Make bolt.Open return the documented errors

- ErrInvalid is returned when a data file is not a Bolt-formatted
  database.
- ErrVersionMismatch is returned when the data file was created with a
  different version of Bolt.
- ErrChecksum is returned when either meta page checksum does not match.

Also:
- Do not wrap errors from os.Stat, so that a caller could handle os.Stat
  errors just like it can handle errors from os.Open that bolt.Open
  might return.
- Name tests consistently, following the pattern "TestOpen_*".
- Remove deferred calls to `os.Remove(path)`.
  The calls are not only unnecessary, but also in all cases `os.Remove`
  returns an error that is ignored. All those calls are meant to remove
  a file that was already removed by `tmpfile()`.
- Combine "bad path" tests and use filepath.Join to build the path.

db.go

@@ -177,7 +177,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 
 	// Initialize the database if it doesn't exist.
 	if info, err := db.file.Stat(); err != nil {
-		return nil, fmt.Errorf("stat error: %s", err)
+		return nil, err
 	} else if info.Size() == 0 {
 		// Initialize new files with meta pages.
 		if err := db.init(); err != nil {
@@ -189,7 +189,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 		if _, err := db.file.ReadAt(buf[:], 0); err == nil {
 			m := db.pageInBuffer(buf[:], 0).meta()
 			if err := m.validate(); err != nil {
-				return nil, fmt.Errorf("meta0 error: %s", err)
+				return nil, err
 			}
 			db.pageSize = int(m.pageSize)
 		}
@@ -253,10 +253,10 @@ func (db *DB) mmap(minsz int) error {
 
 	// Validate the meta pages.
 	if err := db.meta0.validate(); err != nil {
-		return fmt.Errorf("meta0 error: %s", err)
+		return err
 	}
 	if err := db.meta1.validate(); err != nil {
-		return fmt.Errorf("meta1 error: %s", err)
+		return err
 	}
 
 	return nil

db_test.go

@@ -7,29 +7,48 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"regexp"
 	"runtime"
 	"sort"
 	"strings"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/boltdb/bolt"
 )
 
 var statsFlag = flag.Bool("stats", false, "show performance stats")
 
-// Ensure that opening a database with a bad path returns an error.
-func TestOpen_BadPath(t *testing.T) {
-	db, err := bolt.Open("", 0666, nil)
-	assert(t, err != nil, "err: %s", err)
-	assert(t, db == nil, "")
+// version is the data file format version.
+const version = 2
+
+// magic is the marker value to indicate that a file is a Bolt DB.
+const magic uint32 = 0xED0CDAED
+
+// pageSize is the size of one page in the data file.
+const pageSize = 4096
+
+// pageHeaderSize is the size of a page header.
+const pageHeaderSize = 16
+
+// meta represents a simplified version of a database meta page for testing.
+type meta struct {
+	magic    uint32
+	version  uint32
+	_        uint32
+	_        uint32
+	_        [16]byte
+	_        uint64
+	_        uint64
+	_        uint64
+	checksum uint64
 }
 
 // Ensure that a database can be opened without error.
 func TestOpen(t *testing.T) {
 	path := tempfile()
-	defer os.Remove(path)
 	db, err := bolt.Open(path, 0666, nil)
 	assert(t, db != nil, "")
 	ok(t, err)
@@ -37,6 +56,73 @@ func TestOpen(t *testing.T) {
 	ok(t, db.Close())
 }
 
