net/http: context deadline can cause surprising errors

[jhump]

Go version

go1.21.4

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

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/jhumphries/Library/Caches/go-build'
GOENV='/Users/jhumphries/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/jhumphries/go/pkg/mod'
GONOPROXY='buf.build/gen/go,github.com/bufbuild/buf-lang'
GONOSUMDB='buf.build/gen/go,github.com/bufbuild/buf-lang'
GOOS='darwin'
GOPATH='/Users/jhumphries/go'
GOPRIVATE='buf.build/gen/go,github.com/bufbuild/buf-lang'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.21.4/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.21.4/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.4'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/jhumphries/src/connect-go/internal/conformance/go.mod'
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 -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/vy/ws2js8yx4n581x_g8m_cs9jw0000gn/T/go-build1785425850=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

Used a context with a timeout when using http.Client.Do

What did you expect to see?

I would expect any error returned from Do that was caused by the deadline elapsing be identifiable using errors.Is(err, context.DeadlineExceed).

What did you see instead?

There are certain places where dialing can fail due to the deadline that return an underlying error of type internal/poll.DeadlineExceededError. Unlike net.errTimeout, this types does not implement the Is(error) bool method to consider itself equivalent to context.DeadlineExceeded.

So callers must check against both context.DeadlineExceeded and os.ErrDeadlineExceeded in order to determine if the returned error was due to the context deadline.

[dmitshur]

CC @neild.

[gopherbot]

Change https://go.dev/cl/546275 mentions this issue: internal/poll: DeadlineExceededError s/b compatible with context.DeadlineExceeded