net/http: add MaxBytesError

[sslavic]

What version of Go are you using (go version)?

$ go version
go version go1.12 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN="/Users/foo/go/bin"
GOCACHE="/Users/foo/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/foo/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lf/rbfblwvx6rx3xhm68yksmqjwdv1dsf/T/go-build261266046=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Upon limiting the maximum request size (by using MaxBytesReader as in https://stackoverflow.com/a/28292505/381140) I wanted to specially handle too large request body error in a clean way with error type assertion, and return custom response body.
More people asked for the same feature: https://groups.google.com/forum/#!topic/golang-nuts/gMzVGDgPyrY

What did you expect to see?

I expected net/http to have RequestBodyTooLargeError defined and returned by MaxBytesReader so type assertion can be done instead of error message comparison.

What did you see instead?

Trivial error string implementation is returned https://golang.org/src/net/http/request.go#L1114

[perillo]

There is a similar issue in the parsePostForm function: https://golang.org/src/net/http/request.go#L1158.

[s3rj1k]

@rsc At a very least set those string as constants, current code looks ugly and stands against all the things that we love in golang.

const http.StatusRequestEntityTooLargeMessage = "http: request body too large"

[earthboundkid]

@rsc, I think this issue fell off the tracker because it's from before the "Proposal" tag. Can this be tagged? If the proposal is accepted, I'm happy to file the CL.

[ianlancetaylor]

CC @neild