determine_go_toolchain.sh

# This function helps to set proper Go version for a project, leveraging Go's recent
# GOTOOLCHAIN environment variable support. However, unlike rustup, Go does not attempt
# to download older toolchain versions automatically, and must be forced to do so.
# For example, project specifying `go 1.24` and current toolchain being `go 1.22` does
# download a new version, but not vice-versa.
# However for some projects, having older Go is mandatory, especially with dependencies like:
# - older quic-go, which was tied to a specific Go compiler version
# - other dependencies relying on Go internals
# Requires bash, curl, jq, sed

determine_go_toolchain () {
    local gomod="${1:-go.mod}"
    local use_toolchain_directive="${2:-1}"

    local toolchain_version=""
    if (( use_toolchain_directive )); then
        toolchain_version="$(sed -n -E '/^toolchain/s/^\w+\s+(go.*)$/\1/p' < "${gomod}")"
    fi

    if [ -n "${toolchain_version}" ]; then
        echo >&2 "using '${toolchain_version}' as specified by '${gomod}'"
    else
        local go_version
        local releases

        # fall back to finding a latest patch version for given minor.major
        go_version="$(sed -n -E '/^go\s+/s/^go\s+(.+)/\1/p' < "${gomod}")"

        echo >&2 "project specifies go version '${go_version}', finding relevant release"

        releases="$(curl -s -G -d "mode=json" -d "include=all" 'https://go.dev/dl/')"
        toolchain_version="$(jq -cer --arg "wanted" "${go_version}" '[.[] | select(.version | startswith("go\($wanted)")) | .version][0]' <<< "${releases}")"

        echo >&2 "using '${toolchain_version}' as resolved from releases"
    fi

    printf -- '%s' "${toolchain_version}"
}

export GOTOOLCHAIN="$(determine_go_toolchain)"
export GOTOOLCHAIN="$(determine_go_toolchain go.mod 1)"
export GOTOOLCHAIN="$(determine_go_toolchain go.mod 0)"