SPEC: specify only minimum set of environment vars

It is useful for executor implementations to provide various environment
variables to applications, so the specification should only mandate
certain values rather than the complete set.

SPEC.md

@@ -190,14 +190,18 @@ Note that logging mechanisms other than stdout and stderr are not required by th
 
 #### Execution Environment
 
-* **Working directory** defaults to the root of the application image, overridden with "workingDirectory"
+The following environment variables MUST be set for each application's main process and any lifecycle processes:
 * **PATH** `/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin`
 * **USER, LOGNAME** username of the user executing this app
 * **HOME** home directory of the user
 * **SHELL** login shell of the user
 * **AC_APP_NAME** name of the application, as defined in the image manifest
 * **AC_METADATA_URL** URL where the metadata service for this container can be found
 
+An executor MAY set additional environment variables for the application processes.
+
+Additionally, processes must have their **working directory** set to the value of the application's **workingDirectory** option, if specified, or the root of the application image by default.
+
 ### Isolators
 
 Isolators enforce resource constraints rather than namespacing.

ace/validator.go

@@ -177,8 +177,8 @@ func ValidateWorkingDirectory(wwd string) (r results) {
 	return
 }
 
-// ValidateEnvironment ensures that the given environment exactly maps the
-// environment in which this process is running
+// ValidateEnvironment ensures that the given environment contains the
+// necessary/expected environment variables.
 func ValidateEnvironment(wenv map[string]string) (r results) {
 	for wkey, wval := range wenv {
 		gval := os.Getenv(wkey)
@@ -187,16 +187,6 @@ func ValidateEnvironment(wenv map[string]string) (r results) {
 			r = append(r, err)
 		}
 	}
-	for _, s := range os.Environ() {
-		parts := strings.SplitN(s, "=", 2)
-		k := parts[0]
-		_, ok := wenv[k]
-		switch {
-		case k == appNameEnv, k == "PATH", k == "TERM", k == "AC_METADATA_URL":
-		case !ok:
-			r = append(r, fmt.Errorf("unexpected environment variable %q set", k))
-		}
-	}
 	return
 }