[libcamera-devel] [PATCH] libcamera: Print backtrace on fatal errors

[Laurent Pinchart]

When a fatal error occurs the program aborts, and all the logger
provides is the location of the line that caused the error. Extend this
with a full backtrace to help debugging.

The backtrace is generated using the backtrace() call, a GNU extension
to the C library. It is available in glibc and uClibc but not in musl.
Test for availability of the function to condition compilation of the
backtrace printing. Implementing backtrace support with musl is an
exercise left to the reader if desired.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 meson.build           |  4 ++
 src/libcamera/log.cpp | 93 ++++++++++++++++++++++++++++++++++---------
 2 files changed, 79 insertions(+), 18 deletions(-)

diff --git a/meson.build b/meson.build
index 0a222ba96dcb..634488589a46 100644
--- a/meson.build
+++ b/meson.build
@@ -26,6 +26,10 @@ libcamera_version = libcamera_git_version.split('+')[0]
 cc = meson.get_compiler('c')
 config_h = configuration_data()
 
+if cc.has_header_symbol('execinfo.h', 'backtrace')
+    config_h.set('HAVE_BACKTRACE', 1)
+endif
+
 if cc.has_header_symbol('stdlib.h', 'secure_getenv', prefix : '#define _GNU_SOURCE')
     config_h.set('HAVE_SECURE_GETENV', 1)
 endif
diff --git a/src/libcamera/log.cpp b/src/libcamera/log.cpp
index 50f345b98c74..f4eb8c11adc3 100644
--- a/src/libcamera/log.cpp
+++ b/src/libcamera/log.cpp
@@ -7,6 +7,9 @@
 
 #include "log.h"
 
+#if HAVE_BACKTRACE
+#include <execinfo.h>
+#endif
 #include <fstream>
 #include <iostream>
 #include <list>
@@ -108,10 +111,11 @@ public:
 
 	bool isValid() const;
 	void write(const LogMessage &msg);
+	void write(const std::string &msg);
 
 private:
-	void writeSyslog(const LogMessage &msg);
-	void writeStream(const LogMessage &msg);
+	void writeSyslog(LogSeverity severity, const std::string &msg);
+	void writeStream(const std::string &msg);
 
 	std::ostream *stream_;
 	LoggingTarget target_;
@@ -180,34 +184,55 @@ bool LogOutput::isValid() const
  * \param[in] msg Message to write
  */
 void LogOutput::write(const LogMessage &msg)
+{
+	std::string str;
+
+	switch (target_) {
+	case LoggingTargetSyslog:
+		str = std::string(log_severity_name(msg.severity())) + " "
+		    + msg.category().name() + " " + msg.fileInfo() + " "
+		    + msg.msg();
+		writeSyslog(msg.severity(), str);
+		break;
+	case LoggingTargetStream:
+	case LoggingTargetFile:
+		str = "[" + utils::time_point_to_string(msg.timestamp()) + "]"
+		    + log_severity_name(msg.severity()) + " "
+		    + msg.category().name() + " " + msg.fileInfo() + " "
+		    + msg.msg();
+		writeStream(str);
+		break;
+	default:
+		break;
+	}
+}
+
+/**
+ * \brief Write string to log output
+ * \param[in] str String to write
+ */
+void LogOutput::write(const std::string &str)
 {
 	switch (target_) {
 	case LoggingTargetSyslog:
-		writeSyslog(msg);
+		writeSyslog(LogDebug, str);
 		break;
 	case LoggingTargetStream:
 	case LoggingTargetFile:
-		writeStream(msg);
+		writeStream(str);
 		break;
 	default:
 		break;
 	}
 }
 
-void LogOutput::writeSyslog(const LogMessage &msg)
+void LogOutput::writeSyslog(LogSeverity severity, const std::string &str)
 {
-	std::string str = std::string(log_severity_name(msg.severity())) + " " +
-	      		  msg.category().name() + " " + msg.fileInfo() + " " +
-			  msg.msg();
-	syslog(log_severity_to_syslog(msg.severity()), "%s", str.c_str());
+	syslog(log_severity_to_syslog(severity), "%s", str.c_str());
 }
 
-void LogOutput::writeStream(const LogMessage &msg)
+void LogOutput::writeStream(const std::string &str)
 {
-	std::string str = "[" + utils::time_point_to_string(msg.timestamp()) +
-			  "]" + log_severity_name(msg.severity()) + " " +
-			  msg.category().name() + " " + msg.fileInfo() + " " +
-			  msg.msg();
 	stream_->write(str.c_str(), str.size());
 	stream_->flush();
 }
@@ -223,6 +248,7 @@ public:
 	static Logger *instance();
 
 	void write(const LogMessage &msg);
+	void backtrace();
 
 	int logSetFile(const char *path);
 	int logSetStream(std::ostream *stream);
@@ -240,9 +266,6 @@ private:
 	void registerCategory(LogCategory *category);
 	void unregisterCategory(LogCategory *category);
 
-	void writeSyslog(const LogMessage &msg);
-	void writeStream(const LogMessage &msg);
-
 	std::unordered_set<LogCategory *> categories_;
 	std::list<std::pair<std::string, LogSeverity>> levels_;
 
@@ -370,6 +393,38 @@ void Logger::write(const LogMessage &msg)
 	output->write(msg);
 }
 
+/**
+ * \brief Write a backtrace to the log
+ */
+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, ARRAY_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 method and
+	 * ~LogMessage().
+	 */
+	for (int i = 2; i < num_entries; ++i)
+		msg << strings[i] << std::endl;
+
+	output->write(msg.str());
+
+	free(strings);
+#endif
+}
+
 /**
  * \brief Set the log file
  * \param[in] path Full path to the log file
@@ -783,8 +838,10 @@ LogMessage::~LogMessage()
 	if (severity_ >= category_.severity())
 		Logger::instance()->write(*this);
 
-	if (severity_ == LogSeverity::LogFatal)
+	if (severity_ == LogSeverity::LogFatal) {
+		Logger::instance()->backtrace();
 		std::abort();
+	}
 }
 
 /**
-- 
Regards,

Laurent Pinchart