Seed from go-template-repo, renamed to simplexcalc

The template's files at a77fd30, without its history or LICENSE, after
script/rename simplexcalc.

Model: opus-5-5

internal/middleware/recover.go

@@ -0,0 +1,59 @@
+package middleware
+
+import (
+	"net/http"
+	"runtime/debug"
+)
+
+// panicBody is the entire response a recovered panic produces. No
+// template, no detail: the client learns that the request failed, and
+// everything about why goes to the log and to Sentry, where it is not
+// attacker-readable.
+const panicBody = "internal server error"
+
+// Recoverer turns a panicking handler into a 500 rather than a dropped
+// connection.
+//
+// net/http already recovers panics, but what it does is close the
+// connection without a response, so the client sees a transport error
+// and no status. Answering 500 is the difference between "the service
+// is broken" and "the network is broken" for everyone downstream.
+//
+// A panic after the response has started cannot be turned into a 500 —
+// the status is already on the wire — so in that case the connection is
+// deliberately dropped by re-panicking to net/http, which is the only
+// honest signal left that the body is truncated. A truncated 200 that
+// looks complete is worse than a broken connection.
+func (m *Middleware) Recoverer() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			rec := newResponseRecorder(w)
+
+			defer func() {
+				v := recover()
+				if v == nil {
+					return
+				}
+
+				// http.ErrAbortHandler is net/http's documented way for
+				// a handler to abandon a response on purpose. It is not
+				// a bug, so it is not reported; it is re-raised for
+				// net/http to handle as it always does.
+				//nolint:errorlint,err113 // a sentinel value, compared as net/http documents.
+				if v == http.ErrAbortHandler {
+					panic(v)
+				}
+
+				m.params.Sentry.CapturePanic(v, debug.Stack())
+
+				if rec.Written() {
+					panic(v)
+				}
+
+				http.Error(rec, panicBody, http.StatusInternalServerError)
+			}()
+
+			next.ServeHTTP(rec, r)
+		})
+	}
+}