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Merged
Zibbp merged 1 commit into from
Feb 15, 2026
Merged

fix(live): make chat download more crash resistant #1063

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279 changes: 227 additions & 52 deletions internal/exec/twitch.go
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ import (
"fmt"
"io"
"os"
"path/filepath"
"syscall"
"time"

Expand Down Expand Up @@ -126,88 +127,262 @@ func appendMessageToJSONArray(filename string, comment utils.LiveComment) error
if err != nil {
return fmt.Errorf("failed to stat file: %w", err)
}
size := info.Size()

// new file - write a complete array with a single element.
if size == 0 {
if _, err := f.Write([]byte("[\n")); err != nil {
copyFrom, copyPrefixUntil, isEmptyArray, needsOpeningBracket, err := inspectJSONArrayPrefix(f, info.Size())
if err != nil {
return fmt.Errorf("failed to inspect existing JSON array: %w", err)
}

// Write to a temp file and atomically rename it over the original.
// This avoids leaving a partially written/truncated JSON file on crashes.
dir := filepath.Dir(filename)
base := filepath.Base(filename)
tmp, err := os.CreateTemp(dir, base+".tmp-*")
if err != nil {
return fmt.Errorf("failed to create temp file: %w", err)
}
tmpName := tmp.Name()
defer func() {
_ = tmp.Close()
_ = os.Remove(tmpName)
}()

if _, err := f.Seek(copyFrom, io.SeekStart); err != nil {
return fmt.Errorf("failed to seek original file: %w", err)
}

if needsOpeningBracket {
if _, err := tmp.Write([]byte("[\n")); err != nil {
return fmt.Errorf("failed to write opening bracket: %w", err)
}
if _, err := f.Write(msg); err != nil {
return fmt.Errorf("failed to write message: %w", err)
}

if copyPrefixUntil > copyFrom {
if _, err := io.CopyN(tmp, f, copyPrefixUntil-copyFrom); err != nil {
return fmt.Errorf("failed to copy existing JSON prefix: %w", err)
}
if _, err := f.Write([]byte("\n]\n")); err != nil {
return fmt.Errorf("failed to write closing bracket: %w", err)
}

if isEmptyArray {
if _, err := tmp.Write([]byte("\n")); err != nil {
return fmt.Errorf("failed to write newline: %w", err)
}
} else {
if _, err := tmp.Write([]byte(",\n")); err != nil {
return fmt.Errorf("failed to write message separator: %w", err)
}
return f.Sync()
}

// Read only a small tail of the file to find the closing ']' and
// determine whether the array is empty or not.
const tailSize = 1024
bufSize := size
if bufSize > tailSize {
bufSize = tailSize
if _, err := tmp.Write(msg); err != nil {
return fmt.Errorf("failed to write chat message: %w", err)
}
if _, err := tmp.Write([]byte("\n]\n")); err != nil {
return fmt.Errorf("failed to write closing bracket: %w", err)
}
if err := tmp.Sync(); err != nil {
return fmt.Errorf("failed to sync temp file: %w", err)
}
if err := tmp.Close(); err != nil {
return fmt.Errorf("failed to close temp file: %w", err)
}

buf := make([]byte, bufSize)
if _, err := f.ReadAt(buf, size-bufSize); err != nil && err != io.EOF {
return fmt.Errorf("failed to read file tail: %w", err)
if err := os.Rename(tmpName, filename); err != nil {
return fmt.Errorf("failed to atomically replace file: %w", err)
}

// Find last non-whitespace char (should be ']').
i := int(bufSize - 1)
for ; i >= 0 && isSpace(buf[i]); i-- {
// Best-effort sync of parent directory to increase rename durability.
if dirF, err := os.Open(dir); err == nil {
_ = dirF.Sync()
_ = dirF.Close()
}
if i < 0 || buf[i] != ']' {
return fmt.Errorf("file %s is not a JSON array (missing closing ])", filename)

return nil
}

// inspectJSONArrayPrefix inspects the existing file and returns:
// - copyPrefixUntil: number of bytes from the beginning to copy into the temp file
// before appending the next message
// - isEmptyArray: whether the array currently has zero elements
//
// It supports recovery from interrupted writes where trailing commas and/or the
// closing bracket are missing.
func inspectJSONArrayPrefix(f *os.File, size int64) (copyFrom, copyPrefixUntil int64, isEmptyArray, needsOpeningBracket bool, err error) {
if size == 0 {
return 0, 0, true, true, nil
}

// Look backwards to see what’s before the closing ']' to check if array is empty.
j := i - 1
for ; j >= 0 && isSpace(buf[j]); j-- {
firstIdx, firstByte, ok, err := findFirstNonSpaceInRange(f, 0, size)
if err != nil {
return 0, 0, false, false, err
}
if !ok {
// File contains only whitespace; treat as empty/repairable.
return 0, 0, true, true, nil
}

isEmptyArray := false
if j >= 0 && buf[j] == '[' {
isEmptyArray = true
} else if size <= 2 {
isEmptyArray = true
if firstByte != '[' {
// Recovery path: handle files missing the opening '[' due to prior
// interrupted/broken writes.
copyUntil, empty, recErr := inspectMissingOpeningBracketPrefix(f, firstIdx, size)
if recErr != nil {
return 0, 0, false, false, recErr
}
return firstIdx, copyUntil, empty, true, nil
}

lastIdx, lastByte, ok, err := findLastNonSpaceBefore(f, size)
if err != nil {
return 0, 0, false, false, err
}
if !ok {
return 0, 0, true, true, nil
}

