feat: integrate new agentexec pkg (#15609)

- Integrates the `agentexec` pkg into the agent and removes the
legacy system of iterating over the process tree. It adds some linting
rules to hopefully catch future improper uses of `exec.Command` in the package.

agent/agentexec/cli_linux.go

@@ -23,7 +23,8 @@ const unset = -2000
 // CLI runs the agent-exec command. It should only be called by the cli package.
 func CLI() error {
 	// We lock the OS thread here to avoid a race condition where the nice priority
-	// we get is on a different thread from the one we set it on.
+	// we set gets applied to a different thread than the one we exec the provided
+	// command on.
 	runtime.LockOSThread()
 	// Nop on success but we do it anyway in case of an error.
 	defer runtime.UnlockOSThread()
@@ -68,12 +69,18 @@ func CLI() error {
 
 	err = unix.Setpriority(unix.PRIO_PROCESS, 0, *nice)
 	if err != nil {
-		return xerrors.Errorf("set nice score: %w", err)
+		// We alert the user instead of failing the command since it can be difficult to debug
+		// for a template admin otherwise. It's quite possible (and easy) to set an
+		// inappriopriate value for niceness.
+		printfStdErr("failed to adjust niceness to %q: %v", *nice, err)
 	}
 
 	err = writeOOMScoreAdj(*oom)
 	if err != nil {
-		return xerrors.Errorf("set oom score: %w", err)
+		// We alert the user instead of failing the command since it can be difficult to debug
+		// for a template admin otherwise. It's quite possible (and easy) to set an
+		// inappriopriate value for oom_score_adj.
+		printfStdErr("failed to adjust oom score to %q: %v", *nice, err)
 	}
 
 	path, err := exec.LookPath(args[0])
@@ -143,3 +150,7 @@ func execArgs(args []string) []string {
 	}
 	return nil
 }
+
+func printfStdErr(format string, a ...any) {
+	_, _ = fmt.Fprintf(os.Stderr, "coder-agent: %s\n", fmt.Sprintf(format, a...))
+}

agent/agentexec/exec.go

@@ -10,6 +10,8 @@ import (
 	"strconv"
 
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/pty"
 )
 
 const (
@@ -25,19 +27,39 @@ const (
 // is returned. All instances of exec.Cmd should flow through this function to ensure
 // proper resource constraints are applied to the child process.
 func CommandContext(ctx context.Context, cmd string, args ...string) (*exec.Cmd, error) {
+	cmd, args, err := agentExecCmd(cmd, args...)
+	if err != nil {
+		return nil, xerrors.Errorf("agent exec cmd: %w", err)
+	}
+	return exec.CommandContext(ctx, cmd, args...), nil
+}
+
+// PTYCommandContext returns an pty.Cmd that calls "coder agent-exec" prior to exec'ing
+// the provided command if CODER_PROC_PRIO_MGMT is set, otherwise a normal pty.Cmd
+// is returned. All instances of pty.Cmd should flow through this function to ensure
+// proper resource constraints are applied to the child process.
+func PTYCommandContext(ctx context.Context, cmd string, args ...string) (*pty.Cmd, error) {
+	cmd, args, err := agentExecCmd(cmd, args...)
+	if err != nil {
+		return nil, xerrors.Errorf("agent exec cmd: %w", err)
+	}
+	return pty.CommandContext(ctx, cmd, args...), nil
+}
+
+func agentExecCmd(cmd string, args ...string) (string, []string, error) {
 	_, enabled := os.LookupEnv(EnvProcPrioMgmt)
 	if runtime.GOOS != "linux" || !enabled {
-		return exec.CommandContext(ctx, cmd, args...), nil
+		return cmd, args, nil
 	}
 
 	executable, err := os.Executable()
 	if err != nil {
-		return nil, xerrors.Errorf("get executable: %w", err)
+		return "", nil, xerrors.Errorf("get executable: %w", err)
 	}
 
 	bin, err := filepath.EvalSymlinks(executable)
 	if err != nil {
-		return nil, xerrors.Errorf("eval symlinks: %w", err)
+		return "", nil, xerrors.Errorf("eval symlinks: %w", err)
 	}
 
 	execArgs := []string{"agent-exec"}
@@ -51,7 +73,7 @@ func CommandContext(ctx context.Context, cmd string, args ...string) (*exec.Cmd,
 	execArgs = append(execArgs, "--", cmd)
 	execArgs = append(execArgs, args...)
 
-	return exec.CommandContext(ctx, bin, execArgs...), nil
+	return bin, execArgs, nil
 }
 
 // envValInt searches for a key in a list of environment variables and parses it to an int.