Merge branch 'ro'

5 files changed, 124 insertions, 15 deletions

diff --git a/bolt_unix.go b/bolt_unix.go
index 35dce08..266222a 100644
--- a/bolt_unix.go
+++ b/bolt_unix.go
@@ -11,7 +11,7 @@ import (
 )
 
 // flock acquires an advisory lock on a file descriptor.
-func flock(f *os.File, timeout time.Duration) error {
+func flock(f *os.File, exclusive bool, timeout time.Duration) error {
 	var t time.Time
 	for {
 		// If we're beyond our timeout then return an error.
@@ -21,9 +21,13 @@ func flock(f *os.File, timeout time.Duration) error {
 		} else if timeout > 0 && time.Since(t) > timeout {
 			return ErrTimeout
 		}
+		flag := syscall.LOCK_SH
+		if exclusive {
+			flag = syscall.LOCK_EX
+		}
 
 		// Otherwise attempt to obtain an exclusive lock.
-		err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
+		err := syscall.Flock(int(f.Fd()), flag|syscall.LOCK_NB)
 		if err == nil {
 			return nil
 		} else if err != syscall.EWOULDBLOCK {
@@ -44,7 +48,7 @@ func funlock(f *os.File) error {
 func mmap(db *DB, sz int) error {
 	// Truncate and fsync to ensure file size metadata is flushed.
 	// https://github.com/boltdb/bolt/issues/284
-	if !db.NoGrowSync {
+	if !db.NoGrowSync && !db.readOnly {
 		if err := db.file.Truncate(int64(sz)); err != nil {
 			return fmt.Errorf("file resize error: %s", err)
 		}
diff --git a/bolt_windows.go b/bolt_windows.go
index c8539d4..8b782be 100644
--- a/bolt_windows.go
+++ b/bolt_windows.go
@@ -16,7 +16,7 @@ func fdatasync(db *DB) error {
 }
 
 // flock acquires an advisory lock on a file descriptor.
-func flock(f *os.File, _ time.Duration) error {
+func flock(f *os.File, _ bool, _ time.Duration) error {
 	return nil
 }
 
@@ -28,9 +28,11 @@ func funlock(f *os.File) error {
 // mmap memory maps a DB's data file.
 // Based on: https://github.com/edsrzf/mmap-go
 func mmap(db *DB, sz int) error {
-	// Truncate the database to the size of the mmap.
-	if err := db.file.Truncate(int64(sz)); err != nil {
-		return fmt.Errorf("truncate: %s", err)
+	if !db.readOnly {
+		// Truncate the database to the size of the mmap.
+		if err := db.file.Truncate(int64(sz)); err != nil {
+			return fmt.Errorf("truncate: %s", err)
+		}
 	}
 
 	// Open a file mapping handle.
diff --git a/db.go b/db.go
index 5b905be..c415c1b 100644
--- a/db.go
+++ b/db.go
@@ -104,6 +104,10 @@ type DB struct {
 	ops struct {
 		writeAt func(b []byte, off int64) (n int, err error)
 	}
+
+	// Read only mode.
+	// When true, Update() and Begin(true) return ErrDatabaseReadOnly immediately.
+	readOnly bool
 }
 
 // Path returns the path to currently open database file.
@@ -137,19 +141,28 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	db.MaxBatchSize = DefaultMaxBatchSize
 	db.MaxBatchDelay = DefaultMaxBatchDelay
 
+	flag := os.O_RDWR
+	if options.ReadOnly {
+		flag = os.O_RDONLY
+		db.readOnly = true
+	}
+
 	// Open data file and separate sync handler for metadata writes.
 	db.path = path
-
 	var err error
-	if db.file, err = os.OpenFile(db.path, os.O_RDWR|os.O_CREATE, mode); err != nil {
+	if db.file, err = os.OpenFile(db.path, flag|os.O_CREATE, mode); err != nil {
 		_ = db.close()
 		return nil, err
 	}
 
-	// Lock file so that other processes using Bolt cannot use the database
-	// at the same time. This would cause corruption since the two processes
-	// would write meta pages and free pages separately.
-	if err := flock(db.file, options.Timeout); err != nil {
+	// Lock file so that other processes using Bolt in read-write mode cannot
+	// use the database  at the same time. This would cause corruption since
+	// the two processes would write meta pages and free pages separately.
+	// The database file is locked exclusively (only one process can grab the lock)
+	// if !options.ReadOnly.
+	// The database file is locked using the shared lock (more than one process may
+	// hold a lock at the same time) otherwise (options.ReadOnly is set).
+	if err := flock(db.file, !db.readOnly, options.Timeout); err != nil {
 		_ = db.close()
 		return nil, err
 	}
@@ -359,8 +372,11 @@ func (db *DB) close() error {
 
