db: Add Chisel DB v0.1 skeleton

This commit adds the skeleton db package that will grow into the full
Chisel DB implementation with forthcoming commits. The package contains
only New(), Save(), and Load() functions for creating, writing, and
reading the database, respectively.

The API exposes underlying jsonwall interfaces for both reading and
writing. An attempt was made to build an abstraction on top of jsonwall,
but it was later abandoned because of Go type system limitations and
difficulty to test.

The schema is set to 0.1 when writing. An error is returned when the
schema isn't 0.1 when reading. The schema and database location in the
output root directory are implementation details that are not exposed by
the API.

internal/db/db.go

@@ -0,0 +1,80 @@
+package db
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/canonical/chisel/internal/jsonwall"
+	"github.com/klauspost/compress/zstd"
+)
+
+const schema = "0.1"
+
+// New creates a new Chisel DB writer with the proper schema.
+func New() *jsonwall.DBWriter {
+	options := jsonwall.DBWriterOptions{Schema: schema}
+	return jsonwall.NewDBWriter(&options)
+}
+
+func getDBPath(root string) string {
+	return filepath.Join(root, ".chisel.db")
+}
+
+// Save uses the provided writer dbw to write the Chisel DB into the standard
+// path under the provided root directory.
+func Save(dbw *jsonwall.DBWriter, root string) (err error) {
+	dbPath := getDBPath(root)
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("cannot save state to %q: %w", dbPath, err)
+		}
+	}()
+	f, err := os.OpenFile(dbPath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+	if err != nil {
+		return
+	}
+	defer f.Close()
+	// chmod the existing file
+	if err = f.Chmod(0644); err != nil {
+		return
+	}
+	zw, err := zstd.NewWriter(f)
+	if err != nil {
+		return
+	}
+	if _, err = dbw.WriteTo(zw); err != nil {
+		return
+	}
+	return zw.Close()
+}
+
+// Load reads a Chisel DB from the standard path under the provided root
+// directory. If the Chisel DB doesn't exist, the returned error satisfies
+// errors.Is(err, fs.ErrNotExist))
+func Load(root string) (db *jsonwall.DB, err error) {
+	dbPath := getDBPath(root)
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("cannot load state from %q: %w", dbPath, err)
+		}
+	}()
+	f, err := os.Open(dbPath)
+	if err != nil {
+		return
+	}
+	defer f.Close()
+	zr, err := zstd.NewReader(f)
+	if err != nil {
+		return
+	}
+	defer zr.Close()
+	db, err = jsonwall.ReadDB(zr)
+	if err != nil {
+		return nil, err
+	}
+	if s := db.Schema(); s != schema {
+		return nil, fmt.Errorf("invalid schema %#v", s)
+	}
+	return
+}

internal/db/db_test.go

@@ -0,0 +1,74 @@
+package db_test
+
+import (
+	"sort"
+
+	"github.com/canonical/chisel/internal/db"
+	. "gopkg.in/check.v1"
+)
+
+type testEntry struct {
+	S string          `json:"s,omitempty"`
+	I int64           `json:"i,omitempty"`
+	L []string        `json:"l,omitempty"`
+	M map[string]bool `json:"m,omitempty"`
+}
+
+var saveLoadTestCase = []testEntry{
+	{"", 0, nil, nil},
+	{"hello", -1, nil, nil},
+	{"", 0, nil, nil},
+	{"", 100, []string{"a", "b"}, nil},
+	{"", 0, nil, map[string]bool{"a": true, "b": false}},
+	{"abc", 123, []string{"foo", "bar"}, nil},
+}
+
+func (s *S) TestSaveLoadRoundTrip(c *C) {
+	// To compare expected and obtained entries we first wrap the original
+	// entries in wrappers with increasing K. When we read the wrappers back
+	// they may be in different order because jsonwall sorts them serialized
+	// as JSON. So we sort them by K to compare them in the original order.
+
+	type wrapper struct {
+		// test values
+		testEntry
+		// sort key for comparison
+		K int `json:"key"`
+	}
+
+	// wrap the entries with increasing K
+	expected := make([]wrapper, len(saveLoadTestCase))
+	for i, entry := range saveLoadTestCase {
+		expected[i] = wrapper{entry, i}
+	}
+
+	workDir := c.MkDir()
+	dbw := db.New()
+	for _, entry := range expected {
+		err := dbw.Add(entry)
+		c.Assert(err, IsNil)
+	}
+	err := db.Save(dbw, workDir)
+	c.Assert(err, IsNil)
+
+	dbr, err := db.Load(workDir)
+	c.Assert(err, IsNil)
+	c.Assert(dbr.Schema(), Equals, db.Schema)
+
+	iter, err := dbr.Iterate(nil)
+	c.Assert(err, IsNil)
+
+	obtained := make([]wrapper, 0, len(expected))
+	for iter.Next() {
+		var wrapped wrapper
+		err := iter.Get(&wrapped)
+		c.Assert(err, IsNil)
+		obtained = append(obtained, wrapped)
+	}
+
+	// sort the entries by K to get the original order
+	sort.Slice(obtained, func(i, j int) bool {
+		return obtained[i].K < obtained[j].K
+	})
+	c.Assert(obtained, DeepEquals, expected)
+}

internal/db/export_test.go

@@ -0,0 +1,3 @@
+package db
+
+var Schema = schema

internal/db/suite_test.go

@@ -0,0 +1,15 @@
+package db_test
+
+import (
+	"testing"
+
+	. "gopkg.in/check.v1"
+)
+
+func Test(t *testing.T) {
+	TestingT(t)
+}
+
+type S struct{}
+
+var _ = Suite(&S{})