fix(agent): recover OpenResty runtime after invalid PID (#45)

* fix(agent): recover OpenResty runtime after invalid PID

* fix(agent): scope OpenResty shutdown to instance processes

---------

Co-authored-by: Noru Wyrms <wyrmsnoru@gmail.com>

internal/apps/agent/nginx/runtime_process_proc.go

@@ -0,0 +1,118 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package nginx
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+)
+
+//nolint:unused // Used by the Linux process inspector.
+const procParentPIDIndex = 1
+
+//nolint:unused // Used by the Linux process inspector.
+const procStartTimeIndex = 19 // Field 22, counting from state after the process name.
+
+//nolint:unused // Used by the Linux executor; portable for process fixture tests.
+func inspectProcRuntime(root string, executable os.FileInfo, binary, configPath string, uid int) ([]runtimeProcess, error) {
+	entries, err := os.ReadDir(root)
+	if err != nil {
+		return nil, err
+	}
+	var processes []runtimeProcess
+	for _, entry := range entries {
+		pid, err := strconv.Atoi(entry.Name())
+		if err != nil || pid <= 0 || !entry.IsDir() {
+			continue
+		}
+		process, found, err := inspectProcProcess(filepath.Join(root, entry.Name()), pid, executable, binary, configPath, uid)
+		if err != nil {
+			if os.IsNotExist(err) {
+				continue
+			}
+			return nil, err
+		}
+		if found {
+			processes = append(processes, process)
+		}
+	}
+	return processes, nil
+}
+
+//nolint:unused // Used by the Linux process inspector.
+func inspectProcProcess(dir string, pid int, executable os.FileInfo, binary, configPath string, uid int) (runtimeProcess, bool, error) {
+	//nolint:gosec // Read a fixed proc entry under a numeric PID directory.
+	status, err := os.ReadFile(filepath.Join(dir, "status"))
+	if err != nil {
+		return runtimeProcess{}, false, err
+	}
+	owned := effectiveProcessUID(string(status)) == uid
+	//nolint:gosec // Read a fixed proc entry under a numeric PID directory.
+	command, err := os.ReadFile(filepath.Join(dir, "cmdline"))
+	if err != nil {
+		return runtimeProcess{}, false, err
+	}
+	title := strings.ReplaceAll(string(command), "\x00", " ")
+	if !strings.HasPrefix(title, "nginx: ") && !strings.HasPrefix(title, "openresty: ") {
+		return runtimeProcess{}, false, nil
+	}
+	master := owned && matchesRuntimeMaster(title, configPath)
+	exe, err := os.Stat(filepath.Join(dir, "exe"))
+	if err != nil {
+		if !os.IsPermission(err) {
+			return runtimeProcess{}, false, err
+		}
+		// Non-master candidates are only retained by runtimeDescendants when
+		// their parent belongs to this instance. A capability-enabled non-root
+		// master may be non-dumpable, making /proc/PID/exe inaccessible; verify
+		// its invocation and UID instead.
+		fields := strings.Fields(title)
+		if master && (len(fields) < 4 || fields[3] != binary) {
+			return runtimeProcess{}, false, fmt.Errorf("cannot verify executable of openresty master %d", pid)
+		}
+	} else if !os.SameFile(executable, exe) {
+		if master {
+			return runtimeProcess{}, false, fmt.Errorf("executable changed for openresty master %d; refusing duplicate start", pid)
+		}
+		return runtimeProcess{}, false, nil
+	}
+	//nolint:gosec // Read a fixed proc entry under a numeric PID directory.
+	stat, err := os.ReadFile(filepath.Join(dir, "stat"))
+	if err != nil {
+		return runtimeProcess{}, false, err
+	}
+	closeParen := strings.LastIndexByte(string(stat), ')')
+	if closeParen < 0 {
+		return runtimeProcess{}, false, fmt.Errorf("invalid stat for process %d", pid)
+	}
+	fields := strings.Fields(string(stat)[closeParen+1:])
+	if len(fields) <= procStartTimeIndex {
+		return runtimeProcess{}, false, fmt.Errorf("incomplete stat for process %d", pid)
+	}
+	if fields[0] == "Z" {
+		return runtimeProcess{}, false, nil
+	}
+	parentPID, err := strconv.Atoi(fields[procParentPIDIndex])
+	if err != nil {
+		return runtimeProcess{}, false, fmt.Errorf("invalid parent pid for process %d: %w", pid, err)
+	}
+	return runtimeProcess{PID: pid, ParentPID: parentPID, Master: master, StartTime: fields[procStartTimeIndex]}, true, nil
+}
+
+//nolint:unused // Used by the Linux process inspector.
+func effectiveProcessUID(status string) int {
+	for _, line := range strings.Split(status, "\n") {
+		fields := strings.Fields(line)
+		if len(fields) >= 3 && fields[0] == "Uid:" {
+			uid, err := strconv.Atoi(fields[2])
+			if err == nil {
+				return uid
+			}
+		}
+	}
+	return -1
+}