libcamera: base: log: Add LIBCAMERA_LOG_COLOR env var

Replace the `LIBCAMERA_NO_LOG_COLOR` env variable with another environment
variable that recognizes the "auto", "yes", "no" values. When set to "auto",
the messages are only colored if the standard error is a tty (as determined
by `isatty()`).

"auto" is the default value. This ensures that the ansi escape codes won't
litter the output if the stderr is redirected to a file, `tee`, etc.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/base/log.cpp

@@ -19,6 +19,7 @@
 #include <string_view>
 #include <syslog.h>
 #include <time.h>
+#include <unistd.h>
 #include <unordered_set>
 
 #include <libcamera/logging.h>
@@ -582,14 +583,21 @@ void Logger::logSetLevel(const char *category, const char *level)
 /**
  * \brief Construct a logger
  *
- * If the environment variable is not set, log to std::cerr. The log messages
- * are then colored by default. This can be overridden by setting the
- * LIBCAMERA_LOG_NO_COLOR environment variable to disable coloring.
+ * If the environment variable is not set, log to std::cerr. The coloring
+ * of log messages can be controlled using the LIBCAMERA_LOG_COLOR environment
+ * variable.
  */
 Logger::Logger()
 {
-	bool color = !utils::secure_getenv("LIBCAMERA_LOG_NO_COLOR");
-	logSetStream(&std::cerr, color);
+	const char *color = utils::secure_getenv("LIBCAMERA_LOG_COLOR");
+	bool wantColor = false;
+
+	if (!color || strcmp(color, "auto") == 0)
+		wantColor = isatty(STDERR_FILENO) == 1;
+	else if (strcmp(color, "yes") == 0)
+		wantColor = true;
+
+	logSetStream(&std::cerr, wantColor);
 
 	parseLogFile();
 	parseLogLevels();