feat: implement agent process management (#9461)

- An opt-in feature has been added to the agent to allow
   deprioritizing non coder-related processes for CPU by setting their
   niceness level to 10.
- Opting in to the feature requires setting CODER_PROC_PRIO_MGMT to a non-empty value.

agent/agentproc/proc_unix.go

@@ -0,0 +1,109 @@
+//go:build linux
+// +build linux
+
+package agentproc
+
+import (
+	"errors"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"github.com/spf13/afero"
+	"golang.org/x/xerrors"
+)
+
+func List(fs afero.Fs, syscaller Syscaller) ([]*Process, error) {
+	d, err := fs.Open(defaultProcDir)
+	if err != nil {
+		return nil, xerrors.Errorf("open dir %q: %w", defaultProcDir, err)
+	}
+	defer d.Close()
+
+	entries, err := d.Readdirnames(0)
+	if err != nil {
+		return nil, xerrors.Errorf("readdirnames: %w", err)
+	}
+
+	processes := make([]*Process, 0, len(entries))
+	for _, entry := range entries {
+		pid, err := strconv.ParseInt(entry, 10, 32)
+		if err != nil {
+			continue
+		}
+
+		// Check that the process still exists.
+		exists, err := isProcessExist(syscaller, int32(pid))
+		if err != nil {
+			return nil, xerrors.Errorf("check process exists: %w", err)
+		}
+		if !exists {
+			continue
+		}
+
+		cmdline, err := afero.ReadFile(fs, filepath.Join(defaultProcDir, entry, "cmdline"))
+		if err != nil {
+			var errNo syscall.Errno
+			if xerrors.As(err, &errNo) && errNo == syscall.EPERM {
+				continue
+			}
+			return nil, xerrors.Errorf("read cmdline: %w", err)
+		}
+		processes = append(processes, &Process{
+			PID:     int32(pid),
+			CmdLine: string(cmdline),
+			Dir:     filepath.Join(defaultProcDir, entry),
+		})
+	}
+
+	return processes, nil
+}
+
+func isProcessExist(syscaller Syscaller, pid int32) (bool, error) {
+	err := syscaller.Kill(pid, syscall.Signal(0))
+	if err == nil {
+		return true, nil
+	}
+	if err.Error() == "os: process already finished" {
+		return false, nil
+	}
+
+	var errno syscall.Errno
+	if !errors.As(err, &errno) {
+		return false, err
+	}
+
+	switch errno {
+	case syscall.ESRCH:
+		return false, nil
+	case syscall.EPERM:
+		return true, nil
+	}
+
+	return false, xerrors.Errorf("kill: %w", err)
+}
+
+func (p *Process) Niceness(sc Syscaller) (int, error) {
+	nice, err := sc.GetPriority(p.PID)
+	if err != nil {
+		return 0, xerrors.Errorf("get priority for %q: %w", p.CmdLine, err)
+	}
+	return nice, nil
+}
+
+func (p *Process) SetNiceness(sc Syscaller, score int) error {
+	err := sc.SetPriority(p.PID, score)
+	if err != nil {
+		return xerrors.Errorf("set priority for %q: %w", p.CmdLine, err)
+	}
+	return nil
+}
+
+func (p *Process) Cmd() string {
+	return strings.Join(p.cmdLine(), " ")
+}
+
+func (p *Process) cmdLine() []string {
+	return strings.Split(p.CmdLine, "\x00")
+}