lxc exec: Fix exit codes for signaled processes (#14252)

With this, processes spawned by `lxc exec` stopped due to being signaled
now return the proper exit code to LXD.

From the [docs](https://pkg.go.dev/os#ProcessState.Sys):

> Sys returns system-dependent exit information about the process.
Convert it to the appropriate underlying type, such as
[syscall.WaitStatus](https://pkg.go.dev/syscall#WaitStatus) on Unix, to
access its contents.

shared/util_linux.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"strings"
+	"syscall"
 	"time"
 	"unsafe"
 
@@ -397,7 +398,7 @@ func ExitStatus(err error) (int, error) {
 	// Detect and extract ExitError to check the embedded exit status.
 	if errors.As(err, &exitErr) {
 		// If the process was signaled, extract the signal.
-		status, isWaitStatus := exitErr.Sys().(unix.WaitStatus)
+		status, isWaitStatus := exitErr.Sys().(syscall.WaitStatus)
 		if isWaitStatus && status.Signaled() {
 			return 128 + int(status.Signal()), nil // 128 + n == Fatal error signal "n"
 		}

test/suites/exec.sh

@@ -93,19 +93,25 @@ test_exec_exit_code() {
   lxc exec x1 -- invalid-command || exitCode=$?
   [ "${exitCode:-0}" -eq 127 ]
 
-  # Try disconnecting a container stopping forcefully and gracefully to make sure they differ appropriately.
-  (sleep 1 && lxc stop -f x1) &
-  lxc exec x1 -- sleep 10 || exitCode=$?
+  # Signaling the process spawned by lxc exec and checking its exit code.
+  # Simulates what can happen if the container stops in the middle of lxc exec.
+  (sleep 5 && lxc exec x1 -- killall -s SIGTERM sleep) &
+  lxc exec x1 -- sleep 60 || exitCode=$?
+  [ "${exitCode:-0}" -eq 143 ] # 128 + 15(SIGTERM)
+
+  (sleep 5 && lxc exec x1 -- killall -s SIGHUP sleep) &
+  lxc exec x1 -- sleep 60 || exitCode=$?
+  [ "${exitCode:-0}" -eq 129 ] # 128 + 1(SIGHUP)
+
+  (sleep 5 && lxc exec x1 -- killall -s SIGKILL sleep) &
+  lxc exec x1 -- sleep 60 || exitCode=$?
+  [ "${exitCode:-0}" -eq 137 ] # 128 + 9(SIGKILL)
+
+  # Try disconnecting a container stopping forcefully.
+  (sleep 5 && lxc stop -f x1) &
+  lxc exec x1 -- sleep 60 || exitCode=$?
   [ "${exitCode:-0}" -eq 137 ]
-
   wait $!
-  lxc start x1
-  sleep 2
-  (sleep 1 && lxc stop x1) &
-  lxc exec x1 -- sleep 10 || exitCode=$?
-  # Both 129 and 143 have been seen and both make sense here.
-  [ "${exitCode:-0}" -eq 129 ] || [ "${exitCode:-0}" -eq 143 ]
 
-  wait $!
   lxc delete --force x1
 }