Log route handler errors and answer them with a 500 page (#3621)

`run!` hands each request to `HTTP.streamhandler`, so the `try` around it also covers Pluto's route handlers, and `is_disconnection_exception` treats any `IOError` or `SystemError` as the client hanging up. An exception from a handler, such as `POST /notebookupload` failing to write the uploaded file, was dropped without being logged: HTTP.jl 2 answered `200` with an empty body, HTTP.jl 1 a bare `500`.

`internal_error_middleware` catches what the handlers throw, logs it, and answers with Pluto's `500` error page, which the frontend shows when an upload fails. It sits inside `default_headers_middleware`, so the error page gets the same headers as any other response, and the outer `try` only sees failures to read the request or write the response.

Author: pankgeorg

src/webserver/WebServer.jl

@@ -146,6 +146,21 @@ function create_binder_token_middleware(session::ServerSession)
     end
 end
 
+"""
+Middleware that turns an exception thrown while handling a request into a `500` error page,
+and logs it. The details only go to the log, since the request might not be authenticated.
+"""
+function internal_error_middleware(handler)
+    function (request::HTTP.Request)
+        try
+            handler(request)
+        catch e
+            @error "Failed to handle request" request.method path = HTTP.URI(request.target).path exception = (e, catch_backtrace())
+            error_response(500, "Internal server error", "Pluto ran into an unexpected error while handling this request. The details are in the terminal where Pluto is running. Please <a href='https://github.com/JuliaPluto/Pluto.jl/issues'>report this error</a>!")
+        end
+    end
+end
+
 "Middleware that adds the headers that Pluto sets on every HTTP response."
 function default_headers_middleware(handler)
     function (request::HTTP.Request)
@@ -173,7 +188,7 @@ function run!(session::ServerSession)
     pluto_router = http_router_for(session)
     store_session_middleware = create_session_context_middleware(session)
     store_binder_token_middleware = create_binder_token_middleware(session)
-    app = pluto_router |> auth_middleware |> store_session_middleware |> store_binder_token_middleware |> default_headers_middleware
+    app = pluto_router |> auth_middleware |> store_session_middleware |> store_binder_token_middleware |> internal_error_middleware |> default_headers_middleware
     # `HTTP.streamhandler` reads the request body, calls `app`, and writes the response.
     streamed_app = HTTP.streamhandler(app)
 

test/webserver.jl

@@ -108,6 +108,15 @@ end
             @test [c.code for c in notebook.cells] == codes
         end
 
+        @testset "handler error" begin
+            # The upload cannot be saved in a directory that does not exist, so the route handler throws.
+            r = HTTP.post(local_url("notebookupload?name=directory-that-does-not-exist/uploaded"); body="x = 1", status_exception=false)
+            @test r.status == 500
+            @test startswith(HTTP.header(r, "Content-Type"), "text/html")
+            @test HTTP.header(r, "Content-Length") == string(length(r.body))
+            @test length(🍭.notebooks) == 1
+        end
+
         @testset "websocket" begin
             notebook = only(values(🍭.notebooks))
             reply = Ref{Any}(nothing)