fix(util): IsProcessAlive treats zombies as dead on Linux (#1839)

internal/util/process.go

@@ -4,23 +4,82 @@ package util
 
 import (
 	"os"
+	"strings"
 	"syscall"
 )
 
 // IsProcessAlive checks if a process with the given PID is still running.
 // On Unix, it sends signal 0 (no signal, just permission/existence check).
+//
+// #1839: a zombie (exited but not yet reaped by its parent) also answers
+// signal 0, so the raw check would report a dead process as alive. Callers
+// that only decide whether to KEEP state (crash-recovery anchors, instance
+// files) fail safe on residue; but semantics here should be honest - a
+// zombie is not running. When /proc is available we read the process state
+// and treat Z (and X/x, exiting) as dead. /proc-less platforms keep the
+// signal-0-only behavior (documented, conservative).
 func IsProcessAlive(pid int) bool {
 	proc, err := os.FindProcess(pid)
 	if err != nil {
 		return false
 	}
-	return proc.Signal(syscall.Signal(0)) == nil
+	if proc.Signal(syscall.Signal(0)) != nil {
+		return false
+	}
+	return !isZombieUnix(pid)
+}
+
+// isZombieUnix reports whether /proc/<pid>/stat shows the process in an
+// exited (zombie or dying) state. Returns false on any read/parse failure -
+// liveness stays with the signal-0 verdict when /proc is unavailable.
+func isZombieUnix(pid int) bool {
+	if pid <= 0 {
+		return false
+	}
+	data, err := os.ReadFile("/proc/" + itoa(pid) + "/stat")
+	if err != nil {
+		return false
+	}
+	// Layout: pid (comm) state ... - the comm field may contain spaces or
+	// parens, so parse from the LAST ')'.
+	idx := strings.LastIndexByte(string(data), ')')
+	if idx < 0 || idx+2 >= len(data) {
+		return false
+	}
+	state := data[idx+2]
+	switch state {
+	case 'Z', 'X', 'x': // zombie / dead / dying
+		return true
+	}
+	return false
+}
+
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	neg := n < 0
+	if neg {
+		n = -n
+	}
+	var b [20]byte
+	i := len(b)
+	for n > 0 {
+		i--
+		b[i] = byte('0' + n%10)
+		n /= 10
+	}
+	if neg {
+		i--
+		b[i] = '-'
+	}
+	return string(b[i:])
 }
 
 // IsProcessAliveProc checks if the given os.Process is still running.
 func IsProcessAliveProc(proc *os.Process) bool {
 	if proc == nil || proc.Pid <= 0 {
 		return false
 	}
-	return proc.Signal(syscall.Signal(0)) == nil
+	return IsProcessAlive(proc.Pid)
 }

internal/util/process_1839_test.go

@@ -0,0 +1,50 @@
+//go:build !windows
+
+package util
+
+import (
+	"os"
+	"os/exec"
+	"testing"
+	"time"
+)
+
+// #1839: a zombie (exited, unreaped child) must NOT count as alive.
+func Test1839ZombieNotAlive(t *testing.T) {
+	if _, err := os.Stat("/proc/self/stat"); err != nil {
+		t.Skip("/proc unavailable; signal-0-only behavior documented")
+	}
+	cmd := exec.Command("true")
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+	pid := cmd.Process.Pid
+	// Exit without Wait -> the child becomes a zombie owned by us until
+	// test cleanup; poll briefly for the Z state to appear.
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		if isZombieUnix(pid) {
+			break
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	if !isZombieUnix(pid) {
+		_ = cmd.Wait()
+		t.Skip("could not produce a zombie (fast reaping host)")
+	}
+	if IsProcessAlive(pid) {
+		_ = cmd.Wait()
+		t.Fatalf("zombie pid %d must not be reported alive", pid)
+	}
+	_ = cmd.Wait()
+}
+
+// Normal and non-existent processes keep their verdicts.
+func Test1839VerdictsUnchanged(t *testing.T) {
+	if !IsProcessAlive(os.Getpid()) {
+		t.Fatal("self must be alive")
+	}
+	if IsProcessAlive(1 << 22) {
+		t.Fatal("unlikely pid must be dead")
+	}
+}