/
reader.go
161 lines (127 loc) · 3.29 KB
/
reader.go
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// This reader is needed for reading raw windows devices, such as
// \\.\c: On windows, such devices may only be read using sector
// alignment in whole sector numbers. This reader implements page
// aligned reading and adds pages to an LRU cache to make accessing
// various field members faster.
package parser
import (
"fmt"
"io"
"sync"
)
// Keep pages in a free list to avoid allocations.
type FreeList struct {
mu sync.Mutex
pagesize int64
freelist [][]byte
}
func (self *FreeList) Get() []byte {
self.mu.Lock()
defer self.mu.Unlock()
if len(self.freelist) == 0 {
return make([]byte, self.pagesize)
}
// Take the page off the end of the list
result := self.freelist[len(self.freelist)-1]
self.freelist = self.freelist[:len(self.freelist)-1]
return result
}
func (self *FreeList) Put(in []byte) {
self.mu.Lock()
defer self.mu.Unlock()
self.freelist = append(self.freelist, in)
}
type PagedReader struct {
mu sync.Mutex
reader io.ReaderAt
pagesize int64
lru *LRU
freelist *FreeList
Hits int64
Miss int64
}
func (self *PagedReader) IsFixed(offset int64) bool {
return false
}
func (self *PagedReader) VtoP(offset int64) int64 {
return offset
}
func (self *PagedReader) ReadAt(buf []byte, offset int64) (int, error) {
self.mu.Lock()
defer self.mu.Unlock()
buf_idx := 0
for {
// How much is left in this page to read?
to_read := int(self.pagesize - offset%self.pagesize)
// How much do we need to read into the buffer?
if to_read > len(buf)-buf_idx {
to_read = len(buf) - buf_idx
}
// Are we done?
if to_read == 0 {
return buf_idx, nil
}
var page_buf []byte
page := offset - offset%self.pagesize
cached_page_buf, pres := self.lru.Get(int(page))
if !pres {
self.Miss += 1
DebugPrint("Cache miss for %x (%x) (%d)\n", page, self.pagesize,
self.lru.Len())
// Read this page into memory - already holding the lock.
page_buf = self.freelist.Get()
n, err := self.reader.ReadAt(page_buf, page)
if err != nil && err != io.EOF {
return buf_idx, err
}
// Only cache full pages.
if n == int(self.pagesize) {
self.lru.Add(int(page), page_buf)
}
} else {
self.Hits += 1
page_buf = cached_page_buf.([]byte)
}
// Copy the relevant data from the page.
page_offset := int(offset % self.pagesize)
copy(buf[buf_idx:buf_idx+to_read],
page_buf[page_offset:page_offset+to_read])
offset += int64(to_read)
buf_idx += to_read
if debug && (self.Hits+self.Miss)%10000 == 0 {
fmt.Printf("PageCache hit %v miss %v (%v)\n", self.Hits, self.Miss,
float64(self.Hits)/float64(self.Miss))
}
}
}
func (self *PagedReader) Flush() {
self.lru.Purge()
flusher, ok := self.reader.(Flusher)
if ok {
flusher.Flush()
}
}
func NewPagedReader(reader io.ReaderAt, pagesize int64, cache_size int) (*PagedReader, error) {
DebugPrint("Creating cache of size %v\n", cache_size)
self := &PagedReader{
reader: reader,
pagesize: pagesize,
freelist: &FreeList{
pagesize: pagesize,
},
}
// By default 10mb cache.
cache, err := NewLRU(cache_size, func(key int, value interface{}) {
// Put the page back on the free list
self.freelist.Put(value.([]byte))
}, "NewPagedReader")
if err != nil {
return nil, err
}
self.lru = cache
return self, nil
}
// Invalidate the disk cache
type Flusher interface {
Flush()
}