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.

api/runnerpool/runner_pool.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"net/http"
 
+	"github.com/fnproject/fn/api/common"
 	"github.com/fnproject/fn/api/models"
 )
 
@@ -31,9 +32,23 @@ type PKIData struct {
 // MTLSRunnerFactory represents a factory method for constructing runners using mTLS
 type MTLSRunnerFactory func(addr, certCommonName string, pki *PKIData) (Runner, error)
 
+// RunnerStatus is general information on Runner health as returned by Runner::Status() call
+type RunnerStatus struct {
+	ActiveRequestCount int32           // Number of active running requests on Runner
+	StatusFailed       bool            // True if Status execution failed
+	StatusId           string          // Call ID for Status
+	Details            string          // General/Debug Log information
+	ErrorCode          int32           // If StatusFailed, then error code is set
+	ErrorStr           string          // Error details if StatusFailed and ErrorCode is set
+	CreatedAt          common.DateTime // Status creation date at Runner
+	StartedAt          common.DateTime // Status execution date at Runner
+	CompletedAt        common.DateTime // Status completion date at Runner
+}
+
 // Runner is the interface to invoke the execution of a function call on a specific runner
 type Runner interface {
 	TryExec(ctx context.Context, call RunnerCall) (bool, error)
+	Status(ctx context.Context) (*RunnerStatus, error)
 	Close(ctx context.Context) error
 	Address() string
 }