goproxytest: handle /@latest endpoint

The Go module proxy protocol defines a $base/$module/@latest endpoint
that returns the .info JSON for the latest version of a module.
The handler only matched paths containing /@v/, so /@latest got a 404.

Add a check for the /@latest suffix before the /@v/ routing.
When matched, find the highest release version, falling back to
pre-release versions if no release exists, and serve its .info file.

The version selection is in a standalone latestVersion function
that takes an iter.Seq[string], following the same algorithm as
cue-lang/cue's LatestVersion in internal/mod/modload/query.go.

The test is a direct HTTP unit test rather than a testscript like
testdata/list.txt because the go command does not expose /@latest
responses to the user; it uses the endpoint internally as a fallback.
There is no go subcommand whose output we could assert on in a script
to verify that /@latest is served correctly.

Author: mvdan

goproxytest/proxy.go

@@ -28,6 +28,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/fs"
+	"iter"
 	"log"
 	"net"
 	"net/http"
@@ -148,6 +149,44 @@ func (srv *Server) handler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	path := strings.TrimPrefix(r.URL.Path, "/mod/")
+	if enc, ok := strings.CutSuffix(path, "/@latest"); ok {
+		modPath, err := module.UnescapePath(enc)
+		if err != nil {
+			srv.logf("go proxy_test: %v\n", err)
+			http.NotFound(w, r)
+			return
+		}
+		best := latestVersion(func(yield func(string) bool) {
+			for _, m := range srv.modList {
+				if m.Path != modPath {
+					continue
+				}
+				if err := module.Check(m.Path, m.Version); err != nil {
+					continue
+				}
+				if !yield(m.Version) {
+					return
+				}
+			}
+		})
+		if best == "" {
+			http.NotFound(w, r)
+			return
+		}
+		a := srv.readArchive(modPath, best)
+		if a == nil {
+			http.NotFound(w, r)
+			return
+		}
+		for _, f := range a.Files {
+			if f.Name == ".info" {
+				w.Write(f.Data)
+				return
+			}
+		}
+		http.NotFound(w, r)
+		return
+	}
 	i := strings.Index(path, "/@v/")
 	if i < 0 {
 		http.NotFound(w, r)
@@ -347,3 +386,22 @@ func (srv *Server) readArchive(path, vers string) *txtar.Archive {
 	}).(*txtar.Archive)
 	return a
 }
+
+// latestVersion returns the latest version from the given sequence,
+// preferring release versions over pre-release versions.
+// It returns the empty string if the sequence is empty.
+func latestVersion(versions iter.Seq[string]) string {
+	var bestRelease, bestAny string
+	for v := range versions {
+		if semver.Prerelease(v) == "" && (bestRelease == "" || semver.Compare(v, bestRelease) > 0) {
+			bestRelease = v
+		}
+		if bestAny == "" || semver.Compare(v, bestAny) > 0 {
+			bestAny = v
+		}
+	}
+	if bestRelease != "" {
+		return bestRelease
+	}
+	return bestAny
+}

goproxytest/proxy_test.go

@@ -5,6 +5,9 @@
 package goproxytest_test
 
 import (
+	"encoding/json"
+	"io"
+	"net/http"
 	"path/filepath"
 	"testing"
 
@@ -77,3 +80,60 @@ func TestSetup(t *testing.T) {
 		testscript.Run(t, p)
 	})
 }
+
+func TestLatest(t *testing.T) {
+	srv := goproxytest.NewTestServer(t, filepath.Join("testdata", "mod"), "")
+
+	tests := []struct {
+		name   string
+		module string
+		want   string
+	}{
+		{
+			name:   "release preferred over prerelease",
+			module: "fruit.com",
+			want:   "v1.1.0",
+		},
+		{
+			name:   "prerelease only",
+			module: "prerelease.example",
+			want:   "v0.2.0-rc.1",
+		},
+		{
+			name:   "unknown module",
+			module: "noexist.example",
+			want:   "",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			resp, err := http.Get(srv.URL + "/" + test.module + "/@latest")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer resp.Body.Close()
+			if test.want == "" {
+				if resp.StatusCode != 404 {
+					t.Fatalf("got status %d, want 404", resp.StatusCode)
+				}
+				return
+			}
+			if resp.StatusCode != 200 {
+				t.Fatalf("got status %d, want 200", resp.StatusCode)
+			}
+			body, err := io.ReadAll(resp.Body)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var info struct {
+				Version string
+			}
+			if err := json.Unmarshal(body, &info); err != nil {
+				t.Fatalf("invalid JSON response: %v", err)
+			}
+			if info.Version != test.want {
+				t.Fatalf("got version %q, want %q", info.Version, test.want)
+			}
+		})
+	}
+}

goproxytest/testdata/mod/fruit.com_v1.2.0-beta.1.txt

@@ -0,0 +1,13 @@
+-- .mod --
+module fruit.com
+
+-- .info --
+{"Version":"v1.2.0-beta.1","Time":"2018-10-22T18:45:39Z"}
+
+-- go.mod --
+module fruit.com
+
+-- fruit/fruit.go --
+package main
+
+const Name = "Grape"

goproxytest/testdata/mod/prerelease.example_v0.1.0-alpha.1.txt

@@ -0,0 +1,11 @@
+-- .mod --
+module prerelease.example
+
+-- .info --
+{"Version":"v0.1.0-alpha.1","Time":"2018-10-22T18:45:39Z"}
+
+-- go.mod --
+module prerelease.example
+
+-- x.go --
+package x

goproxytest/testdata/mod/prerelease.example_v0.2.0-rc.1.txt

@@ -0,0 +1,11 @@
+-- .mod --
+module prerelease.example
+
+-- .info --
+{"Version":"v0.2.0-rc.1","Time":"2018-10-22T18:45:39Z"}
+
+-- go.mod --
+module prerelease.example
+
+-- x.go --
+package x