feat(kubernetes): kubectl context

Adds support for automatically choosing the right kubectl context, based on data
from spec.json

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/spf13/cobra v0.0.5
 	github.com/spf13/viper v1.4.0
 	github.com/stretchr/objx v0.2.0
+	github.com/thoas/go-funk v0.4.0
 	golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2
 	golang.org/x/sys v0.0.0-20190616124812-15dcb6c0061f // indirect
 	golang.org/x/text v0.3.2 // indirect

go.sum

@@ -143,6 +143,8 @@ github.com/stretchr/objx v0.2.0/go.mod h1:qt09Ya8vawLte6SNmTgCsAVtYtaKzEcn8ATUoH
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
+github.com/thoas/go-funk v0.4.0 h1:KBaa5NL7NMtsFlQaD8nQMbDt1wuM+OOaNQyYNYQFhVo=
+github.com/thoas/go-funk v0.4.0/go.mod h1:mlR+dHGb+4YgXkf13rkQTuzrneeHANxOm6+ZnEV9HsA=
 github.com/tmc/grpc-websocket-proxy v0.0.0-20190109142713-0ad062ec5ee5/go.mod h1:ncp9v5uamzpCO7NfCPTXjqaC+bZgJeR0sMTm6dMHP7U=
 github.com/ugorji/go v1.1.4/go.mod h1:uQMGLiO92mf5W77hV/PUCpI3pbzQx3CRekS0kk+RGrc=
 github.com/ugorji/go/codec v0.0.0-20181204163529-d75b2dcb6bc8/go.mod h1:VFNgLljTbGfSG7qAOspJ7OScBnGdDN/yBr0sguwnwf0=

pkg/provider/kubernetes/kubectl.go

@@ -4,16 +4,74 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"net"
+	"net/url"
 	"os"
 	"os/exec"
+
+	"github.com/stretchr/objx"
+	funk "github.com/thoas/go-funk"
 )
 
 // Kubectl uses the `kubectl` command to operate on a Kubernetes cluster
-type Kubectl struct{}
+type Kubectl struct {
+	context string
+}
+
+func (k Kubectl) setupContext(apiServer, namespace string) error {
+	cmd := exec.Command("kubectl", "config", "view", "-o", "json")
+	cfgJSON := bytes.Buffer{}
+	cmd.Stdout = &cfgJSON
+	if err := cmd.Run(); err != nil {
+		return err
+	}
+	var cfg map[string]interface{}
+	if err := json.Unmarshal(cfgJSON.Bytes(), &cfg); err != nil {
+		return err
+	}
+
+	var err error
+	k.context, err = contextFromKubeconfig(cfg, apiServer, namespace)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func contextFromKubeconfig(kubeconfig map[string]interface{}, apiServer, namespace string) (string, error) {
+	cfg := objx.New(kubeconfig)
+
+	// find the correct cluster
+	cluster := objx.New(funk.Find(cfg.Get("clusters").MustMSISlice(), func(x map[string]interface{}) bool {
+		c := objx.New(x)
+		u, err := url.Parse(c.Get("cluster.server").MustStr())
+		if err != nil {
+			panic(err)
+		}
+		host, _, err := net.SplitHostPort(u.Host)
+		if err != nil {
+			panic(err)
+		}
+		return host == apiServer
+	}))
+
+	// find a context that uses the cluster
+	context := objx.New(funk.Find(cfg.Get("contexts").MustMSISlice(), func(x map[string]interface{}) bool {
+		c := objx.New(x)
+		return c.Get("context.cluster").MustStr() == cluster.Get("name").MustStr()
+	}))
+
+	return context.Get("name").MustStr(), nil
+}
 
 // Get retrieves an Kubernetes object from the API
 func (k Kubectl) Get(namespace, kind, name string) (map[string]interface{}, error) {
-	argv := []string{"get", "-o", "json", "-n", namespace, kind, name}
+	argv := []string{"get",
+		"-o", "json",
+		"-n", namespace,
+		"--context", k.context,
+		kind, name,
+	}
 	cmd := exec.Command("kubectl", argv...)
 	raw := bytes.Buffer{}
 	cmd.Stdout = &raw
@@ -31,7 +89,10 @@ func (k Kubectl) Get(namespace, kind, name string) (map[string]interface{}, erro
 // Diff takes a desired state as yaml and returns the differences
 // to the system in common diff format
 func (k Kubectl) Diff(yaml string) (string, error) {
-	argv := []string{"diff", "-f", "-"}
+	argv := []string{"diff",
+		"--context", k.context,
+		"-f", "-",
+	}
 	cmd := exec.Command("kubectl", argv...)
 	raw := bytes.Buffer{}
 	cmd.Stdout = &raw

pkg/provider/kubernetes/kubernetes.go

@@ -18,6 +18,7 @@ var client = Kubectl{}
 
 // Init makes the provider ready to be used
 func (k *Kubernetes) Init() error {
+	return client.setupContext(k.APIServer, k.Namespace)
 }
 
 // Reconcile receives the raw evaluated jsonnet as a marshaled json dict and