fn: runner status and docker load images (#1116)

* fn: runner status and docker load images

Introducing a function run for pure runner Status
calls. Previously, Status gRPC calls returned active
inflight request counts with the purpose of a simple
health checker. However this is not sufficient since
it does not show if agent or docker is healthy. With
this change, if pure runner is configured with a status
image, that image is executed through docker. The
call uses zero memory/cpu/tmpsize settings to ensure
resource tracker does not block it.

However, operators might not always have a docker
repository accessible/available for status image. Or
operators might not want the status to go over the
network. To allow such cases, and in general possibly
caching docker images, added a new environment variable
FN_DOCKER_LOAD_FILE. If this is set, fn-agent during
startup will load these images that were previously
saved with 'docker save' into docker.

images/fn-status-checker/.gitignore

@@ -0,0 +1 @@
+vendor

images/fn-status-checker/Dockerfile

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+# build stage
+FROM golang:1.10-alpine AS build-env
+RUN apk --no-cache add git
+ENV D=/go/src/github.com/fnproject/fn/images/fn-status-checker
+RUN go get -u github.com/golang/dep/cmd/dep
+ADD Gopkg.* $D/
+RUN cd $D && dep ensure --vendor-only
+ADD . $D
+RUN cd $D && go build -ldflags="-s -w" -o fn-status-checker && cp fn-status-checker /tmp/
+
+# final stage
+FROM alpine
+WORKDIR /function
+COPY --from=build-env /tmp/fn-status-checker /function
+ENTRYPOINT ["./fn-status-checker"]

images/fn-status-checker/Gopkg.lock

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+# This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
+
+
+[[projects]]
+  branch = "master"
+  name = "github.com/fnproject/fdk-go"
+  packages = [
+    ".",
+    "utils"
+  ]
+  revision = "5d768b2006f11737b6a69a758ddd6d2fac04923e"
+
+[solve-meta]
+  analyzer-name = "dep"
+  analyzer-version = 1
+  inputs-digest = "c55f0d3da5ec2e9e5c9a7c563702e4cf28513fa1aaea1c18664ca2cb7d726f89"
+  solver-name = "gps-cdcl"
+  solver-version = 1

images/fn-status-checker/Gopkg.toml

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+[[constraint]]
+  branch = "master"
+  name = "github.com/fnproject/fdk-go"

images/fn-status-checker/build.sh

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+set -e
+docker build --build-arg HTTPS_PROXY --build-arg HTTP_PROXY -t fnproject/fn-status-checker:latest .

images/fn-status-checker/release.sh

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+set -e
+docker push fnproject/fn-status-checker:latest

images/fn-status-checker/status.go

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+package main
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"io"
+	"io/ioutil"
+	"log"
+
+	fdk "github.com/fnproject/fdk-go"
+)
+
+func main() {
+	fdk.Handle(fdk.HandlerFunc(myHandler))
+}
+
+func myHandler(ctx context.Context, in io.Reader, out io.Writer) {
+	var input map[string]interface{}
+
+	body, err := ioutil.ReadAll(in)
+	if err != nil {
+		log.Print("could not read input")
+		fdk.WriteStatus(out, 530)
+		return
+	}
+
+	err = json.Unmarshal(body, &input)
+	if err != nil {
+		log.Print("could not unmarshal json input")
+		fdk.WriteStatus(out, 531)
+		return
+	}
+
+	output, err := json.Marshal(&input)
+	if err != nil {
+		log.Print("could not marshal json output")
+		fdk.WriteStatus(out, 532)
+		return
+	}
+
+	written, err := io.Copy(out, bytes.NewReader(output))
+	if err != nil {
+		log.Print("could not write output")
+		fdk.WriteStatus(out, 533)
+		return
+	}
+
+	if written != int64(len(output)) {
+		log.Print("partial write of output")
+		fdk.WriteStatus(out, 534)
+		return
+	}
+}