[wanda] Add contextOwner type and parser (#479)

Add the contextOwner struct and parseContextOwner function for parsing
"uid:gid" strings. This will be used to allow wanda specs to override
file ownership in the build context tar.

Topic: add-context-owner

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: andrew <andrew@anyscale.com>

Signed-off-by: andrew <andrew@anyscale.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andrew-anyscale

wanda/tar_meta.go

@@ -1,7 +1,10 @@
 package wanda
 
 import (
+	"fmt"
 	"os"
+	"strconv"
+	"strings"
 )
 
 type tarMeta struct {
@@ -18,3 +21,43 @@ func tarMetaFromFileInfo(info os.FileInfo) *tarMeta {
 		GroupID: rootUser,
 	}
 }
+
+// contextOwner holds a uid:gid pair parsed from a wanda spec.
+type contextOwner struct {
+	UserID  int
+	GroupID int
+}
+
+// parseContextOwner parses a "uid:gid" string.
+func parseContextOwner(s string) (*contextOwner, error) {
+	parts := strings.SplitN(s, ":", 2)
+	if len(parts) != 2 {
+		return nil, fmt.Errorf(
+			"context_owner %q: expected uid:gid format",
+			s,
+		)
+	}
+	uid, err := strconv.Atoi(parts[0])
+	if err != nil {
+		return nil, fmt.Errorf(
+			"context_owner uid %q: %w", parts[0], err,
+		)
+	}
+	if uid < 0 {
+		return nil, fmt.Errorf(
+			"context_owner uid %q: must be non-negative", parts[0],
+		)
+	}
+	gid, err := strconv.Atoi(parts[1])
+	if err != nil {
+		return nil, fmt.Errorf(
+			"context_owner gid %q: %w", parts[1], err,
+		)
+	}
+	if gid < 0 {
+		return nil, fmt.Errorf(
+			"context_owner gid %q: must be non-negative", parts[1],
+		)
+	}
+	return &contextOwner{UserID: uid, GroupID: gid}, nil
+}

wanda/tar_meta_test.go

@@ -8,6 +8,34 @@ import (
 	"path/filepath"
 )
 
+func TestParseContextOwner(t *testing.T) {
+	owner, err := parseContextOwner("2000:100")
+	if err != nil {
+		t.Fatalf("parseContextOwner: %v", err)
+	}
+	if owner.UserID != 2000 {
+		t.Errorf("got uid %d, want %d", owner.UserID, 2000)
+	}
+	if owner.GroupID != 100 {
+		t.Errorf("got gid %d, want %d", owner.GroupID, 100)
+	}
+}
+
+func TestParseContextOwnerInvalid(t *testing.T) {
+	for _, input := range []string{
+		"", "2000", "abc:100", "2000:xyz",
+		"-1:100", "100:-1", "100:200:300", " 100:200",
+	} {
+		_, err := parseContextOwner(input)
+		if err == nil {
+			t.Errorf(
+				"parseContextOwner(%q) = nil error, want error",
+				input,
+			)
+		}
+	}
+}
+
 func TestTarMetaFromFileInfo(t *testing.T) {
 	tmp := t.TempDir()