Force a write of all user-space buffered data for stdout in Formatter…

…::print (#1012)

* Force a write of all user-space buffered data for stdout in Formatter::print

When application output is connected to pipe a fully-buffered output is used by
default. In this case the logs of application will not be visible for user
until there are enough logs to fill the buffer.

An example where such behaviour can be observed is launching binary ROS executable
from within Android APK. There is not way to connect stdout to TTY and only
pipe can be used. As a result the user cannot see the application output.

The proposed solution is to force flushing of stdout on every line in ROS logger
when a special environment variable is set.

* minor style change

tools/rosconsole/src/rosconsole/rosconsole.cpp

@@ -107,6 +107,9 @@ std::string g_last_error_message = "Unknown Error";
 #endif
 const char* g_format_string = "[${severity}] [${time}]: ${message}";
 
+bool g_force_stdout_line_buffered = false;
+bool g_stdout_flush_failure_reported = false;
+
 typedef std::map<std::string, std::string> M_string;
 M_string g_extra_fixed_tokens;
 
@@ -393,6 +396,16 @@ void Formatter::print(void* logger_handle, ::ros::console::Level level, const ch
   ss << COLOR_NORMAL;
 
   fprintf(f, "%s\n", ss.str().c_str());
+
+  if (g_force_stdout_line_buffered && f == stdout)
+  {
+    int flush_result = fflush(f);
+    if (flush_result != 0 && !g_stdout_flush_failure_reported)
+    {
+      g_stdout_flush_failure_reported = true;
+      fprintf(stderr, "Error: failed to perform fflush on stdout, fflush return code is %d\n", flush_result);
+    }
+  }
 }
 
 Formatter g_formatter;
@@ -425,6 +438,20 @@ void initialize()
     backend::function_notifyLoggerLevelsChanged = notifyLoggerLevelsChanged;
     backend::function_print = _print;
 
+    std::string line_buffered;
+    if (get_environment_variable(line_buffered, "ROSCONSOLE_STDOUT_LINE_BUFFERED"))
+    {
+      if (line_buffered == "1")
+      {
+        g_force_stdout_line_buffered = true;
+      }
+      else if (line_buffered != "0")
+      {
+        fprintf(stderr, "Warning: unexpected value %s specified for ROSCONSOLE_STDOUT_LINE_BUFFERED. Default value 0 "
+          "will be used. Valid values are 1 or 0.\n", line_buffered.c_str());
+      }
+    }
+
     ::ros::console::impl::initialize();
     g_initialized = true;
   }