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

$ go version
go version go1.21.0 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN='/home/eric/work/bin'
GOCACHE='/home/eric/.cache/go-build'
GOENV='/home/eric/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/eric/work/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/eric/work'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/eric/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/eric/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build978392017=/tmp/go-build -gno-record-gcc-switches'

What did you do?

The net/http.conn.serve method is equipped to handle a special case for when panic is called with http.ErrAbortHandler by downstream http.Handler implementations where it silently recovers and prevents the program from crashing.

When chaining http.Handler implementations, if an http.Handler uses golang.org/x/sync/singleflight.Group, and an http.Handler downstream of that panics with http.ErrAbortHandler, the singleflight package recovers from the panic first, wraps the panic value in its own private error type, and re-panics with its private type. When the new panic gets to net/http.conn.serve, a direct comparison is made to see if err != ErrAbortHandler. Since technically err is not ErrAbortHandler, just another error type that wraps ErrAbortHandler, net/http.conn.serve does not handle the special case and allows the program to crash.

Example where net/http.conn.serve handles the panic and the program does not crash:

• https://go.dev/play/p/kremq3UCgEy

Example using golang.org/x/sync/singleflight.Group where net/http.conn.serve does not handle the panic and the program crashes:

• https://go.dev/play/p/KSRab3EybtA

What did you expect to see?

When using golang.org/x/sync/singleflight.Group in a chain of http.Handler implementations where a panic(http.ErrAbortHandler is called downstream, net/http.conn.serve is able to unwrap errors to determine if the error is actually http.ErrAbortHandler and handle the panic without allowing the program to crash.

What did you see instead?

The program crashes with the message panic: net/http: abort Handler.

This issue is related to #62510