Allow outputs to keep file/program open

Add the ability to keep file/program outputs open (i.e. writing to the
same open file/program for multiple notifications). A new option to the
file/program output "keep_alive", if true, keeps the file/program pipe
open across events.

This makes the need for unbuffered output aka
https://github.com/draios/falco/issues/211 more pressing. Will add that next.

falco.yaml

@@ -53,8 +53,13 @@ outputs:
 syslog_output:
   enabled: true
 
+# If keep_alive is set to true, the file will be opened once and
+# continuously written to, with each output message on its own
+# line. If keep_alive is set to false, the file will be re-opened
+# for each output message.
 file_output:
   enabled: false
+  keep_alive: false
   filename: ./events.txt
 
 stdout_output:
@@ -65,6 +70,8 @@ stdout_output:
 #         program: "jq '{text: .output}' | curl -d @- -X POST https://hooks.slack.com/services/XXX"
 #   - logging (alternate method than syslog):
 #         program: logger -t falco-test
+#   - send over a network connection:
+#         program: nc host.example.com 80
 
 # If keep_alive is set to true, the program will be started once and
 # continuously written to, with each output message on its own

userspace/falco/configuration.cpp

@@ -71,13 +71,17 @@ void falco_configuration::init(string conf_filename, list<string> &cmdline_optio
 	file_output.name = "file";
 	if (m_config->get_scalar<bool>("file_output", "enabled", false))
 	{
-		string filename;
+		string filename, keep_alive;
 		filename = m_config->get_scalar<string>("file_output", "filename", "");
 		if (filename == string(""))
 		{
 			throw invalid_argument("Error reading config file (" + m_config_file + "): file output enabled but no filename in configuration block");
 		}
 		file_output.options["filename"] = filename;
+
+		keep_alive = m_config->get_scalar<string>("file_output", "keep_alive", "");
+		file_output.options["keep_alive"] = keep_alive;
+
 		m_outputs.push_back(file_output);
 	}
 
@@ -99,13 +103,17 @@ void falco_configuration::init(string conf_filename, list<string> &cmdline_optio
 	program_output.name = "program";
 	if (m_config->get_scalar<bool>("program_output", "enabled", false))
 	{
-		string program;
+		string program, keep_alive;
 		program = m_config->get_scalar<string>("program_output", "program", "");
 		if (program == string(""))
 		{
 			throw sinsp_exception("Error reading config file (" + m_config_file + "): program output enabled but no program in configuration block");
 		}
 		program_output.options["program"] = program;
+
+		keep_alive = m_config->get_scalar<string>("program_output", "keep_alive", "");
+		program_output.options["keep_alive"] = keep_alive;
+
 		m_outputs.push_back(program_output);
 	}
 

userspace/falco/lua/output.lua

@@ -29,7 +29,7 @@ function mod.file_validate(options)
       error("File output needs to be configured with a valid filename")
    end
 
-   file, err = io.open(options.filename, "a+")
+   local file, err = io.open(options.filename, "a+")
    if file == nil then
       error("Error with file output: "..err)
    end
@@ -38,9 +38,21 @@ function mod.file_validate(options)
 end
 
 function mod.file(priority, priority_num, msg, options)
-   file = io.open(options.filename, "a+")
+   if options.keep_alive == "true" then
+      if file == nil then
+	 file = io.open(options.filename, "a+")
+      end
+   else
+      file = io.open(options.filename, "a+")
+   end
+
    file:write(msg, "\n")
-   file:close()
+
+   if options.keep_alive == nil or
+      options.keep_alive ~= "true" then
+	 file:close()
+	 file = nil
+   end
 end
 
 function mod.syslog(priority, priority_num, msg, options)
@@ -52,10 +64,22 @@ function mod.program(priority, priority_num, msg, options)
    -- successfully. However, the luajit we're using returns true even
    -- when the shell can't run the program.
 
-   file = io.popen(options.program, "w")
+   -- Note: options are all strings
+   if options.keep_alive == "true" then
+      if file == nil then
+	 file = io.popen(options.program, "w")
+      end
+   else
+      file = io.popen(options.program, "w")
+   end
 
    file:write(msg, "\n")
-   file:close()
+
+   if options.keep_alive == nil or
+      options.keep_alive ~= "true" then
+	 file:close()
+	 file = nil
+   end
 end
 
 function output_event(event, rule, priority, priority_num, format)