Detect imported extvars and TLAs via sentinel path

When a Jsonnet consults an Importer for a path nominated by an
external variable or top-level argument file-based command-line flag,
the "importedFrom" parameter passed to the Importer.Import method is
not a valid URL, but is instead a sentinel placeholder value (namely,
an empty string). Detect these sentinel values and use the current
working directory as the base URL instead when resolving relative
import paths.

cmd/root.go

@@ -252,7 +252,7 @@ func JsonnetVM(cmd *cobra.Command) (*jsonnet.VM, error) {
 		return nil, fmt.Errorf("Unable to determine current working directory: %v", err)
 	}
 
-	vm.Importer(utils.MakeUniversalImporter(searchUrls))
+	vm.Importer(utils.MakeUniversalImporter(searchUrls, dirURL(cwd)))
 
 	for _, spec := range []struct {
 		flagName string

utils/importer.go

@@ -8,7 +8,6 @@ import (
 	"net/http"
 	"net/url"
 	"os"
-	"regexp"
 	"strings"
 	"time"
 
@@ -19,8 +18,6 @@ import (
 
 var errNotFound = errors.New("Not found")
 
-var extVarKindRE = regexp.MustCompile("^<(?:extvar|top-level-arg):.+>$")
-
 //go:generate go-bindata -nometadata -ignore .*_test\.|~$DOLLAR -pkg $GOPACKAGE -o bindata.go -prefix ../ ../lib/...
 func newInternalFS(prefix string) http.FileSystem {
 	// Asset/AssetDir returns `fmt.Errorf("Asset %s not found")`,
@@ -62,7 +59,7 @@ A real-world example:
     will be resolved as https://raw.githubusercontent.com/ksonnet/ksonnet-lib/master/ksonnet.beta.2/k8s.libsonnet
 	and downloaded from that location.
 */
-func MakeUniversalImporter(searchURLs []*url.URL) jsonnet.Importer {
+func MakeUniversalImporter(searchURLs []*url.URL, extVarBaseURL *url.URL) jsonnet.Importer {
 	// Reconstructed copy of http.DefaultTransport (to avoid
 	// modifying the default)
 	t := &http.Transport{
@@ -83,13 +80,15 @@ func MakeUniversalImporter(searchURLs []*url.URL) jsonnet.Importer {
 
 	return &universalImporter{
 		BaseSearchURLs: searchURLs,
+		extVarBaseURL:  extVarBaseURL,
 		HTTPClient:     &http.Client{Transport: t},
 		cache:          map[string]jsonnet.Contents{},
 	}
 }
 
 type universalImporter struct {
 	BaseSearchURLs []*url.URL
+	extVarBaseURL  *url.URL
 	HTTPClient     *http.Client
 	cache          map[string]jsonnet.Contents
 }
@@ -154,9 +153,17 @@ func (importer *universalImporter) expandImportToCandidateURLs(importedFrom, imp
 		return []*url.URL{importedPathURL}, nil
 	}
 
-	importDirURL, err := url.Parse(importedFrom)
-	if err != nil {
-		return nil, fmt.Errorf("Invalid import dir %q: %v", importedFrom, err)
+	var importDirURL *url.URL
+	// Is the import coming from a source nominated by the "--ext-code-file," "--ext-str-file,"
+	// "--tla-code-file," or "--tla-str-file flags"? If so, resolve relative import paths against
+	// the specially configured base directory.
+	if importedFrom == "" {
+		importDirURL = importer.extVarBaseURL
+	} else {
+		importDirURL, err = url.Parse(importedFrom)
+		if err != nil {
+			return nil, fmt.Errorf("Invalid import dir %q", importedFrom)
+		}
 	}
 
 	candidateURLs := make([]*url.URL, 1, len(importer.BaseSearchURLs)+1)

utils/importer_test.go

@@ -31,6 +31,10 @@ func TestExpandImportToCandidateURLs(t *testing.T) {
 		BaseSearchURLs: []*url.URL{
 			{Scheme: "file", Path: "/first/base/search/"},
 		},
+		extVarBaseURL: &url.URL{
+			Scheme: "file",
+			Path:   "/current/working/dir/",
+		},
 	}
 
 	t.Run("Absolute URL in import statement yields a single candidate", func(t *testing.T) {
@@ -53,4 +57,15 @@ func TestExpandImportToCandidateURLs(t *testing.T) {
 			t.Errorf("Expected %v, got %v", expected, urls)
 		}
 	})
+
+	t.Run("Relative URL in import statement used as external variable or TLA yields candidate relative to base URL", func(t *testing.T) {
+		urls, _ := importer.expandImportToCandidateURLs("", "../sought.jsonnet")
+		expected := []*url.URL{
+			{Scheme: "file", Host: "", Path: "/current/working/sought.jsonnet"},
+			{Scheme: "file", Host: "", Path: "/first/base/sought.jsonnet"},
+		}
+		if !reflect.DeepEqual(urls, expected) {
+			t.Errorf("Expected %v, got %v", expected, urls)
+		}
+	})
 }