net/url: roundtripping url which contains certain special characters (`<>{} "`) fails

[JakeChampion]

Go version

go version go1.24.0 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-L/opt/homebrew/opt/openssl@3/lib -L/opt/homebrew/opt/re2/lib'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/jakechampion/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/jakechampion/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/8c/21q0xn6s5d34rp637j4s10f40000gn/T/go-build2305439960=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/jakechampion/code/netlify/stargate/go.mod'
GOMODCACHE='/Users/jakechampion/go/pkg/mod'
GONOPROXY='github.com/netlify'
GONOSUMDB='github.com/netlify'
GOOS='darwin'
GOPATH='/Users/jakechampion/go'
GOPRIVATE='github.com/netlify'
GOPROXY='https://proxy.golang.org/,direct'
GOROOT='/Users/jakechampion/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.0.darwin-arm64'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/jakechampion/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/jakechampion/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.0.darwin-arm64/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

https://go.dev/play/p/aFC9TP6bWBI

What did you see happen?

The output from the playground application:

http://www.google.com/foo<FOO/BAR%2Fbar.txt /foo%3CFOO/BAR/bar.txt http://www.google.com/foo%3CFOO/BAR/bar.txt
http://www.google.com/foo>FOO/BAR%2Fbar.txt /foo%3EFOO/BAR/bar.txt http://www.google.com/foo%3EFOO/BAR/bar.txt
http://www.google.com/foo{FOO/BAR%2Fbar.txt /foo%7BFOO/BAR/bar.txt http://www.google.com/foo%7BFOO/BAR/bar.txt
http://www.google.com/foo}FOO/BAR%2Fbar.txt /foo%7DFOO/BAR/bar.txt http://www.google.com/foo%7DFOO/BAR/bar.txt
http://www.google.com/foo FOO/BAR%2Fbar.txt /foo%20FOO/BAR/bar.txt http://www.google.com/foo%20FOO/BAR/bar.txt
http://www.google.com/foo"FOO/BAR%2Fbar.txt /foo%22FOO/BAR/bar.txt http://www.google.com/foo%22FOO/BAR/bar.txt

What did you expect to see?

I expected to see

http://www.google.com/foo<FOO/BAR%2Fbar.txt /foo%3CFOO/BAR%2Fbar.txt http://www.google.com/foo%3CFOO/BAR%2Fbar.txt
http://www.google.com/foo>FOO/BAR%2Fbar.txt /foo%3EFOO/BAR%2Fbar.txt http://www.google.com/foo%3EFOO/BAR%2Fbar.txt
http://www.google.com/foo{FOO/BAR%2Fbar.txt /foo%7BFOO/BAR%2Fbar.txt http://www.google.com/foo%7BFOO/BAR%2Fbar.txt
http://www.google.com/foo}FOO/BAR%2Fbar.txt /foo%7DFOO/BAR%2Fbar.txt http://www.google.com/foo%7DFOO/BAR%2Fbar.txt
http://www.google.com/foo FOO/BAR%2Fbar.txt /foo%20FOO/BAR%2Fbar.txt http://www.google.com/foo%20FOO/BAR%2Fbar.txt
http://www.google.com/foo"FOO/BAR%2Fbar.txt /foo%22FOO/BAR%2Fbar.txt http://www.google.com/foo%22FOO/BAR%2Fbar.txt

[seankhliao]

I believe this is working as intended. We only guarantee that the output is a valid encoding of the url, not that the exact input will be returned.

[JakeChampion]

@seankhliao I understand, however the returned encoding is semantically different to the input encoding, due to the path segment BAR%2Fbar.txt being transformed into two path segments BAR/bar.txt, this incorrect transformation is also true for the EscapedPath() method, which the Go Playground link in the original issue comment shows. The documentation states that:

This distinction is rarely important, but when it is, the code should use the [URL.EscapedPath] method, which preserves the original encoding of Path`

[seankhliao]

According to RFC 3986, we consider them semantically equivalent https://datatracker.ietf.org/doc/html/rfc3986#section-6.2.2.2

Our position remains unchanged from #14815 (comment) , use RawPath if you wish to use some half decoded form, otherwise Path / EscapedPath will produce fully unescaped or escaped forms.

[JakeChampion]

From datatracker.ietf.org/doc/html/rfc3986#section-6.2.2.2:

These URIs should be normalized by decoding any percent-encoded octet that corresponds to an unreserved character, as described in Section 2.3.

The character in question here is / (percent encoded as %2F) which is not within the unreserved charactes and so should not be getting percent decoded here - however we can see that EscapedPath() may sometimes decode the %2F as shown by the example application