[libcamera-devel] [PATCH v1 1/4] libcamera: base: Add Backtrace class

[Nicolas Dufresne]

Le vendredi 24 septembre 2021 à 13:23 +0300, Laurent Pinchart a écrit :
> Create a new class to abstract generation and access to call stack
> backtraces. The current implementation depends on the glibc backtrace()
> implementation and is copied from the logger. Future development will
> bring support for libunwind, transparently for the users of the class.
> 
> The logger backtrace implementation is dropped, replaced by usage of the
> new Backtrace class.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  include/libcamera/base/backtrace.h |  34 +++++++++
>  include/libcamera/base/meson.build |   1 +
>  meson.build                        |   4 --
>  src/libcamera/base/backtrace.cpp   | 107 +++++++++++++++++++++++++++++
>  src/libcamera/base/log.cpp         |  25 ++-----
>  src/libcamera/base/meson.build     |   5 ++
>  6 files changed, 153 insertions(+), 23 deletions(-)
>  create mode 100644 include/libcamera/base/backtrace.h
>  create mode 100644 src/libcamera/base/backtrace.cpp
> 
> diff --git a/include/libcamera/base/backtrace.h b/include/libcamera/base/backtrace.h
> new file mode 100644
> index 000000000000..aefc76defa83
> --- /dev/null
> +++ b/include/libcamera/base/backtrace.h
> @@ -0,0 +1,34 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2021, Ideas on Board Oy
> + *
> + * backtrace.h - Call stack backtraces
> + */
> +#ifndef __LIBCAMERA_BASE_BACKTRACE_H__
> +#define __LIBCAMERA_BASE_BACKTRACE_H__

In the context of this being modern code base, have you considered using this ?

  #pragma once

> +
> +#include <string>
> +#include <vector>
> +
> +#include <libcamera/base/private.h>
> +
> +#include <libcamera/base/class.h>
> +
> +namespace libcamera {
> +
> +class Backtrace
> +{
> +public:
> +	Backtrace();
> +
> +	std::string toString(unsigned int skipLevels = 0) const;
> +
> +private:
> +	LIBCAMERA_DISABLE_COPY(Backtrace)
> +
> +	std::vector<void *> backtrace_;
> +};
> +
> +} /* namespace libcamera */
> +
> +#endif /* __LIBCAMERA_BASE_BACKTRACE_H__ */
> diff --git a/include/libcamera/base/meson.build b/include/libcamera/base/meson.build
> index 9feb4b9346d5..525aba9d2919 100644
> --- a/include/libcamera/base/meson.build
> +++ b/include/libcamera/base/meson.build
> @@ -3,6 +3,7 @@
>  libcamera_base_include_dir = libcamera_include_dir / 'base'
>  
>  libcamera_base_headers = files([
> +    'backtrace.h',
>      'bound_method.h',
>      'class.h',
>      'event_dispatcher.h',
> diff --git a/meson.build b/meson.build
> index a49c484fe64e..dfed01ba26bc 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -29,10 +29,6 @@ cc = meson.get_compiler('c')
>  cxx = meson.get_compiler('cpp')
>  config_h = configuration_data()
>  
> -if cc.has_header_symbol('execinfo.h', 'backtrace')
> -    config_h.set('HAVE_BACKTRACE', 1)
> -endif
> -
>  if cc.has_header_symbol('unistd.h', 'issetugid')
>      config_h.set('HAVE_ISSETUGID', 1)
>  endif
> diff --git a/src/libcamera/base/backtrace.cpp b/src/libcamera/base/backtrace.cpp
> new file mode 100644
> index 000000000000..913f7ba71b03
> --- /dev/null
> +++ b/src/libcamera/base/backtrace.cpp
> @@ -0,0 +1,107 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2021, Ideas on Board Oy
> + *
> + * backtrace.h - Call stack backtraces
> + */
> +
> +#include <libcamera/base/backtrace.h>
> +
> +#if HAVE_BACKTRACE
> +#include <execinfo.h>
> +#include <stdlib.h>
> +#endif
> +
> +#include <sstream>
> +
> +#include <libcamera/base/span.h>
> +
> +/**
> + * \file backtrace.h
> + * \brief Generate call stack backtraces
                    _s

