use tx.meta during Tx.WriteTo()

This commit changes `Tx.WriteTo()` to use the transaction's
in-memory meta page instead of copying from the disk. This is
needed because the transaction uses the size from its meta page
but writes the current meta page on disk which may have allocated
additional pages since the transaction started.

Fixes boltdb#513

Author: benbjohnson

tx.go

@@ -297,12 +297,34 @@ func (tx *Tx) WriteTo(w io.Writer) (n int64, err error) {
 	}
 	defer func() { _ = f.Close() }()
 
-	// Copy the meta pages.
-	tx.db.metalock.Lock()
-	n, err = io.CopyN(w, f, int64(tx.db.pageSize*2))
-	tx.db.metalock.Unlock()
+	// Generate a meta page. We use the same page data for both meta pages.
+	buf := make([]byte, tx.db.pageSize)
+	page := (*page)(unsafe.Pointer(&buf[0]))
+	page.flags = metaPageFlag
+	*page.meta() = *tx.meta
+
+	// Write meta 0.
+	page.id = 0
+	page.meta().checksum = page.meta().sum64()
+	nn, err := w.Write(buf)
+	n += int64(nn)
+	if err != nil {
+		return n, fmt.Errorf("meta 0 copy: %s", err)
+	}
+
+	// Write meta 1 with a lower transaction id.
+	page.id = 1
+	page.meta().txid -= 1
+	page.meta().checksum = page.meta().sum64()
+	nn, err = w.Write(buf)
+	n += int64(nn)
 	if err != nil {
-		return n, fmt.Errorf("meta copy: %s", err)
+		return n, fmt.Errorf("meta 1 copy: %s", err)
+	}
+
+	// Move past the meta pages in the file.
+	if _, err := f.Seek(int64(tx.db.pageSize*2), os.SEEK_SET); err != nil {
+		return n, fmt.Errorf("seek: %s", err)
 	}
 
 	// Copy data pages.

tx_test.go

@@ -570,7 +570,7 @@ func TestTx_CopyFile_Error_Meta(t *testing.T) {
 
 	if err := db.View(func(tx *bolt.Tx) error {
 		return tx.Copy(&failWriter{})
-	}); err == nil || err.Error() != "meta copy: error injected for tests" {
+	}); err == nil || err.Error() != "meta 0 copy: error injected for tests" {
 		t.Fatalf("unexpected error: %v", err)
 	}
 }