fix: cleanup reaper implementation (#2563)

- Clean up the agent/reaper API to be a more isolated and reusable package.

agent/reaper/reaper_unix.go

@@ -3,7 +3,6 @@
 package reaper
 
 import (
-	"fmt"
 	"os"
 	"syscall"
 
@@ -11,17 +10,6 @@ import (
 	"golang.org/x/xerrors"
 )
 
-// agentEnvMark is a simple environment variable that we use as a marker
-// to indicated that the process is a child as opposed to the reaper.
-// Since we are forkexec'ing we need to be able to differentiate between
-// the two to avoid fork bombing ourselves.
-const agentEnvMark = "CODER_DO_NOT_REAP"
-
-// IsChild returns true if we're the forked process.
-func IsChild() bool {
-	return os.Getenv(agentEnvMark) != ""
-}
-
 // IsInitProcess returns true if the current process's PID is 1.
 func IsInitProcess() bool {
 	return os.Getpid() == 1
@@ -33,19 +21,16 @@ func IsInitProcess() bool {
 // the reaper and an exec.Command waiting for its process to complete.
 // The provided 'pids' channel may be nil if the caller does not care about the
 // reaped children PIDs.
-func ForkReap(pids reap.PidCh) error {
-	// Check if the process is the parent or the child.
-	// If it's the child we want to skip attempting to reap.
-	if IsChild() {
-		return nil
+func ForkReap(opt ...Option) error {
+	opts := &options{
+		ExecArgs: os.Args,
 	}
 
-	go reap.ReapChildren(pids, nil, nil, nil)
+	for _, o := range opt {
+		o(opts)
+	}
 
-	args := os.Args
-	// This is simply done to help identify the real agent process
-	// when viewing in something like 'ps'.
-	args = append(args, "#Agent")
+	go reap.ReapChildren(opts.PIDs, nil, nil, nil)
 
 	pwd, err := os.Getwd()
 	if err != nil {
@@ -54,8 +39,7 @@ func ForkReap(pids reap.PidCh) error {
 
 	pattrs := &syscall.ProcAttr{
 		Dir: pwd,
-		// Add our marker for identifying the child process.
-		Env: append(os.Environ(), fmt.Sprintf("%s=true", agentEnvMark)),
+		Env: os.Environ(),
 		Sys: &syscall.SysProcAttr{
 			Setsid: true,
 		},
@@ -67,7 +51,7 @@ func ForkReap(pids reap.PidCh) error {
 	}
 
 	//#nosec G204
-	pid, _ := syscall.ForkExec(args[0], args, pattrs)
+	pid, _ := syscall.ForkExec(opts.ExecArgs[0], opts.ExecArgs, pattrs)
 
 	var wstatus syscall.WaitStatus
 	_, err = syscall.Wait4(pid, &wstatus, 0, nil)