implement live-reloading dev server

Squashed commit of the following:

commit 02f077432202af4d633eb2cad81dfdaa6921317f
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 21:09:14 2024 +0200

    builder: only remove output directory if set and in dev mode

commit 47001e01c55fa6e74aafeda04ebc3e4e7c47eba0
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 21:03:37 2024 +0200

    implement live reload on dev server

commit 411ec969f61e4b73439f1c54ea29f75135ecc618
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:59:26 2024 +0200

    server: implement graceful shutdown

commit 5400132eb6eb1b638e0b3fd4265f51611c92d473
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:41:07 2024 +0200

    add some debug logs

commit 3c9b678197c044603950232d222f501ef74d7873
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:39:09 2024 +0200

    prefix log output with executable name

commit 300e24c179e390e9d3f5aeab4471c97f17f1fa64
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:29:42 2024 +0200

    don't panic inside internal packages, return error instead

commit fe2715d330402ad67fe866471bed89c7238ad2ec
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Fri Apr 26 01:18:29 2024 +0200

    config: use a table to configure CSP headers

commit d012553aaf78a436fa8871830b5d720a9e292d4b
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 17:13:39 2024 +0200

    dev: create basic dev server to build and serve from a temporary directory

commit a1d11d3e69650d9b43ca1b1d7b7ccc05a808d5c1
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 13:02:22 2024 +0200

    remove unused redirect_other_hostnames config option

commit fd67b19b5c7f76f0c3579e8a05ef20a618e90be7
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 12:58:53 2024 +0200

    server: make port a string, which is what go uses

commit c798e8e736c0649008cade337158399470a9099b
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 12:58:33 2024 +0200

    config: remove unused port variable

commit f94882b9001f3b0855e26b26b4a84b96e3deb22b
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 12:49:10 2024 +0200

