builder/docker: validate export path is not a dir [GH-2105]

CHANGELOG.md

@@ -30,6 +30,7 @@ BUG FIXES:
   * builder/docker: Use `docker exec` for newer versions of Docker for
       running scripts [GH-1993]
   * builder/docker: Fix crash that could occur at certain timed ctrl-c [GH-1838]
+  * builder/docker: validate that `export_path` is not a directory [GH-2105]
   * builder/qemu: Add `disk_discard` option [GH-2120]
   * builder/virtualbox: Added SCSI support
   * builder/vmware: Case-insensitive match of MAC address to find IP [GH-1989]

builder/docker/config.go

@@ -2,6 +2,7 @@ package docker
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/mitchellh/mapstructure"
 	"github.com/mitchellh/packer/common"
@@ -79,6 +80,13 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 			fmt.Errorf("both commit and export_path cannot be set"))
 	}
 
+	if c.ExportPath != "" {
+		if fi, err := os.Stat(c.ExportPath); err == nil && fi.IsDir() {
+			errs = packer.MultiErrorAppend(errs, fmt.Errorf(
+				"export_path must be a file, not a directory"))
+		}
+	}
+
 	if errs != nil && len(errs.Errors) > 0 {
 		return nil, nil, errs
 	}

builder/docker/config_test.go

@@ -1,6 +1,8 @@
 package docker
 
 import (
+	"io/ioutil"
+	"os"
 	"testing"
 )
 
@@ -42,6 +44,12 @@ func testConfigOk(t *testing.T, warns []string, err error) {
 }
 
 func TestConfigPrepare_exportPath(t *testing.T) {
+	td, err := ioutil.TempDir("", "packer")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	defer os.RemoveAll(td)
+
 	raw := testConfig()
 
 	// No export path
@@ -53,6 +61,11 @@ func TestConfigPrepare_exportPath(t *testing.T) {
 	raw["export_path"] = "good"
 	_, warns, errs = NewConfig(raw)
 	testConfigOk(t, warns, errs)
+
+	// Bad export path (directory)
+	raw["export_path"] = td
+	_, warns, errs = NewConfig(raw)
+	testConfigErr(t, warns, errs)
 }
 
 func TestConfigPrepare_exportPathAndCommit(t *testing.T) {