fix : read only user should be able to open db on ReadOnly mode (#2268)

**Description**

Fixes #2234
 
Help appreciated 😄 
Step to reproduce : 
- Open a db and fill it with some data
- run `chmod -w ./db/*`
- Open it again in `WithReadOnly` mode

**Checklist**

- [x] Code compiles correctly and linting passes locally
- [x] Tests added for new functionality, or regression tests for bug
fixes added as applicable

Author: Naelpuissant

db.go

@@ -378,7 +378,7 @@ func Open(opt Options) (*DB, error) {
 	db.closers.writes = z.NewCloser(1)
 	go db.doWrites(db.closers.writes)
 
-	if !db.opt.InMemory {
+	if !db.opt.InMemory && !db.opt.ReadOnly {
 		db.closers.valueGC = z.NewCloser(1)
 		go db.vlog.waitOnGC(db.closers.valueGC)
 	}
@@ -538,7 +538,7 @@ func (db *DB) close() (err error) {
 	db.blockWrites.Store(1)
 	db.isClosed.Store(1)
 
-	if !db.opt.InMemory {
+	if db.closers.valueGC != nil {
 		// Stop value GC first.
 		db.closers.valueGC.SignalAndWait()
 	}
@@ -671,8 +671,8 @@ const (
 // Sync syncs database content to disk. This function provides
 // more control to user to sync data whenever required.
 func (db *DB) Sync() error {
-	if db.opt.InMemory {
-		// InMemory mode does not use WAL/vlog files, so Sync is a no-op.
+	if db.opt.InMemory || db.opt.ReadOnly {
+		// InMemory and read-only modes do not use WAL/vlog files, so Sync is a no-op.
 		return nil
 	}
 
@@ -1235,6 +1235,9 @@ func (db *DB) RunValueLogGC(discardRatio float64) error {
 	if db.opt.InMemory {
 		return ErrGCInMemoryMode
 	}
+	if db.opt.ReadOnly {
+		return ErrGCInReadOnlyMode
+	}
 	if discardRatio >= 1.0 || discardRatio <= 0.0 {
 		return ErrInvalidRequest
 	}
@@ -1355,6 +1358,9 @@ func (db *DB) GetSequence(key []byte, bandwidth uint64) (*Sequence, error) {
 	if db.opt.managedTxns {
 		panic("Cannot use GetSequence with managedDB=true.")
 	}
+	if db.opt.ReadOnly {
+		panic("Cannot use GetSequence in read-only mode.")
+	}
 
 	switch {
 	case len(key) == 0:
@@ -1563,6 +1569,9 @@ func (db *DB) startMemoryFlush() {
 // stopped. Ideally, no writes are going on during Flatten. Otherwise, it would create competition
 // between flattening the tree and new tables being created at level zero.
 func (db *DB) Flatten(workers int) error {
+	if db.opt.ReadOnly {
+		panic("Cannot flatten in read-only mode.")
+	}
 
 	db.stopCompactions()
 	defer db.startCompactions()
@@ -1851,6 +1860,9 @@ func (db *DB) BanNamespace(ns uint64) error {
 	if db.opt.NamespaceOffset < 0 {
 		return ErrNamespaceMode
 	}
+	if db.opt.ReadOnly {
+		panic("Cannot ban namespace in read-only mode.")
+	}
 	db.opt.Infof("Banning namespace: %d", ns)
 	// First set the banned namespaces in DB and then update the in-memory structure.
 	key := y.KeyWithTs(append(bannedNsKey, y.U64ToBytes(ns)...), 1)

db_test.go

@@ -11,6 +11,7 @@ import (
 	"encoding/binary"
 	"flag"
 	"fmt"
+	"io/fs"
 	"math"
 	"math/rand"
 	"os"
@@ -1742,8 +1743,6 @@ func TestTestSequence2(t *testing.T) {
 }
 
 func TestReadOnly(t *testing.T) {
-	t.Skipf("TODO: ReadOnly needs truncation, so this fails")
-
 	dir, err := os.MkdirTemp("", "badger-test")
 	require.NoError(t, err)
 	defer removeDir(dir)
@@ -1771,12 +1770,10 @@ func TestReadOnly(t *testing.T) {
 	opts.ReadOnly = true
 	kv1, err := Open(opts)
 	require.NoError(t, err)
-	defer kv1.Close()
 
 	// Open another read-only
 	kv2, err := Open(opts)
 	require.NoError(t, err)
-	defer kv2.Close()
 
