gus/contrib/cgi/cgi.go

package cgi

import (
	"bytes"
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"io/fs"
	"net"
	"os"
	"os/exec"
	"strings"

	"tildegit.org/tjp/gus"
	"tildegit.org/tjp/gus/gemini"
)

// CGIDirectory runs any executable files relative to a root directory on the file system.
//
// It will also find and run any executables _part way_ through the path, so for example
// a request for /foo/bar/baz can also run an executable found at /foo or  /foo/bar. In
// such a case the PATH_INFO environment variable will include the remaining portion of
// the URI path.
func CGIDirectory(pathRoot, fsRoot string) gus.Handler {
	fsRoot = strings.TrimRight(fsRoot, "/")

	return func(ctx context.Context, req *gus.Request) *gus.Response {
		if !strings.HasPrefix(req.Path, pathRoot) {
			return nil
		}

		path := req.Path[len(pathRoot):]
		segments := strings.Split(strings.TrimLeft(path, "/"), "/")
		for i := range append(segments, "") {
			path := strings.Join(append([]string{fsRoot}, segments[:i]...), "/")
			path = strings.TrimRight(path, "/")
			isDir, isExecutable, err := executableFile(path)
			if err != nil {
				return gemini.Failure(err)
			}

			if isExecutable {
				pathInfo := "/"
				if len(segments) > i+1 {
					pathInfo = strings.Join(segments[i:], "/")
				}
				return RunCGI(ctx, req, path, pathInfo)
			}

			if !isDir {
				break
			}
		}

		return nil
	}
}

func executableFile(path string) (bool, bool, error) {
	file, err := os.Open(path)
	if isNotExistError(err) {
		return false, false, nil
	}
	if err != nil {
		return false, false, err
	}
	defer file.Close()

	info, err := file.Stat()
	if err != nil {
		return false, false, err
	}

	if info.IsDir() {
		return true, false, nil
	}

	// readable + executable by anyone
	return false, info.Mode()&0005 == 0005, nil
}

func isNotExistError(err error) bool {
	if err != nil {
		var pathErr *fs.PathError
		if errors.As(err, &pathErr) {
			e := pathErr.Err
			if errors.Is(e, fs.ErrInvalid) || errors.Is(e, fs.ErrNotExist) {
				return true
			}
		}
	}

	return false
}

// RunCGI runs a specific program as a CGI script.
func RunCGI(
	ctx context.Context,
	req *gus.Request,
	executable string,
	pathInfo string,
) *gus.Response {
	pathSegments := strings.Split(executable, "/")

	dirPath := "."
	if len(pathSegments) > 1 {
		dirPath = strings.Join(pathSegments[:len(pathSegments)-1], "/")
	}
	basename := pathSegments[len(pathSegments)-1]

	scriptName := req.Path[:len(req.Path)-len(pathInfo)]
	if strings.HasSuffix(scriptName, "/") {
		scriptName = scriptName[:len(scriptName)-1]
	}

	cmd := exec.CommandContext(ctx, "./"+basename)
	cmd.Env = prepareCGIEnv(ctx, req, scriptName, pathInfo)
	cmd.Dir = dirPath

	responseBuffer := &bytes.Buffer{}
	cmd.Stdout = responseBuffer

	if err := cmd.Run(); err != nil {
		var exErr *exec.ExitError
		if errors.As(err, &exErr) {
			errMsg := fmt.Sprintf("CGI returned exit code %d", exErr.ExitCode())
			return gemini.CGIError(errMsg)
		}
		return gemini.Failure(err)
	}

	response, err := gemini.ParseResponse(responseBuffer)
	if err != nil {
		return gemini.Failure(err)
	}
	return response
}

func prepareCGIEnv(
	ctx context.Context,
	req *gus.Request,
	scriptName string,
	pathInfo string,
) []string {
	var authType string
	if len(req.TLSState.PeerCertificates) > 0 {
		authType = "Certificate"
	}
	environ := []string{
		"AUTH_TYPE=" + authType,
		"CONTENT_LENGTH=",
		"CONTENT_TYPE=",
		"GATEWAY_INTERFACE=CGI/1.1",
		"PATH_INFO=" + pathInfo,
		"PATH_TRANSLATED=",
		"QUERY_STRING=" + req.RawQuery,
	}

	host, _, _ := net.SplitHostPort(req.RemoteAddr.String())
	environ = append(environ, "REMOTE_ADDR="+host)

	environ = append(
		environ,
		"REMOTE_HOST=",
		"REMOTE_IDENT=",
		"SCRIPT_NAME="+scriptName,
		"SERVER_NAME="+req.Server.Hostname(),
		"SERVER_PORT="+req.Server.Port(),
		"SERVER_PROTOCOL=GEMINI",
		"SERVER_SOFTWARE=GUS",
	)

	if len(req.TLSState.PeerCertificates) > 0 {
		cert := req.TLSState.PeerCertificates[0]
		environ = append(
			environ,
			"TLS_CLIENT_HASH="+fingerprint(cert.Raw),
			"TLS_CLIENT_ISSUER="+cert.Issuer.String(),
			"TLS_CLIENT_ISSUER_CN="+cert.Issuer.CommonName,
			"TLS_CLIENT_SUBJECT="+cert.Subject.String(),
			"TLS_CLIENT_SUBJECT_CN="+cert.Subject.CommonName,
		)
	}

	return environ
}

func fingerprint(raw []byte) string {
	hash := sha256.Sum256(raw)
	return hex.EncodeToString(hash[:])
}