> + */
> +
> +namespace libcamera {
> +
> +/**
> + * \class Backtrace
> + * \brief Representation of a call stack backtrace
> + *
> + * The Backtrace class represents a function call stack. Constructing an
> + * instance captures the call stack at the point the instance is constructed.
> + * The instance can later be used to access the call stack and generate a
                                                                         _s
> + * human-readable representation with the toString() function.
> + *
> + * Depending on the platform, different backends can be used to generate the
> + * backtrace. The Backtrace class provides a best effort to capture accurate
> + * backtraces, but doesn't offer any guarantee of a particular backtrace format.
> + */
> +
> +/**
> + * \brief Construct a backtrace
> + *
> + * The backtrace captures the call stack at the point where it is constructed.
> + * It can later be converted to a string with toString().
> + */
> +Backtrace::Backtrace()
> +{
> +#if HAVE_BACKTRACE
> +	backtrace_.resize(32);
> +
> +	int num_entries = backtrace(backtrace_.data(), backtrace_.size());
> +	if (num_entries < 0) {
> +		backtrace_.clear();
> +		return;
> +	}
> +
> +	backtrace_.resize(num_entries);
> +#endif
> +}
> +
> +/**
> + * \brief Convert a backtrace to a string representation
                   _s
> + * \param[in] skipLevels Number of initial levels to skip in the backtrace
> + *
> + * The string representation of the backtrace is a multi-line string, with one
> + * line per call stack entry. The format of the entries isn't specified and is
> + * platform-dependent.
> + *
> + * The \a skipLevels parameter indicates how many initial entries to skip from
> + * the backtrace. This can be used to hide functions that wrap the construction
> + * of the Backtrace instance from the call stack. The Backtrace constructor
> + * itself is automatically skipped and never shown in the backtrace.
> + *
> + * If backtrace generation fails for any reason (usually because the platform
> + * doesn't support this feature), an empty string is returned.
> + *
> + * \return A string representation of the backtrace, or an empty string if
> + * backtrace generation isn't possible

Just an alternative suggestion for when the backtrace is not possible, perhaps
we could return a string that let user know the backtrace could be be made.
Otherwise, all callers will have to check for empty string in order to give user
feedback.

> + */
> +std::string Backtrace::toString(unsigned int skipLevels) const
> +{
> +	/* Skip the first entry, corresponding to the Backtrace construction. */
> +	skipLevels += 1;
> +
> +	if (backtrace_.size() <= skipLevels)
> +		return std::string();
> +
> +#if HAVE_BACKTRACE
> +	Span<void *const> trace{ backtrace_ };
> +	trace = trace.subspan(skipLevels);
> +
> +	char **strings = backtrace_symbols(trace.data(), trace.size());
> +	if (strings) {
> +		std::ostringstream msg;
> +
> +		for (unsigned int i = 0; i < trace.size(); ++i)

There is no direct way of enumerating that one ? (direct in the sense no index
required)

> +			msg << strings[i] << std::endl;
> +
> +		free(strings);

If you only we had smart pointers like glib does (g_autofree) ;-P

> +		return msg.str();
> +	}
> +#endif
> +
> +	return std::string();
> +}
> +
> +} /* namespace libcamera */
> diff --git a/src/libcamera/base/log.cpp b/src/libcamera/base/log.cpp
> index a3e3f9ea2712..272c440589d4 100644
> --- a/src/libcamera/base/log.cpp
> +++ b/src/libcamera/base/log.cpp
> @@ -8,9 +8,6 @@
>  #include <libcamera/base/log.h>
>  
>  #include <array>
> -#if HAVE_BACKTRACE
> -#include <execinfo.h>
> -#endif
>  #include <fstream>
>  #include <iostream>
>  #include <list>
> @@ -23,6 +20,7 @@
>  
>  #include <libcamera/logging.h>
>  
> +#include <libcamera/base/backtrace.h>
>  #include <libcamera/base/thread.h>
>  #include <libcamera/base/utils.h>
>  
> @@ -418,31 +416,20 @@ void Logger::write(const LogMessage &msg)
>   */
>  void Logger::backtrace()
>  {
> -#if HAVE_BACKTRACE
>  	std::shared_ptr<LogOutput> output = std::atomic_load(&output_);
>  	if (!output)
>  		return;
>  
> -	void *buffer[32];
> -	int num_entries = ::backtrace(buffer, std::size(buffer));
> -	char **strings = backtrace_symbols(buffer, num_entries);
> -	if (!strings)
> -		return;
> -
> -	std::ostringstream msg;
> -	msg << "Backtrace:" << std::endl;
> -
>  	/*
>  	 * Skip the first two entries that correspond to this function and
>  	 * ~LogMessage().
>  	 */
> -	for (int i = 2; i < num_entries; ++i)
> -		msg << strings[i] << std::endl;
> +	std::string backtrace = Backtrace().toString(2);
> +	if (backtrace.empty())
> +		return;

As mention earlier, we have to do empty string check, and then this function
does not even let user know that no backtrace was available. This might be
confusing for leak tracers when the users explicitly ask for backtraces.

>  
> -	output->write(msg.str());
> -
> -	free(strings);
> -#endif
> +	output->write("Backtrace:\n");
> +	output->write(backtrace);
>  }
>  
>  /**
> diff --git a/src/libcamera/base/meson.build b/src/libcamera/base/meson.build
> index 8e125744d823..85af01a19365 100644
> --- a/src/libcamera/base/meson.build
> +++ b/src/libcamera/base/meson.build
> @@ -1,6 +1,7 @@
>  # SPDX-License-Identifier: CC0-1.0
>  
>  libcamera_base_sources = files([
> +    'backtrace.cpp',
>      'class.cpp',
>      'bound_method.cpp',
>      'event_dispatcher.cpp',
> @@ -18,6 +19,10 @@ libcamera_base_sources = files([
>      'utils.cpp',
>  ])
>  
> +if cc.has_header_symbol('execinfo.h', 'backtrace')
> +    config_h.set('HAVE_BACKTRACE', 1)
> +endif
> +
>  libcamera_base_deps = [
>      dependency('threads'),
>      libatomic,