// Compute the absolute offset of the closing ']' in the file.
lastBracketOffset := (size - bufSize) + int64(i)
if lastByte == ']' {
prevIdx, prevByte, ok, err := findLastNonSpaceBefore(f, lastIdx)
if err != nil {
return 0, 0, false, false, err
}
if !ok {
return 0, 0, false, false, fmt.Errorf("malformed JSON array")
}

// Drop the closing ']' (and any trailing whitespace after it).
if err := f.Truncate(lastBracketOffset); err != nil {
return fmt.Errorf("failed to truncate file: %w", err)
return 0, lastIdx, prevIdx == firstIdx && prevByte == '[', false, nil
}

// Seek to the end after truncation.
if _, err := f.Seek(0, io.SeekEnd); err != nil {
return fmt.Errorf("failed to seek: %w", err)
// Recovery path: file likely ended mid-write. Trim trailing commas/whitespace.
searchEnd := size
for {
idx, b, found, err := findLastNonSpaceBefore(f, searchEnd)
if err != nil {
return 0, 0, false, false, err
}
if !found {
return 0, firstIdx + 1, true, false, nil
}

if b == ',' {
searchEnd = idx
continue
}

copyPrefixUntil = idx + 1
break
}

// If the array already has elements, add a comma; otherwise just a newline.
if isEmptyArray {
if _, err := f.Write([]byte("\n")); err != nil {
return fmt.Errorf("failed to write newline: %w", err)
if copyPrefixUntil <= firstIdx {
return 0, firstIdx + 1, true, false, nil
}

_, _, hasContentAfterOpenBracket, err := findFirstNonSpaceInRange(f, firstIdx+1, copyPrefixUntil)
if err != nil {
return 0, 0, false, false, err
}

return 0, copyPrefixUntil, !hasContentAfterOpenBracket, false, nil
}

func inspectMissingOpeningBracketPrefix(f *os.File, firstIdx, size int64) (copyPrefixUntil int64, isEmptyArray bool, err error) {
searchEnd := size

// If a trailing closing bracket exists, drop it first.
if idx, b, found, err := findLastNonSpaceBefore(f, searchEnd); err != nil {
return 0, false, err
} else if !found {
return firstIdx, true, nil
} else if b == ']' {
searchEnd = idx
}

for {
idx, b, found, err := findLastNonSpaceBefore(f, searchEnd)
if err != nil {
return 0, false, err
}
} else {
if _, err := f.Write([]byte(",\n")); err != nil {
return fmt.Errorf("failed to write comma: %w", err)
if !found {
return firstIdx, true, nil
}

if b == ',' {
searchEnd = idx
continue
}

if idx < firstIdx {
return firstIdx, true, nil
}

return idx + 1, false, nil
}
}

// Write the new message and close the array again.
if _, err := f.Write(msg); err != nil {
return fmt.Errorf("failed to write message: %w", err)
func findFirstNonSpaceInRange(f *os.File, start, end int64) (int64, byte, bool, error) {
if start >= end {
return 0, 0, false, nil
}
if _, err := f.Write([]byte("\n]\n")); err != nil {
return fmt.Errorf("failed to write closing bracket: %w", err)

const chunkSize int64 = 4096
buf := make([]byte, chunkSize)

for offset := start; offset < end; {
toRead := end - offset
if toRead > chunkSize {
toRead = chunkSize
}

n, err := f.ReadAt(buf[:toRead], offset)
if err != nil && err != io.EOF {
return 0, 0, false, err
}

for i := 0; i < n; i++ {
if !isSpace(buf[i]) {
return offset + int64(i), buf[i], true, nil
}
}

offset += int64(n)
if n == 0 {
break
}
}

return 0, 0, false, nil
}

func findLastNonSpaceBefore(f *os.File, end int64) (int64, byte, bool, error) {
if end <= 0 {
return 0, 0, false, nil
}

const chunkSize int64 = 4096
buf := make([]byte, chunkSize)

for right := end; right > 0; {
left := right - chunkSize
if left < 0 {
left = 0
}

toRead := right - left
n, err := f.ReadAt(buf[:toRead], left)
if err != nil && err != io.EOF {
return 0, 0, false, err
}

for i := n - 1; i >= 0; i-- {
if !isSpace(buf[i]) {
return left + int64(i), buf[i], true, nil
}
}

right = left
if n == 0 {
break
}
}

return f.Sync()
return 0, 0, false, nil
}

// isSpace is sufficient for JSON whitespace around the closing bracket.
Expand Down
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