Fix Windows mmap sizing.

This commit fixes an issue on Windows where the database was doubling
when it was re-opened. This occurred because Windows has to truncate the
file to the mmap size and the mmap resizing code was doubling the size
whenever the DB size was at the next threshold. This has been changed so
that the DB size will double only when the DB size is above the next
threshold.

db.go

@@ -212,7 +212,7 @@ 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 {
-	if size < minMmapSize {
+	if size <= minMmapSize {
 		return minMmapSize
 	} else if size < maxMmapStep {
 		size *= 2

db_test.go

@@ -297,7 +297,8 @@ func TestDB_mmapSize(t *testing.T) {
 	assert.Equal(t, db.mmapSize(0), minMmapSize)
 	assert.Equal(t, db.mmapSize(16384), minMmapSize)
 	assert.Equal(t, db.mmapSize(minMmapSize-1), minMmapSize)
-	assert.Equal(t, db.mmapSize(minMmapSize), minMmapSize*2)
+	assert.Equal(t, db.mmapSize(minMmapSize), minMmapSize)
+	assert.Equal(t, db.mmapSize(minMmapSize+1), (minMmapSize*2)+4096)
 	assert.Equal(t, db.mmapSize(10000000), 20000768)
 	assert.Equal(t, db.mmapSize((1<<30)-1), 2147483648)
 	assert.Equal(t, db.mmapSize(1<<30), 1<<31)