Run a verb from the controller's own running image, and refuse what it cannot run as a handover (hq issue 289)

The witness moves a running build aside into a directory the controller's user
cannot enter, then deletes it; a verb exec'd from os.Executable() in that window
failed with permission denied. /proc/self/exe stays valid while the process lives.
A verb that still cannot start, or arrives while the controller stops, is refused
with link.ErrHandingOver and marked retry: handing-over.

cmd/mesh-controller/selfexec.go

@@ -0,0 +1,46 @@
+package main
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"runtime"
+	"sync/atomic"
+)
+
+// startedFrom is the path this process's executable had when it started: what a verb's command line
+// is named in a process listing, and what is run where the image cannot be named otherwise.
+var startedFrom, _ = os.Executable()
+
+// ownImage is how a process names the executable it is running, as long as it runs, wherever the file
+// has gone since. On Linux the kernel keeps it: /proc/self/exe is the running image itself, not a path,
+// so it is valid after the file is renamed into a directory this process may not enter, or deleted —
+// which is what the node-engine's witness does to a build it replaces (novox/hq issue 289). Read in the
+// child, it names the child's image, which until the exec is this process's.
+//
+// os.Executable reads the same link and returns the path it points at *now*: correct at start and
+// wrong the moment the file moves, which is the fault. A variable so a test can name another.
+var ownImage = func() string {
+	if runtime.GOOS == "linux" {
+		if _, err := os.Stat("/proc/self/exe"); err == nil {
+			return "/proc/self/exe"
+		}
+	}
+	return startedFrom
+}
+
+// selfCommand is this binary run with a command line: the image this process runs, named in a process
+// listing as the path it started from.
+func selfCommand(ctx context.Context, argv []string) *exec.Cmd {
+	cmd := exec.CommandContext(ctx, ownImage(), argv...)
+	if startedFrom != "" {
+		cmd.Args[0] = startedFrom
+	}
+	return cmd
+}
+
+// handingOver is set when the serving controller begins to stop — a signal from its supervisor, a lease
+// lost. From then a verb that would run a command is refused as a handover rather than started and
+// killed with this process: the caller asks again, and the controller after this one answers
+// (novox/hq issue 289).
+var handingOver atomic.Bool