forgejo-runner-optimiser/internal/proc/stat.go

package proc

import (
	"fmt"
	"os"
	"strconv"
	"strings"
)

// ProcStat holds CPU-related information from /proc/[pid]/stat
type ProcStat struct {
	PID        int
	Comm       string // Process name (executable filename)
	State      byte   // Process state (R, S, D, Z, T, etc.)
	PPID       int    // Parent PID
	UTime      uint64 // User mode CPU time (in clock ticks)
	STime      uint64 // Kernel mode CPU time (in clock ticks)
	CUTime     int64  // Children user mode CPU time
	CSTime     int64  // Children system mode CPU time
	NumThreads int64  // Number of threads
	StartTime  uint64 // Time process started after boot (in clock ticks)
}

// ReadStat reads and parses /proc/[pid]/stat for the given PID
func ReadStat(procPath string, pid int) (*ProcStat, error) {
	path := fmt.Sprintf("%s/%d/stat", procPath, pid)
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("reading stat file: %w", err)
	}

	return parseStat(pid, string(data))
}

// parseStat parses the content of /proc/[pid]/stat
// The format is complex because comm (field 2) can contain spaces and parentheses
func parseStat(pid int, data string) (*ProcStat, error) {
	// Find the comm field which is enclosed in parentheses
	// This handles cases where comm contains spaces or other special characters
	start := strings.Index(data, "(")
	end := strings.LastIndex(data, ")")
	if start == -1 || end == -1 || end <= start {
		return nil, fmt.Errorf("invalid stat format: cannot find comm field")
	}

	comm := data[start+1 : end]

	// Fields after the comm field
	remainder := strings.TrimSpace(data[end+1:])
	fields := strings.Fields(remainder)

	// We need at least 20 fields after comm (fields 3-22)
	if len(fields) < 20 {
		return nil, fmt.Errorf("invalid stat format: expected at least 20 fields after comm, got %d", len(fields))
	}

	stat := &ProcStat{
		PID:  pid,
		Comm: comm,
	}

	// Field 3: state (index 0 after comm)
	if len(fields[0]) > 0 {
		stat.State = fields[0][0]
	}

	// Field 4: ppid (index 1)
	ppid, err := strconv.Atoi(fields[1])
	if err != nil {
		return nil, fmt.Errorf("parsing ppid: %w", err)
	}
	stat.PPID = ppid

	// Field 14: utime (index 11) - user mode CPU time
	utime, err := strconv.ParseUint(fields[11], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("parsing utime: %w", err)
	}
	stat.UTime = utime

	// Field 15: stime (index 12) - kernel mode CPU time
	stime, err := strconv.ParseUint(fields[12], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("parsing stime: %w", err)
	}
	stat.STime = stime

	// Field 16: cutime (index 13) - children user mode CPU time
	cutime, err := strconv.ParseInt(fields[13], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("parsing cutime: %w", err)
	}
	stat.CUTime = cutime

	// Field 17: cstime (index 14) - children system mode CPU time
	cstime, err := strconv.ParseInt(fields[14], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("parsing cstime: %w", err)
	}
	stat.CSTime = cstime

	// Field 20: num_threads (index 17)
	numThreads, err := strconv.ParseInt(fields[17], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("parsing num_threads: %w", err)
	}
	stat.NumThreads = numThreads

	// Field 22: starttime (index 19) - time process started after boot
	startTime, err := strconv.ParseUint(fields[19], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("parsing starttime: %w", err)
	}
	stat.StartTime = startTime

	return stat, nil
}

// TotalTime returns the total CPU time (user + system) in clock ticks
func (s *ProcStat) TotalTime() uint64 {
	return s.UTime + s.STime
}

// TotalTimeWithChildren returns total CPU time including children
func (s *ProcStat) TotalTimeWithChildren() uint64 {
	total := int64(s.UTime) + int64(s.STime) + s.CUTime + s.CSTime
	if total < 0 {
		return 0
	}
	return uint64(total)
}