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128 lines
4.4 KiB
128 lines
4.4 KiB
package common
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import (
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fmt "fmt"
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"io"
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"os"
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"path/filepath"
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"strconv"
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"strings"
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"sync"
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"time"
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)
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const (
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atomicWriteFilePrefix = "write-file-atomic-"
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// Maximum number of atomic write file conflicts before we start reseeding
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// (reduced from golang's default 10 due to using an increased randomness space)
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atomicWriteFileMaxNumConflicts = 5
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// Maximum number of attempts to make at writing the write file before giving up
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// (reduced from golang's default 10000 due to using an increased randomness space)
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atomicWriteFileMaxNumWriteAttempts = 1000
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// LCG constants from Donald Knuth MMIX
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// This LCG's has a period equal to 2**64
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lcgA = 6364136223846793005
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lcgC = 1442695040888963407
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// Create in case it doesn't exist and force kernel
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// flush, which still leaves the potential of lingering disk cache.
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// Never overwrites files
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atomicWriteFileFlag = os.O_WRONLY | os.O_CREATE | os.O_SYNC | os.O_TRUNC | os.O_EXCL
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)
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var (
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atomicWriteFileRand uint64
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atomicWriteFileRandMu sync.Mutex
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)
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func writeFileRandReseed() uint64 {
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// Scale the PID, to minimize the chance that two processes seeded at similar times
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// don't get the same seed. Note that PID typically ranges in [0, 2**15), but can be
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// up to 2**22 under certain configurations. We left bit-shift the PID by 20, so that
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// a PID difference of one corresponds to a time difference of 2048 seconds.
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// The important thing here is that now for a seed conflict, they would both have to be on
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// the correct nanosecond offset, and second-based offset, which is much less likely than
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// just a conflict with the correct nanosecond offset.
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return uint64(time.Now().UnixNano() + int64(os.Getpid()<<20))
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}
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// Use a fast thread safe LCG for atomic write file names.
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// Returns a string corresponding to a 64 bit int.
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// If it was a negative int, the leading number is a 0.
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func randWriteFileSuffix() string {
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atomicWriteFileRandMu.Lock()
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r := atomicWriteFileRand
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if r == 0 {
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r = writeFileRandReseed()
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}
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// Update randomness according to lcg
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r = r*lcgA + lcgC
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atomicWriteFileRand = r
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atomicWriteFileRandMu.Unlock()
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// Can have a negative name, replace this in the following
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suffix := strconv.Itoa(int(r))
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if string(suffix[0]) == "-" {
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// Replace first "-" with "0". This is purely for UI clarity,
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// as otherwhise there would be two `-` in a row.
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suffix = strings.Replace(suffix, "-", "0", 1)
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}
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return suffix
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}
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// WriteFileAtomic creates a temporary file with data and provided perm and
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// swaps it atomically with filename if successful.
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func WriteFileAtomic(filename string, data []byte, perm os.FileMode) (err error) {
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// This implementation is inspired by the golang stdlibs method of creating
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// tempfiles. Notable differences are that we use different flags, a 64 bit LCG
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// and handle negatives differently.
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// The core reason we can't use golang's TempFile is that we must write
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// to the file synchronously, as we need this to persist to disk.
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// We also open it in write-only mode, to avoid concerns that arise with read.
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var (
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dir = filepath.Dir(filename)
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f *os.File
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)
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nconflict := 0
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// Limit the number of attempts to create a file. Something is seriously
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// wrong if it didn't get created after 1000 attempts, and we don't want
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// an infinite loop
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i := 0
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for ; i < atomicWriteFileMaxNumWriteAttempts; i++ {
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name := filepath.Join(dir, atomicWriteFilePrefix+randWriteFileSuffix())
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f, err = os.OpenFile(name, atomicWriteFileFlag, perm)
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// If the file already exists, try a new file
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if os.IsExist(err) {
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// If the files exists too many times, start reseeding as we've
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// likely hit another instances seed.
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if nconflict++; nconflict > atomicWriteFileMaxNumConflicts {
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atomicWriteFileRandMu.Lock()
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atomicWriteFileRand = writeFileRandReseed()
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atomicWriteFileRandMu.Unlock()
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}
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continue
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} else if err != nil {
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return err
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}
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break
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}
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if i == atomicWriteFileMaxNumWriteAttempts {
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return fmt.Errorf("could not create atomic write file after %d attempts", i)
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}
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// Clean up in any case. Defer stacking order is last-in-first-out.
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defer os.Remove(f.Name())
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defer f.Close()
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if n, err := f.Write(data); err != nil {
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return err
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} else if n < len(data) {
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return io.ErrShortWrite
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}
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// Close the file before renaming it, otherwise it will cause "The process
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// cannot access the file because it is being used by another process." on windows.
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f.Close()
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return os.Rename(f.Name(), filename)
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}
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