    re-organise module layout

3 files changed, 353 insertions, 0 deletions

diff --git a/internal/server/filemap.go b/internal/server/filemap.go
new file mode 100644
index 0000000..466db49
--- /dev/null
+++ b/internal/server/filemap.go
@@ -0,0 +1,77 @@
+package server
+
+import (
+	"fmt"
+	"hash/fnv"
+	"io"
+	"io/fs"
+	"log"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+type File struct {
+	filename string
+	etag     string
+}
+
+var files = map[string]File{}
+
+func hashFile(filename string) (string, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	hash := fnv.New64a()
+	if _, err := io.Copy(hash, f); err != nil {
+		return "", err
+	}
+	return fmt.Sprintf(`W/"%x"`, hash.Sum(nil)), nil
+}
+
+func registerFile(urlpath string, filepath string) error {
+	if files[urlpath] != (File{}) {
+		log.Printf("registerFile called with duplicate file, urlPath: %s", urlpath)
+		return nil
+	}
+	hash, err := hashFile(filepath)
+	if err != nil {
+		return err
+	}
+	files[urlpath] = File{
+		filename: filepath,
+		etag:     hash,
+	}
+	return nil
+}
+
+func registerContentFiles(root string) error {
+	err := filepath.WalkDir(root, func(filePath string, f fs.DirEntry, err error) error {
+		if err != nil {
+			return errors.WithMessagef(err, "failed to access path %s", filePath)
+		}
+		relPath, err := filepath.Rel(root, filePath)
+		if err != nil {
+			return errors.WithMessagef(err, "failed to make path relative, path: %s", filePath)
+		}
+		urlPath, _ := strings.CutSuffix(relPath, "index.html")
+		if !f.IsDir() {
+			slog.Debug("registering file", "urlpath", "/"+urlPath)
+			return registerFile("/"+urlPath, filePath)
+		}
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func GetFile(urlPath string) File {
+	return files[urlPath]
+}
diff --git a/internal/server/logging.go b/internal/server/logging.go
new file mode 100644
index 0000000..135f06e
--- /dev/null
+++ b/internal/server/logging.go
@@ -0,0 +1,55 @@
+package server
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+)
+
+type loggingResponseWriter struct {
+	http.ResponseWriter
+	statusCode int
+}
+
+func (lrw *loggingResponseWriter) WriteHeader(code int) {
+	lrw.statusCode = code
+	// avoids warning: superfluous response.WriteHeader call
+	if lrw.statusCode != http.StatusOK {
+		lrw.ResponseWriter.WriteHeader(code)
+	}
+}
+
+func NewLoggingResponseWriter(w http.ResponseWriter) *loggingResponseWriter {
+	return &loggingResponseWriter{w, http.StatusOK}
+}
+
+type wrappedHandlerOptions struct {
+	defaultHostname string
+	logger          io.Writer
+}
+
+func wrapHandlerWithLogging(wrappedHandler http.Handler, opts wrappedHandlerOptions) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		scheme := r.Header.Get("X-Forwarded-Proto")
+		if scheme == "" {
+			scheme = "http"
+		}
+		host := r.Header.Get("Host")
+		if host == "" {
+			host = opts.defaultHostname
+		}
+		lw := NewLoggingResponseWriter(w)
+		wrappedHandler.ServeHTTP(lw, r)
+		statusCode := lw.statusCode
+		fmt.Fprintf(
+			opts.logger,
+			"%s %s %d %s %s %s\n",
+			scheme,
+			r.Method,
+			statusCode,
+			host,
+			r.URL.Path,
+			lw.Header().Get("Location"),
+		)
+	})
+}
diff --git a/internal/server/server.go b/internal/server/server.go
new file mode 100644
index 0000000..2e9796d
--- /dev/null
+++ b/internal/server/server.go
@@ -0,0 +1,221 @@
+package server
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log"
+	"log/slog"
+	"mime"
+	"net"
+	"net/http"
+	"os"
+	"path"
+	"slices"
+	"strings"
+	"time"
+
+	cfg "website/internal/config"
+
+	"github.com/getsentry/sentry-go"
+	sentryhttp "github.com/getsentry/sentry-go/http"
+	"github.com/pkg/errors"
+	"github.com/shengyanli1982/law"
+)
+
+var config *cfg.Config
+
+type Config struct {
+	Production    bool    `conf:"default:false"`
+	InDevServer   bool    `conf:"default:false"`
+	Root          string  `conf:"default:website"`
+	ListenAddress string  `conf:"default:localhost"`
+	Port          string  `conf:"default:3000,short:p"`
+	BaseURL       cfg.URL `conf:"default:http://localhost:3000,short:b"`
+}
+
+type HTTPError struct {
+	Error   error
+	Message string
+	Code    int
+}
+
+type Server struct {
+	*http.Server
+}
+
+func canonicalisePath(path string) (cPath string, differs bool) {
+	cPath = path
+	if strings.HasSuffix(path, "/index.html") {
+		cPath, differs = strings.CutSuffix(path, "index.html")
+	} else if !strings.HasSuffix(path, "/") && files[path+"/"] != (File{}) {
+		cPath, differs = path+"/", true
