redirect mode prepends pathPrefix to redirected URL, causing errors

[chennin]

Describe the bug
With ATHENS_PATH_PREFIX and mode = "redirect" set, the redirected URL includes the path prefix, which is probably invalid.

Error Message

bad request: invalid escaped module path "go/github.com/openshift/api": malformed module path "go/github.com/openshift/api": missing dot in first path element

where "/go" is my path prefix

To Reproduce

Set up Athens with a path prefix, I am using ATHENS_PATH_PREFIX: "/go"

Set a download mode file such as

mode = "sync"
downloadURL = "https://proxy.golang.org"
download "github.com/openshift/api" {
  mode = "redirect"
  downloadURL = "https://proxy.golang.org"
}

with GOPROXY set to the Athens, run go mod download github.com/openshift/api@v3.9.0+incompatible

or, curl the URL

$ curl -Lv "http://MY_ATHENS_HOST/go/github.com/openshift/api/@v/v3.9.0+incompatible.info"

* Host MY_ATHENS_HOST:80 was resolved.
* IPv6: (none)
* IPv4: 10.0.1.3
*   Trying 10.0.1.3:80...
* Connected to MY_ATHENS_HOST (10.0.1.3) port 80
* using HTTP/1.x
> GET /go/github.com/openshift/api/@v/v3.9.0+incompatible.info HTTP/1.1
> Host: MY_ATHENS_HOST
> User-Agent: curl/8.11.1
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 301 Moved Permanently
< Server: nginx
< Date: Sat, 25 Jan 2025 04:14:43 GMT
< Content-Type: application/json; charset=utf-8
< Content-Length: 0
< Connection: keep-alive
< Location: https://proxy.golang.org/go/github.com/openshift/api/@v/v3.9.0+incompatible.info ### <-- includes pathprefix which won't be valid
* Ignoring the response-body
* setting size while ignoring
<
* Connection #0 to host MY_ATHENS_HOST left intact
* Clear auth, redirects to port from 80 to 443
* Issue another request to this URL: 'https://proxy.golang.org/go/github.com/openshift/api/@v/v3.9.0+incompatible.info'
* Host proxy.golang.org:443 was resolved.
* IPv6: 2607:f8b0:4006:809::2011
* IPv4: 142.250.80.113
*   Trying [2607:f8b0:4006:809::2011]:443...
* ALPN: curl offers h2,http/1.1
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: none
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (IN), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384 / x25519 / id-ecPublicKey
* ALPN: server accepted h2
* Server certificate:
*  subject: CN=misc-sni.google.com
*  start date: Jan  6 08:36:24 2025 GMT
*  expire date: Mar 31 08:36:23 2025 GMT
*  subjectAltName: host "proxy.golang.org" matched cert's "*.golang.org"
*  issuer: C=US; O=Google Trust Services; CN=WR2
*  SSL certificate verify ok.
*   Certificate level 0: Public key type EC/prime256v1 (256/128 Bits/secBits), signed using sha256WithRSAEncryption
*   Certificate level 1: Public key type RSA (2048/112 Bits/secBits), signed using sha256WithRSAEncryption
*   Certificate level 2: Public key type RSA (4096/152 Bits/secBits), signed using sha384WithRSAEncryption
* Connected to proxy.golang.org (2607:f8b0:4006:809::2011) port 443
* using HTTP/2
* [HTTP/2] [1] OPENED stream for https://proxy.golang.org/go/github.com/openshift/api/@v/v3.9.0+incompatible.info
* [HTTP/2] [1] [:method: GET]
* [HTTP/2] [1] [:scheme: https]
* [HTTP/2] [1] [:authority: proxy.golang.org]
* [HTTP/2] [1] [:path: /go/github.com/openshift/api/@v/v3.9.0+incompatible.info]
* [HTTP/2] [1] [user-agent: curl/8.11.1]
* [HTTP/2] [1] [accept: */*]
> GET /go/github.com/openshift/api/@v/v3.9.0+incompatible.info HTTP/2
> Host: proxy.golang.org
> User-Agent: curl/8.11.1
> Accept: */*
>
* Request completely sent off
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
< HTTP/2 404
< access-control-allow-origin: *
< content-security-policy-report-only: script-src 'none'; form-action 'none'; frame-src 'none'; report-uri https://csp.withgoogle.com/csp/goa-fa2dfb7c_2
< content-type: text/plain; charset=UTF-8
< cross-origin-opener-policy: same-origin
< vary: Sec-Fetch-Site
< vary: Sec-Fetch-Mode
< vary: Sec-Fetch-Dest
< x-content-type-options: nosniff
< x-frame-options: SAMEORIGIN
< x-xss-protection: 0
< date: Sat, 25 Jan 2025 04:14:44 GMT
< content-length: 158
< alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000
<
* Connection #1 to host proxy.golang.org left intact
bad request: invalid escaped module path "go/github.com/openshift/api": malformed module path "go/github.com/openshift/api": missing dot in first path element

Expected behavior
Athens redirects to a URL without my local path prefix

Environment (please complete the following information):

• OS: Linux 64bit
• Go version : go1.23.5
• Proxy version : v0.15.4
• Storage :fs

Additional information

Attempting to work around a retracted tag openshift/api#1661 where sync does not work