net: lookupIPAddr mapErr causes panic

[lyouthzzz]

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

$ go version
go1.18.10

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
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_arm64"
GOVCS=""
GOVERSION="go1.18.10"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3128295910=/tmp/go-build -gno-record-gcc-switches"

What did you do?

parse dns domain when send http request

What did you expect to see?

no panic

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference

 0  0x000000000046d1e1 in runtime.raise
    at /usr/local/go/src/runtime/sys_linux_amd64.s:168
 1  0x000000000044e8a5 in runtime.dieFromSignal
    at /usr/local/go/src/runtime/signal_unix.go:852
 2  0x000000000044f276 in runtime.sigfwdgo
    at /usr/local/go/src/runtime/signal_unix.go:1066
 3  0x000000000044d607 in runtime.sigtrampgo
    at /usr/local/go/src/runtime/signal_unix.go:430
 4  0x000000000046d1e1 in runtime.raise
    at /usr/local/go/src/runtime/sys_linux_amd64.s:167
 5  0x000000000044e8a5 in runtime.dieFromSignal
    at /usr/local/go/src/runtime/signal_unix.go:852
 6  0x0000000000438969 in runtime.crash
    at /usr/local/go/src/runtime/signal_unix.go:944
 7  0x0000000000438969 in runtime.fatalpanic
    at /usr/local/go/src/runtime/panic.go:1092
 8  0x0000000000438137 in runtime.gopanic
    at /usr/local/go/src/runtime/panic.go:941
 9  0x000000000044e77d in runtime.panicmem
    at /usr/local/go/src/runtime/panic.go:220
10  0x000000000044e77d in runtime.sigpanic
    at /usr/local/go/src/runtime/signal_unix.go:818
11  0x00000000005306b3 in net.(*Resolver).lookupIPAddr
    at /usr/local/go/src/net/lookup.go:344
12  0x000000000052d99a in net.(*Resolver).internetAddrList
    at /usr/local/go/src/net/ipsock.go:288
13  0x00000000005151bb in net.(*Resolver).resolveAddrList
    at /usr/local/go/src/net/dial.go:221
14  0x0000000000516128 in net.(*Dialer).DialContext
    at /usr/local/go/src/net/dial.go:406
15  0x000000000072b6db in net.(*Dialer).DialContext-fm
    at <autogenerated>:1
16  0x000000000071509a in net/http.(*Transport).dial
    at /usr/local/go/src/net/http/transport.go:1169
17  0x00000000007181bf in net/http.(*Transport).dialConn
    at /usr/local/go/src/net/http/transport.go:1607
18  0x0000000000716a50 in net/http.(*Transport).dialConnFor
    at /usr/local/go/src/net/http/transport.go:1449
19  0x000000000071696a in net/http.(*Transport).queueForDial.func1
    at /usr/local/go/src/net/http/transport.go:1418
20  0x000000000046b8a1 in runtime.goexit
    at /usr/local/go/src/runtime/asm_amd64.s:1571

[lyouthzzz]

It's similar to this issue. #60029

[lyouthzzz]

Ctx is done and then ctx.Error() returns nil. It's a random occurrence in our production environment. we use ip to avoid resolving domain，but this is not a long-term solution. we use custom context,but we can promise the context has no problem.

[bradfitz]

Ctx is done and then ctx.Error() returns nil. [...] we use custom context

That's not a valid implementation of context.Context. The interface docs say:

https://pkg.go.dev/context#Context

// If Done is closed, Err returns a non-nil error explaining why:

Therefore I don't think there's anything to fix on Go's side.

[bradfitz]

Sorry, I hit the wrong button closing. Reopening. But I still don't know what we'd fix.

[bcmills]

On the one hand, mapErr could perhaps panic with a more helpful message in this case.

On the other hand, in general Go APIs do panic if the program violates invariants that are supposed to always hold in a correct program.

If you have a Context implementation that may close its Done channel but return nil from Err, that's not a valid implementation and it's likely to break way more than just the net package. Fundamentally you need to fix your Context implementation.

[lyouthzzz]

On the one hand, mapErr could perhaps panic with a more helpful message in this case.

On the other hand, in general Go APIs do panic if the program violates invariants that are supposed to always hold in a correct program.

If you have a Context implementation that may close its Done channel but return nil from Err, that's not a valid implementation and it's likely to break way more than just the net package. Fundamentally you need to fix your Context implementation.

Thank you for your patient reply. I will colse my issue.