Default to PuluginFolders is KnownPluginFolders is unset

* PACKER_PLUGIN_PATH takes precedence over all

packer/plugin.go

@@ -6,7 +6,6 @@ package packer
 import (
 	"crypto/sha256"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -81,7 +80,7 @@ func (c *PluginConfig) Discover() error {
 	}
 
 	if len(c.KnownPluginFolders) == 0 {
-		return errors.New("no known plugin folders defined")
+		c.KnownPluginFolders = PluginFolders()
 	}
 
 	// TODO after JSON is deprecated remove support for legacy component plugins.

packer/plugin_discover_test.go

@@ -24,7 +24,6 @@ func newPluginConfig() PluginConfig {
 	var conf PluginConfig
 	conf.PluginMinPort = 10000
 	conf.PluginMaxPort = 25000
-	conf.KnownPluginFolders = []string{os.TempDir()}
 	return conf
 }
 

packer/plugin_folders.go

@@ -16,6 +16,11 @@ import (
 func PluginFolders(dirs ...string) []string {
 	res := []string{}
 
+	if packerPluginPath := os.Getenv("PACKER_PLUGIN_PATH"); packerPluginPath != "" {
+		res = append(res, strings.Split(packerPluginPath, string(os.PathListSeparator))...)
+		return res
+	}
+
 	if path, err := os.Executable(); err != nil {
 		log.Printf("[ERR] Error finding executable: %v", err)
 	} else {
@@ -30,9 +35,5 @@ func PluginFolders(dirs ...string) []string {
 		res = append(res, filepath.Join(cd, "plugins"))
 	}
 
-	if packerPluginPath := os.Getenv("PACKER_PLUGIN_PATH"); packerPluginPath != "" {
-		res = append(res, strings.Split(packerPluginPath, string(os.PathListSeparator))...)
-	}
-
 	return res
 }