Stream stderr to logs for streaming fork mode

This mode is not expected to be used by any customers in
production, however we received a message from a customer who
had decided to use this mode.

They reported not being able to log to stderr, I was able to
reproduce the problem and have added bindLoggingPipe to
stderr as per the other modes in the of-watchdog.

After testing with a simple Python program, the logging is
now available.

We only recommend HTTP mode for production use.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

executor/logging.go

@@ -10,7 +10,6 @@ import (
 )
 
 // bindLoggingPipe spawns a goroutine for passing through logging of the given output pipe.
-//
 func bindLoggingPipe(name string, pipe io.Reader, output io.Writer, logPrefix bool, maxBufferSize int) {
 	log.Printf("Started logging: %s from function.", name)
 

executor/serializing_fork_runner.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
 
 	units "github.com/docker/go-units"
 
@@ -21,8 +22,9 @@ import (
 
 // SerializingForkFunctionRunner forks a process for each invocation
 type SerializingForkFunctionRunner struct {
-	ExecTimeout time.Duration
-	LogPrefix   bool
+	ExecTimeout   time.Duration
+	LogPrefix     bool
+	LogBufferSize int
 }
 
 // Run run a fork for each invocation
@@ -99,11 +101,14 @@ func serializeFunction(req FunctionRequest, f *SerializingForkFunctionRunner) (*
 
 	stdout, _ := cmd.StdoutPipe()
 	stdin, _ := cmd.StdinPipe()
+	stderr, _ := cmd.StderrPipe()
 
 	if err := cmd.Start(); err != nil {
 		return nil, err
 	}
 
+	bindLoggingPipe("stderr", stderr, os.Stderr, f.LogPrefix, f.LogBufferSize)
+
 	functionRes, errors := pipeToProcess(stdin, stdout, &data)
 	if len(errors) > 0 {
 		return nil, errors[0]

main.go

@@ -233,8 +233,9 @@ func createLockFile() (string, error) {
 
 func makeSerializingForkRequestHandler(watchdogConfig config.WatchdogConfig, logPrefix bool) func(http.ResponseWriter, *http.Request) {
 	functionInvoker := executor.SerializingForkFunctionRunner{
-		ExecTimeout: watchdogConfig.ExecTimeout,
-		LogPrefix:   logPrefix,
+		ExecTimeout:   watchdogConfig.ExecTimeout,
+		LogPrefix:     logPrefix,
+		LogBufferSize: watchdogConfig.LogBufferSize,
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {