Add non-mmap fallback for unsupported file systems (jffs2)

bolt_openbsd.go

@@ -9,7 +9,7 @@ func msync(db *DB) error {
 }
 
 func fdatasync(db *DB) error {
-	if db.data != nil {
+	if db.data != nil && !db.mmapFallback {
 		return msync(db)
 	}
 	return db.file.Sync()

db.go

@@ -116,15 +116,16 @@ type DB struct {
 	// `dataref` isn't used at all on Windows, and the golangci-lint
 	// always fails on Windows platform.
 	//nolint
-	dataref  []byte // mmap'ed readonly, write throws SEGV
-	data     *[maxMapSize]byte
-	datasz   int
-	meta0    *common.Meta
-	meta1    *common.Meta
-	pageSize int
-	opened   bool
-	rwtx     *Tx
-	txs      []*Tx
+	dataref      []byte // mmap'ed readonly, write throws SEGV
+	data         *[maxMapSize]byte
+	datasz       int
+	meta0        *common.Meta
+	meta1        *common.Meta
+	pageSize     int
+	opened       bool
+	mmapFallback bool
+	rwtx         *Tx
+	txs          []*Tx
 
 	freelist     *freelist
 	freelistLoad sync.Once
@@ -450,7 +451,13 @@ func (db *DB) mmap(minsz int) (err error) {
 	// gofail: var mapError string
 	// return errors.New(mapError)
 	if err = mmap(db, size); err != nil {
-		return err
+		if !isMmapUnsupported(err) {
+			return err
+		}
+		mmapErr := err
+		if err = mmapFallback(db, size); err != nil {
+			return fmt.Errorf("mmap unsupported: %v; fallback error: %w", mmapErr, err)
+		}
 	}
 
 	// Perform unmmap on any error to reset all data fields:
@@ -490,6 +497,7 @@ func (db *DB) invalidate() {
 	db.dataref = nil
 	db.data = nil
 	db.datasz = 0
+	db.mmapFallback = false
 
 	db.meta0 = nil
 	db.meta1 = nil
@@ -501,13 +509,31 @@ func (db *DB) munmap() error {
 
 	// gofail: var unmapError string
 	// return errors.New(unmapError)
-	if err := munmap(db); err != nil {
+	var err error
+	if db.mmapFallback {
+		err = munmapFallback(db)
+	} else {
+		err = munmap(db)
+	}
+	if err != nil {
 		return fmt.Errorf("unmap error: " + err.Error())
 	}
 
 	return nil
 }
 
+// writeAt writes bytes to the backing file and, in fallback mode, mirrors the
+// write into the heap buffer so the in-memory page view stays consistent.
+func (db *DB) writeAt(b []byte, off int64) (int, error) {
+	n, err := db.ops.writeAt(b, off)
+	if n > 0 && db.mmapFallback {
+		db.mmaplock.Lock()
+		db.copyToMmapFallback(b[:n], off)
+		db.mmaplock.Unlock()
+	}
+	return n, err
+}
+
 // mmapSize determines the appropriate size for the mmap given the current size
 // of the database. The minimum size is 32KB and doubles until it reaches 1GB.
 // Returns an error if the new mmap size is greater than the max allowed.
@@ -607,7 +633,7 @@ func (db *DB) init() error {
 	p.SetCount(0)
 
 	// Write the buffer to our data file.
-	if _, err := db.ops.writeAt(buf, 0); err != nil {
+	if _, err := db.writeAt(buf, 0); err != nil {
 		return err
 	}
 	if err := fdatasync(db); err != nil {

mmap_error_unix.go

@@ -0,0 +1,19 @@
+//go:build !windows && !plan9
+
+package bbolt
+
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+)
+
+// isMmapUnsupported reports whether err indicates the underlying file system
+// does not support mmap (e.g. jffs2 returns EINVAL for MAP_SHARED).
+func isMmapUnsupported(err error) bool {
+	return errors.Is(err, unix.ENOSYS) ||
+		errors.Is(err, unix.ENODEV) ||
+		errors.Is(err, unix.EOPNOTSUPP) ||
+		errors.Is(err, unix.ENOTSUP) ||
+		errors.Is(err, unix.EINVAL)
+}

mmap_error_windows.go

@@ -0,0 +1,12 @@
+package bbolt
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+func isMmapUnsupported(err error) bool {
+	return errors.Is(err, windows.ERROR_INVALID_FUNCTION) ||
+		errors.Is(err, windows.ERROR_NOT_SUPPORTED)
+}

mmap_fallback.go

@@ -0,0 +1,51 @@
+package bbolt
+
+import (
+	"fmt"
+	"io"
+	"unsafe"
+)
+
+// mmapFallback provides a heap-backed mirror of the data file as a substitute
+// for mmap on file systems that do not support memory mapping (e.g. jffs2).
+func mmapFallback(db *DB, sz int) error {
+	b := make([]byte, sz)
+
+	info, err := db.file.Stat()
+	if err != nil {
+		return fmt.Errorf("file stat: %w", err)
+	}
+
+	readSize := int64(sz)
+	if info.Size() < readSize {
+		readSize = info.Size()
+	}
+	if readSize > 0 {
+		if _, err := db.file.ReadAt(b[:readSize], 0); err != nil && err != io.EOF {
+			return fmt.Errorf("file read: %w", err)
+		}
+	}
+
+	db.dataref = b
+	db.data = (*[maxMapSize]byte)(unsafe.Pointer(&b[0]))
+	db.datasz = sz
+	db.mmapFallback = true
+	return nil
+}
+
+func munmapFallback(db *DB) error {
+	db.dataref = nil
+	db.data = nil
+	db.datasz = 0
+	db.mmapFallback = false
+	return nil
+}
+
+// copyToMmapFallback copies written bytes into the heap mirror so the in-memory
+// page view stays consistent with the file in fallback mode.
+func (db *DB) copyToMmapFallback(b []byte, off int64) {
+	if off < 0 || off >= int64(len(db.dataref)) {
+		return
+	}
+	copy(db.dataref[off:], b)
+}

tx.go

@@ -421,7 +421,7 @@ func (tx *Tx) write() error {
 			}
 			buf := common.UnsafeByteSlice(unsafe.Pointer(p), written, 0, int(sz))
 
-			if _, err := tx.db.ops.writeAt(buf, offset); err != nil {
+			if _, err := tx.db.writeAt(buf, offset); err != nil {
 				return err
 			}
 
@@ -472,7 +472,7 @@ func (tx *Tx) writeMeta() error {
 	tx.meta.Write(p)
 
 	// Write the meta page to file.
-	if _, err := tx.db.ops.writeAt(buf, int64(p.Id())*int64(tx.db.pageSize)); err != nil {
+	if _, err := tx.db.writeAt(buf, int64(p.Id())*int64(tx.db.pageSize)); err != nil {
 		return err
 	}
 	if !tx.db.NoSync || common.IgnoreNoSync {