go/internal/xtesting += ZTestData & friends

ZTestData is Go counterpart of ztestdata on zodbtools/py side from
https://lab.nexedi.com/nexedi/zodbtools/-/blob/f9d36ba7/zodbtools/test/conftest.py#L31-65
that was recently added in nexedi/zodbtools@bf772ce0.

On Go side each ZTestData describes testdata files on the filesystem +
carry arbitrary extra information. For example FileStorage tests will
use it to place there "what is expected to load" information
corresponding to on-filesystem database.

go/internal/xtesting/ztestdata.go

+// Copyright (C) 2024  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package xtesting
+// ZTestData + friends
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"lab.nexedi.com/kirr/neo/go/internal/xmaps"
+)
+
+
+// ZTestData[T] represents one ZODB testdata case.
+//
+// Test files are located at .Prefix on the filesystem and were generated with
+// specified zkind (python major and pickle protocol versions). T represents
+// additional information about test data, for example which values to expect
+// when loading data from ZODB files.
+type ZTestData[T any] struct {
+	Kind   string // e.g. py2_pickle3
+	Prefix string // e.g. testdata/py2_pickle3/
+	Misc   *T     // better embed, but https://github.com/golang/go/issues/49030
+}
+
+
+// ZTestDataRegistry maintains registry of available ZTestData.
+//
+// Each testdata case needs to be registered via .Register method.
+type ZTestDataRegistry[T any] struct {
+	dir map[string/*zkind*/]*ZTestData[T]
+}
+
+// Register adds ZODB testdata case to the registry.
+//
+// ZODB test files are of specified zkind are located under prefix on the filesystem.
+// Additional information about testdata goes in misc.
+//
+// It is invalid to register the same zkind twice.
+func (r *ZTestDataRegistry[T]) Register(zkind, prefix string, misc *T) {
+	if r.dir == nil {
+		r.dir = make(map[string]*ZTestData[T])
+	}
+	_, already := r.dir[zkind]
+	if already {
+		panic(fmt.Sprintf("zkind %q already registered, zkind", zkind))
+	}
+	r.dir[zkind] = &ZTestData[T]{Kind: zkind, Prefix: prefix, Misc: misc}
+}
+
+// RunWithEach tests f with each registered ztestdata separately.
+func (r *ZTestDataRegistry[T]) RunWithEach(t *testing.T, f func(t *testing.T, ztestdata *ZTestData[T])) {
+	for _, zkind := range xmaps.SortedKeys(r.dir) {
+		ztestdata := r.dir[zkind]
+		t.Run(zkind, func(t *testing.T) {
+			f(t, ztestdata)
+		})
+	}
+}
+
+// BenchWithEach benchmarks f with each registered ztestdata separately.
+func (r *ZTestDataRegistry[T]) BenchWithEach(b *testing.B, f func(b *testing.B, ztestdata *ZTestData[T])) {
+	for _, zkind := range xmaps.SortedKeys(r.dir) {
+		ztestdata := r.dir[zkind]
+		b.Run(zkind, func(b *testing.B) {
+			f(b, ztestdata)
+		})
+	}
+}
+
+// Path returns filesystem path for a file located as path under ztestdata prefix.
+func (z *ZTestData[T]) Path(path string) string {
+	return fmt.Sprintf("%s/%s", z.Prefix, path)
+}
+
+
+// LoadZTestData loads ZODB testdata cases from the filesystem.
+//
+// Every subdirectory under dir counts as one testdata case.
+func LoadZTestData(dir string) ZTestDataRegistry[struct{}] {
+	ztestdataReg := ZTestDataRegistry[struct{}]{}
+	zprefixv, err := filepath.Glob(dir + "/[^.]*")
+	if err != nil {
+		panic(err) // the only possible error is ErrBadPattern
+	}
+	for _, zprefix := range zprefixv {
+		i, err := os.Stat(zprefix)
+		if err != nil {
+			continue // same behaviour as glob
+		}
+		if !i.IsDir() {
+			continue
+		}
+		zkind := filepath.Base(zprefix)
+		ztestdataReg.Register(zkind, zprefix, nil)
+	}
+	return ztestdataReg
+}