Fix mmap step & max size check.

This commit adds fixes suggested by @tv42 for the mmap step fix in 834b38e:

* Check max size before calculating the new the mmap size.
* Fix mmap step loop to go to 1GB instead of 512MB.

db.go

@@ -171,11 +171,9 @@ func (db *DB) mmap(minsz int) error {
 	if size < minsz {
 		size = minsz
 	}
-	size = db.mmapSize(size)
-
-	// Verify the map size is not above the maximum allowed.
-	if size > maxMapSize {
-		return fmt.Errorf("mmap too large")
+	size, err = db.mmapSize(size)
+	if err != nil {
+		return err
 	}
 
 	// Dereference all mmap references before unmapping.
@@ -218,14 +216,20 @@ func (db *DB) munmap() error {
 
 // mmapSize determines the appropriate size for the mmap given the current size
 // of the database. The minimum size is 4MB and doubles until it reaches 1GB.
-func (db *DB) mmapSize(size int) int {
+// Returns an error if the new mmap size is greater than the max allowed.
+func (db *DB) mmapSize(size int) (int, error) {
 	// Double the size from 1MB until 1GB.
-	for i := uint(20); i < 30; i++ {
+	for i := uint(20); i <= 30; i++ {
 		if size <= 1<<i {
-			return 1 << i
+			return 1 << i, nil
 		}
 	}
 
+	// Verify the map size is not above the maximum allowed.
+	if size > maxMapSize-maxMmapStep {
+		return 0, fmt.Errorf("mmap too large")
+	}
+
 	// If larger than 1GB then grow by 1GB at a time.
 	size += maxMmapStep
 	if remainder := size % maxMmapStep; remainder > 0 {
@@ -238,7 +242,7 @@ func (db *DB) mmapSize(size int) int {
 		size = ((size / db.pageSize) + 1) * db.pageSize
 	}
 
-	return size
+	return size, nil
 }
 
 // init creates a new database file and initializes its meta pages.