exec2: more tweaks to driver harness (#20221)

Also add an explicit exit code to subproc package for when a child
process is instructed to run an unrunnable command (i.e. cannot be
found or is not executable) - with the 127 return code folks using bash
are familiar with

helper/subproc/subproc.go

@@ -15,13 +15,16 @@ import (
 
 const (
 	// ExitSuccess indicates the subprocess completed successfully.
-	ExitSuccess = iota
+	ExitSuccess = 0
 
 	// ExitFailure indicates the subprocess terminated unsuccessfully.
-	ExitFailure
+	ExitFailure = 1
 
 	// ExitTimeout indicates the subprocess timed out before completion.
-	ExitTimeout
+	ExitTimeout = 2
+
+	// ExitNotRunnable indicates a command cannot be run.
+	ExitNotRunnable = 127 // bash-ism
 )
 
 // MainFunc is the function that runs for this sub-process.

plugins/drivers/testutils/testing.go

@@ -83,7 +83,11 @@ func (h *DriverHarness) MkAllocDir(t *drivers.TaskConfig, enableLogs bool) func(
 	dir, err := os.MkdirTemp("", "nomad_driver_harness-")
 	must.NoError(h.t, err)
 
-	allocDir := allocdir.NewAllocDir(h.logger, dir, dir, t.AllocID)
+	mountsDir, err := os.MkdirTemp("", "nomad_driver_harness-mounts-")
+	must.NoError(h.t, err)
+	must.NoError(h.t, os.Chmod(mountsDir, 0755))
+
+	allocDir := allocdir.NewAllocDir(h.logger, dir, mountsDir, t.AllocID)
 	must.NoError(h.t, allocDir.Build())
 
 	t.AllocDir = allocDir.AllocDir
@@ -261,7 +265,12 @@ func SetEnvvars(envBuilder *taskenv.Builder, fsmode fsisolation.Mode, taskDir *a
 
 	// Set driver-specific environment variables
 	switch fsmode {
-	case fsisolation.None, fsisolation.Unveil:
+	case fsisolation.Unveil:
+		// Use mounts host paths
+		envBuilder.SetAllocDir(filepath.Join(taskDir.MountsAllocDir, "alloc"))
+		envBuilder.SetTaskLocalDir(filepath.Join(taskDir.MountsTaskDir, "local"))
+		envBuilder.SetSecretsDir(filepath.Join(taskDir.SecretsDir, "secrets"))
+	case fsisolation.None:
 		// Use host paths
 		envBuilder.SetAllocDir(taskDir.SharedAllocDir)
 		envBuilder.SetTaskLocalDir(taskDir.LocalDir)