test(functional): factor busybox skip check into requireBusybox

Six tests had a copy-pasted `[[ $busybox =~ busybox ]]` check that only
verifies the variable contains the substring "busybox" — it passes even
when the path doesn't exist. Factor into `requireBusybox` in functions.sh
using `[[ -x "$busybox" ]]` instead, which at least catches an unset
variable, a nonexistent path, or a non-executable file.

Note this still doesn't probe whether a nested build sandbox can actually
exec the path — on some remote builders the host path is valid but the
chroot-store sandbox cannot reach it. The comment in requireBusybox
documents that limitation; such environments must skip these tests
explicitly.

Author: lovesegfault

tests/functional/build-remote-trustless-should-fail-0.sh

@@ -9,7 +9,7 @@ restartDaemon
 
 requireSandboxSupport
 requiresUnprivilegedUserNamespaces
-[[ $busybox =~ busybox ]] || skipTest "no busybox"
+requireBusybox
 
 unset NIX_STORE_DIR
 

tests/functional/build-remote-trustless.sh

@@ -6,7 +6,7 @@
 
 requireSandboxSupport
 requiresUnprivilegedUserNamespaces
-[[ "$busybox" =~ busybox ]] || skipTest "no busybox"
+requireBusybox
 
 unset NIX_STORE_DIR
 

tests/functional/build-remote-with-mounted-ssh-ng.sh

@@ -3,7 +3,7 @@
 source common.sh
 
 requireSandboxSupport
-[[ $busybox =~ busybox ]] || skipTest "no busybox"
+requireBusybox
 
 enableFeatures mounted-ssh-store
 

tests/functional/build-remote.sh

@@ -1,10 +1,11 @@
 # shellcheck shell=bash
 
+# shellcheck disable=SC2154
 : "${file?must be defined by caller (remote building test case using this)}"
 
 requireSandboxSupport
 requiresUnprivilegedUserNamespaces
-[[ "${busybox-}" =~ busybox ]] || skipTest "no busybox"
+requireBusybox
 
 # Avoid store dir being inside sandbox build-dir
 unset NIX_STORE_DIR

tests/functional/common/functions.sh

@@ -162,6 +162,17 @@ requireSandboxSupport () {
     canUseSandbox || skipTest "Sandboxing not supported"
 }
 
+requireBusybox() {
+    # Check executability, not just that the path contains "busybox" — the
+    # variable is @sandbox_shell@ from meson and is empty when not found.
+    # Note: this does not probe whether a nested build sandbox can exec the
+    # path; on some remote builders the host path is valid but the sandbox
+    # built from a chroot store cannot reach it. Such environments must skip
+    # these tests explicitly.
+    # shellcheck disable=SC2154 # set by subst-vars.sh
+    [[ -x "$busybox" ]] || skipTest "no busybox"
+}
+
 requireGit() {
     [[ $(type -p git) ]] || skipTest "Git not installed"
 }

tests/functional/local-overlay-store/common.sh

@@ -29,7 +29,7 @@ export LIBMOUNT_FORCE_MOUNT2=always
 
 requireEnvironment () {
   requireSandboxSupport
-  [[ $busybox =~ busybox ]] || skipTest "no busybox"
+  requireBusybox
   if [[ $(uname) != Linux ]]; then skipTest "Need Linux for overlayfs"; fi
   needLocalStore "The test uses --store always so we would just be bypassing the daemon"
 }

tests/functional/supplementary-groups.sh

@@ -3,7 +3,7 @@
 source common.sh
 
 requireSandboxSupport
-[[ $busybox =~ busybox ]] || skipTest "no busybox"
+requireBusybox
 if ! command -p -v unshare; then skipTest "Need unshare"; fi
 needLocalStore "The test uses --store always so we would just be bypassing the daemon"