 	// Attempt a read-write open while it's open for read-only
 	opts.ReadOnly = false
@@ -1793,6 +1790,10 @@ func TestReadOnly(t *testing.T) {
 	require.Equal(t, b1, []byte("value1"))
 	err = txn1.Commit()
 	require.NoError(t, err)
+	err = kv1.RunValueLogGC(0.5)
+	require.Error(t, err, ErrGCInReadOnlyMode)
+	err = kv1.Sync()
+	require.NoError(t, err)
 
 	// Get a thing from the DB via the other connection
 	txn2 := kv2.NewTransaction(true)
@@ -1811,6 +1812,32 @@ func TestReadOnly(t *testing.T) {
 	require.Contains(t, err.Error(), "No sets or deletes are allowed in a read-only transaction")
 	err = txn.Commit()
 	require.NoError(t, err)
+
+	// Close
+	require.NoError(t, kv1.Close())
+	require.NoError(t, kv2.Close())
+
+	// Test os permission read-only open
+	err = filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+		require.NoError(t, err)
+		if path == dir {
+			return os.Chmod(path, 0o500)
+		}
+		return os.Chmod(path, 0o400)
+	})
+	require.NoError(t, err)
+
+	opts.ReadOnly = true
+	kv3, err := Open(opts)
+	require.NoError(t, err)
+	txn3 := kv3.NewTransaction(true)
+	_, err = txn3.Get([]byte("key1"))
+	require.NoError(t, err)
+	require.NoError(t, txn3.Commit())
+	require.NoError(t, kv3.Close())
+
+	// Restore permissions for cleanup
+	require.NoError(t, os.Chmod(dir, 0o700))
 }
 
 func TestLSMOnly(t *testing.T) {
@@ -1845,7 +1872,6 @@ func TestLSMOnly(t *testing.T) {
 		txnSet(t, db, []byte(fmt.Sprintf("key%d", i)), value, 0x00)
 	}
 	require.NoError(t, db.Close())
-
 }
 
 // This test function is doing some intricate sorcery.

errors.go

@@ -109,6 +109,9 @@ var (
 	// ErrGCInMemoryMode is returned when db.RunValueLogGC is called in in-memory mode.
 	ErrGCInMemoryMode = stderrors.New("Cannot run value log GC when DB is opened in InMemory mode")
 
+	// ErrGCInReadOnlyMode is returned when db.RunValueLogGC is called in read-only mode.
+	ErrGCInReadOnlyMode = stderrors.New("Cannot run value log GC when DB is opened in ReadOnly mode")
+
 	// ErrDBClosed is returned when a get operation is performed after closing the DB.
 	ErrDBClosed = stderrors.New("DB Closed")
 )

value.go

@@ -544,6 +544,10 @@ func (vlog *valueLog) init(db *DB) {
 	}
 	vlog.dirPath = vlog.opt.ValueDir
 
+	if vlog.opt.ReadOnly {
+		return
+	}
+
 	vlog.garbageCh = make(chan struct{}, 1) // Only allow one GC at a time.
 	lf, err := InitDiscardStats(vlog.opt)
 	y.Check(err)
@@ -575,14 +579,17 @@ func (vlog *valueLog) open(db *DB) error {
 		lf, ok := vlog.filesMap[fid]
 		y.AssertTrue(ok)
 
-		// Just open in RDWR mode. This should not create a new log file.
 		lf.opt = vlog.opt
-		if err := lf.open(vlog.fpath(fid), os.O_RDWR,
+		flags := os.O_RDWR
+		if vlog.opt.ReadOnly {
+			flags = os.O_RDONLY
+		}
+		if err := lf.open(vlog.fpath(fid), flags,
 			2*vlog.opt.ValueLogFileSize); err != nil {
 			return y.Wrapf(err, "Open existing file: %q", lf.path)
 		}
 		// We shouldn't delete the maxFid file.
-		if lf.size.Load() == vlogHeaderSize && fid != vlog.maxFid {
+		if lf.size.Load() == vlogHeaderSize && fid != vlog.maxFid && !vlog.opt.ReadOnly {
 			vlog.opt.Infof("Deleting empty file: %s", lf.path)
 			if err := lf.Delete(); err != nil {
 				return y.Wrapf(err, "while trying to delete empty file: %s", lf.path)
@@ -1093,7 +1100,7 @@ func (vlog *valueLog) runGC(discardRatio float64) error {
 }
 
 func (vlog *valueLog) updateDiscardStats(stats map[uint32]int64) {
-	if vlog.opt.InMemory {
+	if vlog.opt.InMemory || vlog.opt.ReadOnly {
 		return
 	}
 	for fid, discard := range stats {