[#4] Introduce developer guide with log documentation

Author: elfenpiff

examples/rust/publish_subscribe/publisher.rs

@@ -17,6 +17,7 @@ use iceoryx2::prelude::*;
 const CYCLE_TIME: Duration = Duration::from_secs(1);
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
+    set_log_level(LogLevel::Trace);
     let node = NodeBuilder::new().create::<ipc::Service>()?;
 
     let service = node

internal/cpp_doc_generator/developer_guide.rst

@@ -0,0 +1,7 @@
+Developer Guide
+===============
+
+.. toctree::
+   :maxdepth: 1
+
+  developer_guide_logging

internal/cpp_doc_generator/developer_guide_logging.rst

@@ -0,0 +1,304 @@
+Logging
+-------
+
+In this section, the log mechanisms of iceoryx2 are described in detail. We
+begin by analyzing the structure of log messages and how they can be utilized
+when debugging a process. Next, we describe the error-handling strategy, how to
+log messages in general, and provide a table that outlines when to use each log
+level. The final section explains how a user can implement a custom logger.
+
+Understanding The Log Output
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In iceoryx2, a log message typically consists of two parts: the origin and the
+actual message. The origin refers to the debug output of the object that
+emitted the log message.
+
+Including the origin is important because a backtrace, which points to a
+specific line in the code, is often not enough. A typical setup might instantiate
+many objects of the same type in different states, and a bug could be caused by
+a subtle detail that affects a later statement.
+
+Therefore, the full object state is added as the origin. In the following example,
+we see the log output of the `FileBuilder`, the class responsible for constructing
+a `File` object. By including the object's state, we gain valuable details about
+the file that was opened for reading.
+
+.. code-block::
+
+   +--------- monotonic counter
+   |  +------ log level (T = Trace, D = Debug, I = Info, W = Warn, E = Error, F = Fatal)
+   |  |  +--- origin: the state (debug output) of the object that emitted the log
+   |  |  |            message
+   |  |  |                                the actual log message ----------------+
+   |  |  |                                                                       |
+   |  |  |                                                                       |
+   0 [T] FileBuilder { file_path: FilePath { value: FixedSizeByteString<255> {   |
+         len: 20, data: "config/iceoryx2.toml" } }, access_mode: Read,           |
+         permission: Permission(448), has_ownership: false, owner: None,         |
+         group: None, truncate_size: None, creation_mode: None }                 |
+         | opened ---------------------------------------------------------------+
+
+How To Handle Errors
+^^^^^^^^^^^^^^^^^^^^
+
+In this section, we describe the error handling of internal iceoryx2 Rust code.
+The language bindings may diverge from this approach when it does not align
+with the idioms of the respective language.
+
+Non-Recoverable Errors
+""""""""""""""""""""""
+
+Non-recoverable errors describe scenarios where the program cannot continue and
+would either encounter undefined behavior or crash in the next statement. For
+example, this can occur when accessing an out-of-bounds array index or when a
+corruption in the underlying OS is detected due to illegally modified managed
+resources.
+
+In these cases, the only viable option is to terminate the program and provide
+a detailed error message.
+
+Only the `fatal_panic!` macro should be used for such scenarios; **never** use
+Rust's default `panic!` macro. While `fatal_panic!` utilizes `panic!` under the
+hood, it also adds a final log message to the logger to inform the user of the
+impending crash.
+
+.. code-block:: Rust
+
+    // call from within an object
+    fn do_stuff(&self) {
+        // self must implement Debug
+        fatal_panic!(from self, "Something horrible has happened!");
+    }
+
+    // from a free function
+    fatal_panic!(from "some_context_here", "Something horrible has happened!");
+
+    // only fail when a function fails
+    fn do_stuff() -> Result<(), SomeError>;
+    fatal_panic!(from self, when do_stuff(), "Something horrible has happened!");
+
+Recoverable Errors
+""""""""""""""""""
+
+Recoverable errors occur when something fails but with appropriate error
+handling, the program can continue. For instance, attempting to open a file