+// Ensure that opening a database with a bad path returns an error.
+func TestOpen_BadPath(t *testing.T) {
+	for _, path := range []string{
+		"",
+		filepath.Join(tempfile(), "youre-not-my-real-parent"),
+	} {
+		t.Logf("path = %q", path)
+		db, err := bolt.Open(path, 0666, nil)
+		assert(t, err != nil, "err: %s", err)
+		equals(t, path, err.(*os.PathError).Path)
+		equals(t, "open", err.(*os.PathError).Op)
+		equals(t, (*bolt.DB)(nil), db)
+	}
+}
+
+// Ensure that opening a file with wrong checksum returns ErrChecksum.
+func TestOpen_ErrChecksum(t *testing.T) {
+	buf := make([]byte, pageSize)
+	meta := (*meta)(unsafe.Pointer(&buf[0]))
+	meta.magic = magic
+	meta.version = version
+	meta.checksum = 123
+
+	path := tempfile()
+	f, err := os.Create(path)
+	equals(t, nil, err)
+	f.WriteAt(buf, pageHeaderSize)
+	f.Close()
+	defer os.Remove(path)
+
+	_, err = bolt.Open(path, 0666, nil)
+	equals(t, bolt.ErrChecksum, err)
+}
+
+// Ensure that opening a file that is not a Bolt database returns ErrInvalid.
+func TestOpen_ErrInvalid(t *testing.T) {
+	path := tempfile()
+
+	f, err := os.Create(path)
+	equals(t, nil, err)
+	fmt.Fprintln(f, "this is not a bolt database")
+	f.Close()
+	defer os.Remove(path)
+
+	_, err = bolt.Open(path, 0666, nil)
+	equals(t, bolt.ErrInvalid, err)
+}
+
+// Ensure that opening a file created with a different version of Bolt returns
+// ErrVersionMismatch.
+func TestOpen_ErrVersionMismatch(t *testing.T) {
+	buf := make([]byte, pageSize)
+	meta := (*meta)(unsafe.Pointer(&buf[0]))
+	meta.magic = magic
+	meta.version = version + 100
+
+	path := tempfile()
+	f, err := os.Create(path)
+	equals(t, nil, err)
+	f.WriteAt(buf, pageHeaderSize)
+	f.Close()
+	defer os.Remove(path)
+
+	_, err = bolt.Open(path, 0666, nil)
+	equals(t, bolt.ErrVersionMismatch, err)
+}
+
 // Ensure that opening an already open database file will timeout.
 func TestOpen_Timeout(t *testing.T) {
 	if runtime.GOOS == "solaris" {
@@ -44,7 +130,6 @@ func TestOpen_Timeout(t *testing.T) {
 	}
 
 	path := tempfile()
-	defer os.Remove(path)
 
 	// Open a data file.
 	db0, err := bolt.Open(path, 0666, nil)
@@ -68,7 +153,6 @@ func TestOpen_Wait(t *testing.T) {
 	}
 
 	path := tempfile()
-	defer os.Remove(path)
 
 	// Open a data file.
 	db0, err := bolt.Open(path, 0666, nil)
@@ -179,7 +263,6 @@ func TestOpen_Size_Large(t *testing.T) {
 // Ensure that a re-opened database is consistent.
 func TestOpen_Check(t *testing.T) {
 	path := tempfile()
-	defer os.Remove(path)
 
 	db, err := bolt.Open(path, 0666, nil)
 	ok(t, err)
@@ -192,26 +275,14 @@ func TestOpen_Check(t *testing.T) {
 	db.Close()
 }
 
-// Ensure that the database returns an error if the file handle cannot be open.
-func TestDB_Open_FileError(t *testing.T) {
-	path := tempfile()
-	defer os.Remove(path)
-
-	_, err := bolt.Open(path+"/youre-not-my-real-parent", 0666, nil)
-	assert(t, err.(*os.PathError) != nil, "")
-	equals(t, path+"/youre-not-my-real-parent", err.(*os.PathError).Path)
-	equals(t, "open", err.(*os.PathError).Op)
-}
-
 // Ensure that write errors to the meta file handler during initialization are returned.
-func TestDB_Open_MetaInitWriteError(t *testing.T) {
+func TestOpen_MetaInitWriteError(t *testing.T) {
 	t.Skip("pending")
 }
 
 // Ensure that a database that is too small returns an error.
-func TestDB_Open_FileTooSmall(t *testing.T) {
+func TestOpen_FileTooSmall(t *testing.T) {
 	path := tempfile()
-	defer os.Remove(path)
 
 	db, err := bolt.Open(path, 0666, nil)
 	ok(t, err)
@@ -235,7 +306,6 @@ 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)