// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package scan provides functionality for running govulncheck.

See [cmd/govulncheck/main.go] as a usage example.

[cmd/govulncheck/main.go]: https://go.googlesource.com/vuln/+/master/cmd/govulncheck/main.go
*/
package scan

import (
	"context"
	"errors"
	"io"
	"os"

	"golang.org/x/vuln/internal/scan"
)

// Cmd represents an external govulncheck command being prepared or run,
// similar to exec.Cmd.
type Cmd struct {
	// Stdin specifies the standard input. If provided, it is expected to be
	// the output of govulncheck -json.
	Stdin io.Reader

	// Stdout specifies the standard output. If nil, Run connects os.Stdout.
	Stdout io.Writer

	// Stderr specifies the standard error. If nil, Run connects os.Stderr.
	Stderr io.Writer

	// Env is the environment to use.
	// If Env is nil, the current environment is used.
	// As in os/exec's Cmd, only the last value in the slice for
	// each environment key is used. To specify the setting of only
	// a few variables, append to the current environment, as in:
	//
	//	opt.Env = append(os.Environ(), "GOOS=plan9", "GOARCH=386")
	//
	Env []string

	ctx  context.Context
	args []string
	done chan struct{}
	err  error
}

// Command returns the Cmd struct to execute govulncheck with the given
// arguments.
func Command(ctx context.Context, arg ...string) *Cmd {
	return &Cmd{
		ctx:  ctx,
		args: arg,
	}
}

// Start starts the specified command but does not wait for it to complete.
//
// After a successful call to Start the Wait method must be called in order to
// release associated system resources.
func (c *Cmd) Start() error {
	if c.done != nil {
		return errors.New("vuln: already started")
	}
	if c.Stdin == nil {
		c.Stdin = os.Stdin
	}
	if c.Stdout == nil {
		c.Stdout = os.Stdout
	}
	if c.Stderr == nil {
		c.Stderr = os.Stderr
	}
	if c.Env == nil {
		c.Env = os.Environ()
	}
	c.done = make(chan struct{})
	go func() {
		defer close(c.done)
		c.err = c.scan()
	}()
	return nil
}

// Wait waits for the command to exit. The command must have been started by
// Start.
//
// If the command fails to run or does not complete successfully, the returned
// error wraps an error implementing:
//
//	interface {
//		ExitCode() int
//	}
//
// Callers can use errors.As to retrieve the exit code. Other errors may be
// returned for other problems (e.g. I/O issues).
//
// Wait releases any resources associated with the Cmd.
func (c *Cmd) Wait() error {
	if c.done == nil {
		return errors.New("vuln: start must be called before wait")
	}
	<-c.done
	return c.err
}

func (c *Cmd) scan() error {
	if err := c.ctx.Err(); err != nil {
		return err
	}
	return scan.RunGovulncheck(c.ctx, c.Env, c.Stdin, c.Stdout, c.Stderr, c.args)
}
