[feature] More consistent API error handling (#637)

* update templates

* start reworking api error handling

* update template

* return AP status at web endpoint if negotiated

* start making api error handling much more consistent

* update account endpoints to new error handling

* use new api error handling in admin endpoints

* go fmt ./...

* use api error logic in app

* use generic error handling in auth

* don't export generic error handler

* don't defer clearing session

* user nicer error handling on oidc callback handler

* tidy up the sign in handler

* tidy up the token handler

* use nicer error handling in blocksget

* auth emojis endpoint

* fix up remaining api endpoints

* fix whoopsie during login flow

* regenerate swagger docs

* change http error logging to debug

internal/api/errorhandling.go

@@ -0,0 +1,127 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package api
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/sirupsen/logrus"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+// TODO: add more templated html pages here for different error types
+
+// NotFoundHandler serves a 404 html page through the provided gin context,
+// if accept is 'text/html', or just returns a json error if 'accept' is empty
+// or application/json.
+//
+// When serving html, NotFoundHandler calls the provided InstanceGet function
+// to fetch the apimodel representation of the instance, for serving in the
+// 404 header and footer.
+//
+// If an error is returned by InstanceGet, the function will panic.
+func NotFoundHandler(c *gin.Context, instanceGet func(ctx context.Context, domain string) (*apimodel.Instance, gtserror.WithCode), accept string) {
+	switch accept {
+	case string(TextHTML):
+		host := config.GetHost()
+		instance, err := instanceGet(c.Request.Context(), host)
+		if err != nil {
+			panic(err)
+		}
+
+		c.HTML(http.StatusNotFound, "404.tmpl", gin.H{
+			"instance": instance,
+		})
+	default:
+		c.JSON(http.StatusNotFound, gin.H{"error": http.StatusText(http.StatusNotFound)})
+	}
+}
+
+// genericErrorHandler is a more general version of the NotFoundHandler, which can
+// be used for serving either generic error pages with some rendered help text,
+// or just some error json if the caller prefers (or has no preference).
+func genericErrorHandler(c *gin.Context, instanceGet func(ctx context.Context, domain string) (*apimodel.Instance, gtserror.WithCode), accept string, errWithCode gtserror.WithCode) {
+	switch accept {
+	case string(TextHTML):
+		host := config.GetHost()
+		instance, err := instanceGet(c.Request.Context(), host)
+		if err != nil {
+			panic(err)
+		}
+
+		c.HTML(errWithCode.Code(), "error.tmpl", gin.H{
+			"instance": instance,
+			"code":     errWithCode.Code(),
+			"error":    errWithCode.Safe(),
+		})
+	default:
+		c.JSON(errWithCode.Code(), gin.H{"error": errWithCode.Safe()})
+	}
+}
+
+// ErrorHandler takes the provided gin context and errWithCode and tries to serve
+// a helpful error to the caller. It will do content negotiation to figure out if
+// the caller prefers to see an html page with the error rendered there. If not, or
+// if something goes wrong during the function, it will recover and just try to serve
+// an appropriate application/json content-type error.
+func ErrorHandler(c *gin.Context, errWithCode gtserror.WithCode, instanceGet func(ctx context.Context, domain string) (*apimodel.Instance, gtserror.WithCode)) {
+	path := c.Request.URL.Path
+	if raw := c.Request.URL.RawQuery; raw != "" {
+		path = path + "?" + raw
+	}
+
+	l := logrus.WithFields(logrus.Fields{
+		"path":  path,
+		"error": errWithCode.Error(),
+	})
+
+	statusCode := errWithCode.Code()
+
+	if statusCode == http.StatusInternalServerError {
+		l.Error("Internal Server Error")
+	} else {
+		l.Debug("handling error")
+	}
+
+	// if we panic for any reason during error handling,
+	// we should still try to return a basic code
+	defer func() {
+		if p := recover(); p != nil {
+			l.Warnf("recovered from panic: %s", p)
+			c.JSON(statusCode, gin.H{"error": errWithCode.Safe()})
+		}
+	}()
+
+	// discover if we're allowed to serve a nice html error page,
+	// or if we should just use a json. Normally we would want to
+	// check for a returned error, but if an error occurs here we
+	// can just fall back to default behavior (serve json error).
+	accept, _ := NegotiateAccept(c, HTMLOrJSONAcceptHeaders...)
+
+	if statusCode == http.StatusNotFound {
+		// use our special not found handler with useful status text
+		NotFoundHandler(c, instanceGet, accept)
+	} else {
+		genericErrorHandler(c, instanceGet, accept, errWithCode)
+	}
+}