+without sufficient permissions would result in a recoverable error. In such
+cases, the function would return an error wrapped inside a `Result`.
+
+In iceoryx2, we follow the rule that every `Err(..)` must be accompanied by a
+debug log message, ensuring a complete error trace for easier debugging.
+
+To enforce the correct log level and message, iceoryx2 introduces the `fail!`
+macro, which requires the origin, a log message, and optionally the error value.
+**Never** use a combination of `return Err(..);` with a separate `debug!` log
+message.
+
+.. code-block:: Rust
+
+   // call from within an object
+   fn do_stuff(&self) -> Result<(), SomeError> {
+        // self must implement Debug
+        fail!(from self, 
+              with SomeError::WhatEver, 
+              "An error has occurred.");
+   }
+
+    // from a free function
+    fn do_stuff() -> Result<(), SomeError> {
+        fail!(from "some_context_here", 
+              with SomeError::WhatEver, 
+              "An error has occurred.");
+    }
+
+    // only fail when function fails, otherwise use value
+    fn call_me() -> Result<i32, SomeError>;
+    fn do_stuff() -> Result<(), SomeError> {
+        let number = fail!(from "some_context_here", 
+              when call_me(),
+              "An error has occurred.");
+    }
+
+A more complex example might involve a user trying to open a service, where the
+following sequence of events happens under the hood:
+
+1. The static configuration file of the service is opened.
+   - Trace log message.
+2. The deserialization fails because a field is missing.
+   - Debug log message informing the next layer about the issue.
+3. The `ServiceBuilder` fails because the static details exist but cannot be read.
+   - Debug log message informing the user that the service appears to be in a corrupted state.
+
+This sequence allows us to trace exactly why the service is in a corrupted state,
+specifically due to a deserialization failure. The following snippet demonstrates
+a case where the file
+`/tmp/iceoryx2/services/iox2_4eacadf2695a3f4b2eb95485759246ce1a2aa906.service`
+of the service `My/Funk/ServiceName` cannot be deserialized because the field
+`max_subscribers` is missing.
+
+Had we only relied on a stack trace pointing to the lines of code where the log
+messages originated, we wouldn't have known which service was affected or which
+underlying file had the issue.
+
+.. code-block::
+
+    12 [T] FileBuilder { file_path: FilePath { value: FixedSizeByteString<255> { 
+           len: 76, data: "/tmp/iceoryx2/services/iox2_4eacadf2695a3f4b2eb954857
+           59246ce1a2aa906.service" } }, access_mode: Read, 
+           permission: Permission(448), has_ownership: false, owner: None, 
+           group: None, truncate_size: None, creation_mode: None }
+           | opened
+    13 [D] "Toml::deserialize"
+           | Failed to deserialize object (TOML parse error at line 5, column 1
+           |
+         5 | [messaging_pattern]
+           | ^^^^^^^^^^^^^^^^^^^
+         missing field `max_subscribers`
+         ).
+    14 [D] BuilderWithServiceType { service_config: StaticConfig { service_id: 
+           ServiceId(RestrictedFileName { value: FixedSizeByteString<64> { len: 
+           40, data: "4eacadf2695a3f4b2eb95485759246ce1a2aa906" } }), 
+           service_name: ServiceName { value: "My/Funk/ServiceName" }, .... 
+           | Unable to deserialize the service config. Is the service corrupted?
+
+How To Log - What LogLevel To Use When
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+======== ========= ======================================================================================================
+LogLevel Recipient Description
+======== ========= ======================================================================================================
+Trace    Developer For interesting application events, e.g., whenever a resource like a file is created or destroyed.
+Debug    Developer Only used when a function that returns a `Result` encounters an error.
+Info     User      For messages that are relevant to the user, such as "ready to perform tasks."
+Warn     User      When functionality is restricted due to a recoverable error that has been mitigated.
+Error    User      For severe failures where parts of the application cannot operate and no mitigation is possible.
+Fatal    User      The last message before the application crashes.
