docs: provide custom sinks example

CMakeLists.txt

@@ -32,6 +32,7 @@ include (GetCacheVariables)
 include (GNUInstallDirs)
 
 option (BUILD_SHARED_LIBS "Build shared libraries" ON)
+option (BUILD_EXAMPLES "Build examples" ON)
 option (PRINT_UNSYMBOLIZED_STACK_TRACES
   "Print file offsets in traces instead of symbolizing" OFF)
 option (WITH_GFLAGS "Use gflags" ON)
@@ -960,6 +961,11 @@ if (BUILD_TESTING)
   )
 endif (BUILD_TESTING)
 
+if (BUILD_EXAMPLES)
+  add_executable (custom_sink_example examples/custom_sink.cc)
+  target_link_libraries (custom_sink_example PRIVATE glog::glog)
+endif (BUILD_EXAMPLES)
+
 install (TARGETS glog
   EXPORT glog-targets
   RUNTIME DESTINATION ${_glog_CMake_BINDIR}

docs/sinks.md

@@ -0,0 +1,84 @@
+# Custom Sinks
+
+Under certain circumstances, it is useful to send the log output to a
+destination other than a file, `stderr` and/or `stdout`. In case, the library
+provides the `#!cpp google::LogSink` interface whose implementations can be used
+to write the log output to arbitrary locations.
+
+## Basic Interface
+
+The sink interface is defined as follows:
+
+``` cpp
+class LogSink {
+ public:
+  virtual void send(LogSeverity severity, const char* full_filename,
+                    const char* base_filename, int line,
+                    const LogMessageTime& time, const char* message,
+                    size_t message_len);
+};
+```
+
+The user must implement `#!cpp google::LogSink::send`, which is called by the
+library every time a message is logged.
+
+!!! warning "Possible deadlock due to nested logging"
+    This method can't use `LOG()` or `CHECK()` as logging system mutex(s) are
+    held during this call.
+
+## Registering Log Sinks
+
+To use the custom sink and instance of the above interface implementation must
+be registered using `google::AddLogSink` which expects a pointer to the
+`google::LogSink` instance. To unregister use `google::RemoveLogSink`. Both
+functions are thread-safe.
+
+!!! danger "`LogSink` ownership"
+    The `google::LogSink` instance must not be destroyed until the referencing
+    pointer is unregistered.
+
+## Direct Logging
+
+Instead of registering the sink, we can directly use to log messages. While `#!
+LOG_TO_SINK(sink, severity)` allows to log both to the sink and to a global log
+registry, e.g., a file, `#!cpp LOG_TO_SINK_BUT_NOT_TO_LOGFILE(sink, severity)`
+will avoid the latter.
+
+!!! example "Using a custom sink"
+    ``` cpp title="custom_sink.cc"
+    -8<- "examples/custom_sink.cc:33:"
+    ```
+
+    1. `MySink` implements a custom sink that sends log messages to `std::cout`.
+    2. The custom sink must be registered to for use with existing logging
+       macros.
+    3. Once the custom sink is no longer needed we remove it from the registry.
+    4. A sink does not need to be registered globally. However, then, messages
+       must be logged using dedicated macros.
+
+    Running the above example as `#!bash GLOG_log_dir=. ./custom_sink_example`
+    will produce
+
+    <div class="annotate" markdown>
+
+    ``` title="Custom sink output"
+    INFO custom_sink.cc:63 logging to MySink
+    INFO custom_sink.cc:68 direct logging
+    INFO custom_sink.cc:69 direct logging but not to file (1)
+    ```
+
+    </div>
+
+    1. This line is not present in the log file because we used
+       `LOG_TO_SINK_BUT_NOT_TO_LOGFILE` to log the message.
+
+    and the corresponding log file will contain
+
+    ``` title="Log file generated with the custom sink"
+    Log file created at: 2024/06/11 13:24:27
+    Running on machine: pc
+    Running duration (h:mm:ss): 0:00:00
+    Log line format: [IWEF]yyyymmdd hh:mm:ss.uuuuuu threadid file:line] msg
+    I20240611 13:24:27.476620 126237946035776 custom_sink.cc:63] logging to MySink
+    I20240611 13:24:27.476796 126237946035776 custom_sink.cc:68] direct logging
+    ```

examples/custom_sink.cc

@@ -0,0 +1,71 @@
+// Copyright (c) 2024, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: Sergiu Deitsch
+//
+
+#include <glog/logging.h>
+
+#include <algorithm>
+#include <cstddef>
+#include <iostream>
+#include <iterator>
+
+namespace {
+
+struct MyLogSink : google::LogSink { // (1)!
+  void send(google::LogSeverity severity, const char* full_filename,
+            const char* base_filename, int line,
+            const google::LogMessageTime& time, const char* message,
+            std::size_t message_len) override {
+    std::cout << google::GetLogSeverityName(severity) << ' ' << base_filename
+              << ':' << line << ' ';
+    std::copy_n(message, message_len,
+                std::ostreambuf_iterator<char>{std::cout});
+    std::cout << '\n';
+  }
+};
+
+}  // namespace
+
+int main(int argc, char** argv) {
+  google::InitGoogleLogging(argv[0]);
+
+  MyLogSink sink;
+  google::AddLogSink(&sink); // (2)!
+
+  LOG(INFO) << "logging to MySink";
+
+  google::RemoveLogSink(&sink); // (3)!
+
+  // We can directly log to a sink without registering it
+  LOG_TO_SINK(&sink, INFO) << "direct logging"; // (4)!
+  LOG_TO_SINK_BUT_NOT_TO_LOGFILE(&sink, INFO)
+      << "direct logging but not to file";
+}

mkdocs.yml

@@ -7,7 +7,9 @@ edit_uri: edit/0.7.x/docs/
 copyright: Copyright &copy; 2024 Google Inc. &amp; contributors - <a href="#__consent">Change cookie settings</a>
 markdown_extensions:
   - admonition
+  - attr_list
   - def_list
+  - md_in_html
   - pymdownx.details
   - pymdownx.highlight:
       anchor_linenums: true
@@ -26,8 +28,9 @@ theme:
   name: material
   custom_dir: docs/overrides
   icon:
-    repo: fontawesome/brands/git-alt
+    annotation: material/chevron-right-circle
     edit: material/pencil
+    repo: fontawesome/brands/git-alt
     view: material/eye
   language: en
   features:
@@ -107,6 +110,7 @@ nav:
       - Installation using Package Managers: packages.md
   - User Guide:
       - Logging: logging.md
+      - Custom Sinks: sinks.md
       - Failure Handler: failures.md
       - Log Removal: log_cleaner.md
       - Stripping Log Messages: log_stripping.md