time: LoadLocationFromTZData with slim tzdata uses wrong cached zone #42216
Description
What version of Go are you using (go version)?
$ go version go version devel +146dd2e134 Mon Oct 26 13:44:44 2020 +0100 darwin/amd64
Does this issue reproduce with the latest release?
Yes, it can be reproduced with 1.15: see #42138 .
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/Users/hlubek/Library/Caches/go-build" GOENV="/Users/hlubek/Library/Application Support/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOINSECURE="" GOMODCACHE="/Users/hlubek/Dev/Go/local/pkg/mod" GONOPROXY="" GONOSUMDB="" GOOS="darwin" GOPATH="/Users/hlubek/Dev/Go/local" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/Users/hlubek/Dev/Go/tip" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/Users/hlubek/Dev/Go/tip/pkg/tool/darwin_amd64" GCCGO="gccgo" AR="ar" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD="/dev/null" 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/tq/c8vr5k7928n1vpxb0cdw276c0000gn/T/go-build497574509=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
- Compile Go binary (without embedded
tzdata) - Add to Alpine Docker image with installed
tzdatapackage (which uses slim zoneinfo with IANA 2020b) - Times in e.g. Europe/Berlin have wrong zone (CEST instead of CET) after October, 25th
See https://gist.github.com/hlubek/f46a73bc9d150cf1f2af585b0849e3d9 on how to reproduce.
What did you expect to see?
The correct time zone for the dates in CET.
What did you see instead?
The zone CEST.
Notes
It seems to be the case, that the cached zone which is calculated from the transitions in LoadLocationFromTZData just uses the last transition zone instead of checking for the correct zone from the result of tzset when using the extend data from the time zone file.