feat(helm): Require Charts to be locally available (#370)

This modifies the `helmTemplate` native function in a way that it
expectes a relative path instead of the `repo/name` identifier for
Charts.

Before, this feature relied on the system wide Helm cache, which is
independent from the Tanka project and not controlled by git at all.

This does not play well with out aim for hermeticity and strong
reproducibility.

To overcome this, `helmTemplate` resolves the chart **relative to the
calling file**. This suggests to keep Helm Charts as close to the
consuming source code as possible.

pkg/helm/template.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"strings"
 
 	jsonnet "github.com/google/go-jsonnet"
@@ -18,9 +19,14 @@ import (
 
 // TemplateOpts defines additional parameters that can be passed to the
 // Helm.Template action
+// TODO: Isolate between Helm.Template and NativeFunc
 type TemplateOpts struct {
-	Values map[string]interface{}
-	Flags  []string
+	// general
+	Values map[string]interface{} `json:"values"`
+	Flags  []string               `json:"flags"`
+
+	// native func related
+	CalledFrom string `json:"calledFrom"`
 }
 
 func confToArgs(conf TemplateOpts) ([]string, []string, error) {
@@ -93,19 +99,23 @@ func (h ExecHelm) Template(name, chart string, opts TemplateOpts) (manifest.List
 }
 
 // NativeFunc returns a jsonnet native function that provides the same
-// functionality as `Helm.Template` of this package
+// functionality as `Helm.Template` of this package. Charts are required to be
+// present on the local filesystem, at a relative location to the file that
+// calls `helm.template()` / `std.native('helmTemplate')`. This guarantees
+// hermeticity
 func NativeFunc() *jsonnet.NativeFunction {
 	return &jsonnet.NativeFunction{
 		Name: "helmTemplate",
-		// Lines up with `helm template [NAME] [CHART] [flags]` except 'conf' is a bit more elaborate
+		// Similar to `helm template [NAME] [CHART] [flags]` except 'conf' is a
+		// bit more elaborate and chart is a local path
 		Params: ast.Identifiers{"name", "chart", "conf"},
 		Func: func(data []interface{}) (interface{}, error) {
 			name, ok := data[0].(string)
 			if !ok {
 				return nil, fmt.Errorf("First argument 'name' must be of 'string' type, got '%T' instead", data[0])
 			}
 
-			chart, ok := data[1].(string)
+			chartpath, ok := data[1].(string)
 			if !ok {
 				return nil, fmt.Errorf("Second argument 'chart' must be of 'string' type, got '%T' instead", data[1])
 			}
@@ -120,6 +130,20 @@ func NativeFunc() *jsonnet.NativeFunction {
 				return "", err
 			}
 
+			// Charts are only allowed at relative paths. Use conf.CalledFrom to find the callers directory
+			if conf.CalledFrom == "" {
+				// TODO: rephrase and move lengthy explanation to website
+				return nil, fmt.Errorf("helmTemplate: 'conf.calledFrom' is unset or empty.\nHowever, Tanka must know where helmTemplate was called from, to resolve the Helm Chart relative to that.\n")
+			}
+			callerDir := filepath.Dir(conf.CalledFrom)
+
+			// resolve the Chart relative to the caller
+			chart := filepath.Join(callerDir, chartpath)
+			if _, err := os.Stat(chart); err != nil {
+				// TODO: add website link for explanation
+				return nil, fmt.Errorf("helmTemplate: Failed to find a Chart at '%s': %s", chart, err)
+			}
+
 			// TODO: Define Template on the Helm interface instead
 			var h ExecHelm
 			list, err := h.Template(name, chart, conf)