+======== ========= ======================================================================================================
+
+Each log level has its corresponding macro and can be used in a similar manner
+to the `fail!` and `fatal_panic!` macros. Below, we demonstrate the usage with
+the `trace!` log macro. The `debug!`, `info!`, `warn!`, `error!`, and `fatal_panic!`
+macros work in an identical way.
+
+.. code-block:: Rust
+
+    // call from within an object
+    fn do_stuff(&self) {
+        // self must implement Debug
+        trace!(from self, "Something horrible has happened!");
+    }
+
+    // from a free function
+    trace!(from "some_context_here", "Something horrible has happened!");
+
+    // only log when a function fails
+    fn do_stuff() -> Result<(), SomeError>;
+    trace!(from self, when do_stuff(), "Something horrible has happened!");
+
+Custom Logger
+^^^^^^^^^^^^^
+
+iceoryx2 allows users to set up their own custom logger. All approaches have two
+things in common: the logger can only be set once, and once set, it will remain
+the active logger until the process exits.
+
+Additionally, the logger must be set before the first log message is created.
+If log messages have already been generated, the default logger is automatically
+set and can no longer be changed.
+
+To integrate iceoryx2 with other Rust libraries, iceoryx2 provides support for
+`log <https://crates.io/crates/log>`_ and
+`tracing <https://crates.io/crates/tracing>`_. This can be enabled by using
+either the `logger_log` or the `logger_tracing` feature flags.
+
+.. code-block:: Toml
+
+    # Cargo.toml
+    [dependencies]
+    iceoryx2 = { version = "0.4", features = ["logger_log"] }
+
+Language: C
+"""""""""""
+
+The C API provides the function `iox2_set_logger(iox2_log_callback logger)`, where
+`iox2_log_callback` is a function pointer with the signature 
+`void (*iox2_log_callback)(enum iox2_log_level_e, const char* origin, const char* message)`.
+
+The following code snippet demonstrates how to implement a simple `printf` logger:
+
+.. code-block:: C
+
+    #include "iceoryx2.h"
+
+    void custom_logger(enum iox2_log_level_e, const char* origin, const char* message) {
+        printf("origin: %s, message: %s\n", origin, message);
+    }
+
+    int main() {
+        if ( !iox2_set_logger(custom_logger) ) {
+            printf("Failed to set logger\n");
+        }
+    }
+
+Language: C++
+"""""""""""""
+
+The C++ API provides the function `auto set_logger(Log& logger) -> bool`. Users
+must provide a custom logger that implements the `Log` interface. This logger can
+be attached, but it requires a static lifetime to ensure logging during the
+application's shutdown phase.
+
+The following code snippet demonstrates how to implement a simple `std::cout` logger:
+
+.. code-block:: C++
+
+    #include "iox2/log.hpp"
+
+    class CoutLogger : public iox2::Log {
+      public:
+        void log(LogLevel log_level, const char* origin, const char* message) override {
+            std::cout << "origin: " << origin << ", message: " << message << std::endl;
+        }
+    };
+
+    int main() {
+        static CoutLogger LOGGER;
+
+        if ( !iox2::set_logger(&logger) ) {
+            std::cerr << "Failed to set logger" << std::endl;
+        }
+    }
+
+Language: Rust
+""""""""""""""
+
+The Rust API provides the function 
+`pub fn set_logger<T: Log + 'static>(value: &'static T) -> bool`. 
+A custom logger must implement the `Log` trait and requires a static lifetime to 
+ensure logging during the application's shutdown.
+
+The following code snippet demonstrates how to implement a simple `println!` logger:
+
+.. code-block:: Rust
+
+    use iceoryx2_bb_log::{set_logger, Log, LogLevel};
+    use std::sync::LazyLock;
+
+    #[derive(default)]
+    struct PrintLogger {}
+
+    impl Log for PrintLogger {
+        fn log(&self, log_level: LogLevel, origin: std::fmt::Arguments, message: std::fmt::Arguments ) {
+            println!("log level: {:?}, origin: {}, message: {}",
+                    log_level, origin.to_string(), message.to_string());
+        }
+    }
+
+    static LOGGER: LazyLock<PrintLogger> = LazyLock::new(|| PrintLogger::default());
+
+    fn main() {
+        if !set_logger(&*LOGGER) {
+            println!("Failed to set logger");
+        }
+    }
+

internal/cpp_doc_generator/index.rst

@@ -10,6 +10,8 @@ Welcome to iceoryx2's C / C++ documentation!
 
    getting_started
 
+   developer_guide
+
    iceoryx_hoofs_api
 
    iceoryx2_c_api