request 241])
+* Fixed incorrect file counter used by `text_file_backend` when the backend was configured to append to an existing file and the actively written file name pattern didn't have a counter placeholder but the target file name pattern did, and the log files were written directly into the target storage. ([github_issue 245])
 
 [heading 2.30, Boost 1.87]
 

src/text_file_backend.cpp

@@ -1,5 +1,5 @@
 /*
- *          Copyright Andrey Semashev 2007 - 2015.
+ *          Copyright Andrey Semashev 2007 - 2025.
  * Distributed under the Boost Software License, Version 1.0.
  *    (See accompanying file LICENSE_1_0.txt or copy at
  *          http://www.boost.org/LICENSE_1_0.txt)
@@ -507,7 +507,14 @@ BOOST_LOG_ANONYMOUS_NAMESPACE {
     }
 
     //! The function parses file name pattern and splits it into path and filename and creates a function object that will generate the actual filename from the pattern
-    void parse_file_name_pattern(filesystem::path const& pattern, filesystem::path& storage_dir, filesystem::path& file_name_pattern, boost::log::aux::light_function< path_string_type (unsigned int) >& file_name_generator)
+    void parse_file_name_pattern
+    (
+        filesystem::path const& pattern,
+        filesystem::path& storage_dir,
+        filesystem::path& file_name_pattern,
+        boost::log::aux::light_function< path_string_type (unsigned int) >& file_name_generator,
+        bool& has_file_counter
+    )
     {
         // Note: avoid calling Boost.Filesystem functions that involve path::codecvt()
         // https://svn.boost.org/trac/boost/ticket/9119
@@ -606,6 +613,8 @@ BOOST_LOG_ANONYMOUS_NAMESPACE {
             // No placeholders detected
             file_name_generator = empty_formatter(name_pattern);
         }
+
+        has_file_counter = counter_found;
     }
 
 
@@ -1271,6 +1280,8 @@ struct text_file_backend::implementation
     filesystem::path m_StorageDir;
     //! File name generator (according to m_FileNamePattern)
     boost::log::aux::light_function< path_string_type (unsigned int) > m_FileNameGenerator;
+    //! The flag indicates whether m_FileNamePattern has a file counter placeholder
+    bool m_FileNamePatternHasCounter;
 
     //! Target file name pattern
     filesystem::path m_TargetFileNamePattern;
@@ -1316,6 +1327,7 @@ struct text_file_backend::implementation
     bool m_IsFirstFile;
 
     implementation(uintmax_t rotation_size, auto_newline_mode auto_newline, bool auto_flush, bool enable_final_rotation) :
+        m_FileNamePatternHasCounter(false),
         m_FileCounter(0u),
         m_FileOpenMode(std::ios_base::trunc | std::ios_base::out),
         m_CharactersWritten(0u),
@@ -1449,11 +1461,11 @@ BOOST_LOG_API void text_file_backend::consume(record_view const& rec, string_typ
             unsigned int file_counter = m_pImpl->m_FileCounter;
             if (BOOST_LIKELY(m_pImpl->m_FileCounterIsLastUsed))
             {
-                // If the sink backend is configured to append to a previously written file, don't
-                // increment the file counter and try to open the existing file. Only do this if the
-                // file is not moved to a different storage location by the file collector.
+                // If the sink backend is configured to append to a previously written file and the file pattern
+                // includes a file counter, don't increment the counter and try to open the existing file, with the last
+                // used counter value. Only do this if the file is not moved to a different storage location by the file collector.
                 bool increment_file_counter = true;
-                if (BOOST_UNLIKELY(m_pImpl->m_IsFirstFile && (m_pImpl->m_FileOpenMode & std::ios_base::app) != 0))
+                if (BOOST_UNLIKELY(m_pImpl->m_IsFirstFile && (m_pImpl->m_FileOpenMode & std::ios_base::app) != 0 && m_pImpl->m_FileNamePatternHasCounter))
                 {
                     filesystem::path last_file_name = m_pImpl->m_StorageDir / m_pImpl->m_FileNameGenerator(file_counter);
                     if (!!m_pImpl->m_pFileCollector)
@@ -1575,7 +1587,8 @@ BOOST_LOG_API void text_file_backend::set_file_name_pattern_internal(filesystem:
         !pattern.empty() ? pattern : filesystem::path(traits_t::default_file_name_pattern()),
         m_pImpl->m_StorageDir,
         m_pImpl->m_FileNamePattern,
-        m_pImpl->m_FileNameGenerator
+        m_pImpl->m_FileNameGenerator,
+        m_pImpl->m_FileNamePatternHasCounter
     );
 }
 
@@ -1584,7 +1597,8 @@ BOOST_LOG_API void text_file_backend::set_target_file_name_pattern_internal(file
 {
     if (!pattern.empty())
     {
-        parse_file_name_pattern(pattern, m_pImpl->m_TargetStorageDir, m_pImpl->m_TargetFileNamePattern, m_pImpl->m_TargetFileNameGenerator);
+        bool has_file_counter = false;
+        parse_file_name_pattern(pattern, m_pImpl->m_TargetStorageDir, m_pImpl->m_TargetFileNamePattern, m_pImpl->m_TargetFileNameGenerator, has_file_counter);
     }
     else
     {