 	// Close file handles.
 	if db.file != nil {
-		// Unlock the file.
-		_ = funlock(db.file)
+		// No need to unlock read-only file.
+		if !db.readOnly {
+			// Unlock the file.
+			_ = funlock(db.file)
+		}
 
 		// Close the file descriptor.
 		if err := db.file.Close(); err != nil {
@@ -426,6 +442,11 @@ func (db *DB) beginTx() (*Tx, error) {
 }
 
 func (db *DB) beginRWTx() (*Tx, error) {
+	// If the database was opened with Options.ReadOnly, return an error.
+	if db.readOnly {
+		return nil, ErrDatabaseReadOnly
+	}
+
 	// Obtain writer lock. This is released by the transaction when it closes.
 	// This enforces only one writer transaction at a time.
 	db.rwlock.Lock()
@@ -622,6 +643,10 @@ func (db *DB) allocate(count int) (*page, error) {
 	return p, nil
 }
 
+func (db *DB) IsReadOnly() bool {
+	return db.readOnly
+}
+
 // Options represents the options that can be set when opening a database.
 type Options struct {
 	// Timeout is the amount of time to wait to obtain a file lock.
@@ -631,6 +656,10 @@ type Options struct {
 
 	// Sets the DB.NoGrowSync flag before memory mapping the file.
 	NoGrowSync bool
+
+	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
+	// grab a shared lock (UNIX).
+	ReadOnly bool
 }
 
 // DefaultOptions represent the options used if nil options are passed into Open().
diff --git a/db_test.go b/db_test.go
index ad17e87..6af6423 100644
--- a/db_test.go
+++ b/db_test.go
@@ -224,6 +224,76 @@ func TestDB_Open_FileTooSmall(t *testing.T) {
 	equals(t, errors.New("file size too small"), err)
 }
 
+// Ensure that a database can be opened in read-only mode by multiple processes
+// and that a database can not be opened in read-write mode and in read-only
+// mode at the same time.
+func TestOpen_ReadOnly(t *testing.T) {
+	bucket, key, value := []byte(`bucket`), []byte(`key`), []byte(`value`)
+
+	path := tempfile()
+	defer os.Remove(path)
+
+	// Open in read-write mode.
+	db, err := bolt.Open(path, 0666, nil)
+	ok(t, db.Update(func(tx *bolt.Tx) error {
+		b, err := tx.CreateBucket(bucket)
+		if err != nil {
+			return err
+		}
+		return b.Put(key, value)
+	}))
+	assert(t, db != nil, "")
+	assert(t, !db.IsReadOnly(), "")
+	ok(t, err)
+	ok(t, db.Close())
+
+	// Open in read-only mode.
+	db0, err := bolt.Open(path, 0666, &bolt.Options{ReadOnly: true})
+	ok(t, err)
+	defer db0.Close()
+
+	// Opening in read-write mode should return an error.
+	_, err = bolt.Open(path, 0666, &bolt.Options{Timeout: time.Millisecond * 100})
+	assert(t, err != nil, "")
+
+	// And again (in read-only mode).
+	db1, err := bolt.Open(path, 0666, &bolt.Options{ReadOnly: true})
+	ok(t, err)
+	defer db1.Close()
+
+	// Verify both read-only databases are accessible.
+	for _, db := range []*bolt.DB{db0, db1} {
+		// Verify is is in read only mode indeed.
+		assert(t, db.IsReadOnly(), "")
+
+		// Read-only databases should not allow updates.
+		assert(t,
+			bolt.ErrDatabaseReadOnly == db.Update(func(*bolt.Tx) error {
+				panic(`should never get here`)
+			}),
+			"")
+
+		// Read-only databases should not allow beginning writable txns.
+		_, err = db.Begin(true)
+		assert(t, bolt.ErrDatabaseReadOnly == err, "")
+
+		// Verify the data.
+		ok(t, db.View(func(tx *bolt.Tx) error {
+			b := tx.Bucket(bucket)
+			if b == nil {
+				return fmt.Errorf("expected bucket `%s`", string(bucket))
+			}
+
+			got := string(b.Get(key))
+			expected := string(value)
+			if got != expected {
+				return fmt.Errorf("expected `%s`, got `%s`", expected, got)
+			}
+			return nil
+		}))
+	}
+}
+
 // TODO(benbjohnson): Test corruption at every byte of the first two pages.
 
 // Ensure that a database cannot open a transaction when it's not open.
diff --git a/errors.go b/errors.go
index aa504f1..6883786 100644
--- a/errors.go
+++ b/errors.go
@@ -36,6 +36,10 @@ var (
 	// ErrTxClosed is returned when committing or rolling back a transaction
 	// that has already been committed or rolled back.
 	ErrTxClosed = errors.New("tx closed")
+
+	// ErrDatabaseReadOnly is returned when a mutating transaction is started on a
+	// read-only database.
+	ErrDatabaseReadOnly = errors.New("database is in read-only mode")
 )
 
 // These errors can occur when putting or deleting a value or a bucket.