Add a CGI contrib

1 files changed, 178 insertions, 0 deletions

diff --git a/contrib/cgi/cgi.go b/contrib/cgi/cgi.go
new file mode 100644
index 0000000..2e20485
--- /dev/null
+++ b/contrib/cgi/cgi.go
@@ -0,0 +1,178 @@
+package cgi
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io/fs"
+	"net"
+	"os"
+	"os/exec"
+	"strings"
+
+	"tildegit.org/tjp/gus/gemini"
+)
+
+func CGIHandler(pathPrefix, rootDir string) gemini.Handler {
+	rootDir = strings.TrimRight(rootDir, "/")
+
+	return func(ctx context.Context, req *gemini.Request) *gemini.Response {
+		if !strings.HasPrefix(req.Path, pathPrefix) {
+			return gemini.NotFound("Resource does not exist.")
+		}
+
+		path := req.Path[len(pathPrefix):]
+		segments := strings.Split(strings.TrimLeft(path, "/"), "/")
+		for i := range append(segments, "") {
+			path := strings.Join(append([]string{rootDir}, 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+1:], "/")
+				}
+				return runCGI(ctx, req.Server, req, path, pathInfo)
+			}
+
+			if !isDir {
+				break
+			}
+		}
+
+		return gemini.NotFound("Resource does not exist.")
+	}
+}
+
+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
+}
+
+func runCGI(
+	ctx context.Context,
+	server *gemini.Server,
+	req *gemini.Request,
+	filePath string,
+	pathInfo string,
+) *gemini.Response {
+	pathSegments := strings.Split(filePath, "/")
+
+	dirPath := "."
+	if len(pathSegments) > 1 {
+		dirPath = strings.Join(pathSegments[:len(pathSegments)-1], "/")
+	}
+	filePath = "./" + pathSegments[len(pathSegments)-1]
+
+	cmd := exec.CommandContext(ctx, filePath)
+	cmd.Env = prepareCGIEnv(ctx, server, req, filePath, 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) {
+			return gemini.CGIError(fmt.Sprintf("CGI returned with exit code %d", exErr.ExitCode()))
+		}
+		return gemini.Failure(err)
+	}
+
+	response, err := gemini.ParseResponse(responseBuffer)
+	if err != nil {
+		return gemini.Failure(err)
+	}
+	return response
+}
+
+func prepareCGIEnv(
+	ctx context.Context,
+	server *gemini.Server,
+	req *gemini.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="+server.Hostname(),
+		"SERVER_PORT="+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),
+		)
+	}
+
+	return environ
+}
+
+func fingerprint(raw []byte) string {
+	hash := sha256.Sum256(raw)
+	return hex.EncodeToString(hash[:])
+}