fn: introduce agent config and minor ghostreader tweak (#797)

* fn: introduce agent config and minor ghostreader tweak

TODO: move all constants/tweaks in agent to agent config.

* fn: json convention

api/agent/config.go

@@ -0,0 +1,60 @@
+package agent
+
+import (
+	"errors"
+	"math"
+	"os"
+	"strconv"
+	"time"
+)
+
+type AgentConfig struct {
+	MinDockerVersion string        `json:"min_docker_version"`
+	FreezeIdleMsecs  time.Duration `json:"freeze_idle_msecs"`
+	EjectIdleMsecs   time.Duration `json:"eject_idle_msecs"`
+}
+
+func NewAgentConfig() (*AgentConfig, error) {
+
+	var err error
+
+	cfg := &AgentConfig{
+		MinDockerVersion: "17.06.0-ce",
+	}
+
+	cfg.FreezeIdleMsecs, err = getEnvMsecs("FN_FREEZE_IDLE_MSECS", 50*time.Millisecond)
+	if err != nil {
+		return cfg, errors.New("error initializing freeze idle delay")
+	}
+
+	cfg.EjectIdleMsecs, err = getEnvMsecs("FN_EJECT_IDLE_MSECS", 1000*time.Millisecond)
+	if err != nil {
+		return cfg, errors.New("error initializing eject idle delay")
+	}
+
+	if cfg.EjectIdleMsecs == time.Duration(0) {
+		return cfg, errors.New("error eject idle delay cannot be zero")
+	}
+
+	return cfg, nil
+}
+
+func getEnvMsecs(name string, defaultVal time.Duration) (time.Duration, error) {
+
+	delay := defaultVal
+
+	if dur := os.Getenv(name); dur != "" {
+		durInt, err := strconv.ParseInt(dur, 10, 64)
+		if err != nil {
+			return defaultVal, err
+		}
+		// disable if negative or set to msecs specified.
+		if durInt < 0 || time.Duration(durInt) >= math.MaxInt64/time.Millisecond {
+			delay = math.MaxInt64
+		} else {
+			delay = time.Duration(durInt) * time.Millisecond
+		}
+	}
+
+	return delay, nil
+}