internal/postgres: create GetLatestMajorVersion in postgres package

This change completes the GetLatestMajorVersion function in the
DataSource interface that retrieves the latest major version of a module
path by adding the implementation for the postgres package.

Updates golang/go#37765

Change-Id: I48eadab8b71ffe73cec49f5c8bcd4f48f13b490d
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/250937
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

internal/postgres/version.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"strings"
 
+	"golang.org/x/mod/module"
 	"golang.org/x/pkgsite/internal"
 	"golang.org/x/pkgsite/internal/derrors"
 	"golang.org/x/pkgsite/internal/version"
@@ -230,3 +231,32 @@ func getPathVersions(ctx context.Context, db *DB, path string, versionTypes ...v
 	}
 	return versions, nil
 }
+
+// GetLatestMajorVersion returns the latest major version string of a module
+// path. For example, in the module path "github.com/casbin/casbin", there
+// is another path with a greater major version
+// "github.com/casbin/casbin/v3". This function will return "/v3" or an
+// empty string if there is no major version string at the end.
+func (db *DB) GetLatestMajorVersion(ctx context.Context, seriesPath string) (_ string, err error) {
+	defer derrors.Wrap(&err, "DB.GetLatestMajorVersion(ctx, %q)", seriesPath)
+
+	var latestPath string
+	latestModulePathQuery := fmt.Sprintf(`
+		SELECT
+			m.module_path
+		FROM
+			modules m
+		WHERE
+			m.series_path = $1
+		%s
+		LIMIT 1;`, orderByLatest)
+	row := db.db.QueryRow(ctx, latestModulePathQuery, seriesPath)
+	if err := row.Scan(&latestPath); err != nil {
+		return "", err
+	}
+	_, majorPath, ok := module.SplitPathVersion(latestPath)
+	if !ok {
+		return "", fmt.Errorf("module.SplitPathVersion(%q): %v", latestPath, majorPath)
+	}
+	return majorPath, nil
+}

internal/postgres/version_test.go

@@ -6,6 +6,8 @@ package postgres
 
 import (
 	"context"
+	"database/sql"
+	"errors"
 	"fmt"
 	"testing"
 
@@ -364,3 +366,52 @@ func TestGetVersions(t *testing.T) {
 		})
 	}
 }
+func TestGetLatestMajorVersion(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), testTimeout)
+	defer cancel()
+
+	defer ResetTestDB(testDB, t)
+	for _, modulePath := range []string{
+		"foo.com/bar",
+		"foo.com/bar/v2",
+		"foo.com/bar/v3",
+		"bar.com/foo",
+	} {
+		m := sample.Module(modulePath, sample.VersionString, sample.Suffix)
+		if err := testDB.InsertModule(ctx, m); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	for _, tc := range []struct {
+		seriesPath  string
+		wantVersion string
+		wantErr     error
+	}{
+		{
+			seriesPath:  "foo.com/bar",
+			wantVersion: "/v3",
+		},
+		{
+			seriesPath:  "bar.com/foo",
+			wantVersion: "",
+		},
+		{
+			seriesPath: "boo.com/far",
+			wantErr:    sql.ErrNoRows,
+		},
+	} {
+		gotVersion, err := testDB.GetLatestMajorVersion(ctx, tc.seriesPath)
+		if err != nil {
+			if tc.wantErr == nil {
+				t.Fatalf("got unexpected error %v", err)
+			}
+			if !errors.Is(err, tc.wantErr) {
+				t.Errorf("got err = %v, want Is(%v)", err, tc.wantErr)
+			}
+		}
+		if gotVersion != tc.wantVersion {
+			t.Errorf("testDB.GetLatestMajorVersion(%v) = %v, want = %v", tc.seriesPath, gotVersion, tc.wantVersion)
+		}
+	}
+}