+	}
+	return cPath, differs
+}
+
+func serveFile(w http.ResponseWriter, r *http.Request) *HTTPError {
+	urlPath, shouldRedirect := canonicalisePath(r.URL.Path)
+	if shouldRedirect {
+		http.Redirect(w, r, urlPath, 302)
+		return nil
+	}
+	file := GetFile(urlPath)
+	if file == (File{}) {
+		return &HTTPError{
+			Message: "File not found",
+			Code:    http.StatusNotFound,
+		}
+	}
+	w.Header().Add("ETag", file.etag)
+	w.Header().Add("Vary", "Accept-Encoding")
+	w.Header().Add("Content-Security-Policy", config.CSP.String())
+	for k, v := range config.Extra.Headers {
+		w.Header().Add(k, v)
+	}
+
+	http.ServeFile(w, r, files[urlPath].filename)
+	return nil
+}
+
+type webHandler func(http.ResponseWriter, *http.Request) *HTTPError
+
+func (fn webHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	defer func() {
+		if fail := recover(); fail != nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			slog.Error("runtime panic!", "error", fail)
+		}
+	}()
+	w.Header().Set("Server", fmt.Sprintf("website (%s)", ShortSHA))
+	if err := fn(w, r); err != nil {
+		if strings.Contains(r.Header.Get("Accept"), "text/html") {
+			w.WriteHeader(err.Code)
+			notFoundPage := "website/private/404.html"
+			http.ServeFile(w, r, notFoundPage)
+		} else {
+			http.Error(w, err.Message, err.Code)
+		}
+	}
+}
+
+var newMIMEs = map[string]string{
+	".xsl": "text/xsl",
+}
+
+func fixupMIMETypes() {
+	for ext, newType := range newMIMEs {
+		if err := mime.AddExtensionType(ext, newType); err != nil {
+			slog.Error("could not update mime type", "ext", ext, "mime", newType)
+		}
+	}
+}
+
+func applyDevModeOverrides(config *cfg.Config) {
+	config.CSP.ScriptSrc = slices.Insert(config.CSP.ScriptSrc, 0, "'unsafe-inline'")
+	config.CSP.ConnectSrc = slices.Insert(config.CSP.ConnectSrc, 0, "'self'")
+}
+
+func New(runtimeConfig *Config) (*Server, error) {
+	fixupMIMETypes()
+
+	var err error
+	config, err = cfg.GetConfig()
+	if err != nil {
+		return nil, errors.WithMessage(err, "error parsing configuration file")
+	}
+	if runtimeConfig.InDevServer {
+		applyDevModeOverrides(config)
+	}
+
+	prefix := path.Join(runtimeConfig.Root, "public")
+	slog.Debug("registering content files", "prefix", prefix)
+	err = registerContentFiles(prefix)
+	if err != nil {
+		return nil, errors.WithMessagef(err, "registering content files")
+	}
+
+	env := "development"
+	if runtimeConfig.Production {
+		env = "production"
+	}
+	err = sentry.Init(sentry.ClientOptions{
+		EnableTracing:    true,
+		TracesSampleRate: 1.0,
+		Dsn:              os.Getenv("SENTRY_DSN"),
+		Release:          CommitSHA,
+		Environment:      env,
+	})
+	if err != nil {
+		return nil, errors.WithMessage(err, "could not set up sentry")
+	}
+	defer sentry.Flush(2 * time.Second)
+	sentryHandler := sentryhttp.New(sentryhttp.Options{
+		Repanic: true,
+	})
+
+	top := http.NewServeMux()
+	mux := http.NewServeMux()
+	slog.Debug("binding main handler to", "host", runtimeConfig.BaseURL.Hostname()+"/")
+	mux.Handle(runtimeConfig.BaseURL.Hostname()+"/", webHandler(serveFile))
+
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		newURL := runtimeConfig.BaseURL.String() + r.URL.String()
+		http.Redirect(w, r, newURL, 301)
+	})
+
+	var logWriter io.Writer
+	if runtimeConfig.Production {
+		logWriter = law.NewWriteAsyncer(os.Stdout, nil)
+	} else {
+		logWriter = os.Stdout
+	}
+	top.Handle("/",
+		sentryHandler.Handle(
+			wrapHandlerWithLogging(mux, wrappedHandlerOptions{
+				defaultHostname: runtimeConfig.BaseURL.Hostname(),
+				logger:          logWriter,
+			}),
+		),
+	)
+	// no logging, no sentry
+	top.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	listenAddress := net.JoinHostPort(runtimeConfig.ListenAddress, runtimeConfig.Port)
+	return &Server{
+		&http.Server{
+			Addr:    listenAddress,
+			Handler: top,
+		},
+	}, nil
+}
+
+func (s *Server) Start() error {
+	if err := s.ListenAndServe(); err != http.ErrServerClosed {
+		return err
+	}
+	return nil
+}
+
+func (s *Server) Stop() chan struct{} {
+	slog.Debug("stop called")
+
+	idleConnsClosed := make(chan struct{})
+
+	go func() {
+		slog.Debug("shutting down server")
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		err := s.Server.Shutdown(ctx)
+		slog.Debug("server shut down")
+		if err != nil {
+			// Error from closing listeners, or context timeout:
+			log.Printf("HTTP server Shutdown: %v", err)
+		}
+		close(idleConnsClosed)
+	}()
+
+	return idleConnsClosed
+}