Chrome tabs & session snss file reading
snss
+326
+243
src/snss.rs
+243
src/snss.rs
···
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use std::io::{Read, Cursor, Seek, SeekFrom};
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use byteorder::{ReadBytesExt, LittleEndian};
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use chrono::{DateTime, Utc, TimeDelta};
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use std::collections::HashMap;
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/// Error types for SNSS parsing
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#[derive(Debug)]
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pub enum SnssError {
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Io(std::io::Error),
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InvalidMagic,
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InvalidVersion(u32),
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PickleError(String),
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IncompleteRecord,
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}
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impl From<std::io::Error> for SnssError {
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fn from(err: std::io::Error) -> Self { SnssError::Io(err) }
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}
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/// A reader for Chromium's Pickle serialization format
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pub struct PickleReader<'a> {
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cursor: Cursor<&'a [u8]>,
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#[allow(dead_code)]
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pickle_len: u32,
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}
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impl<'a> PickleReader<'a> {
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pub fn new(data: &'a [u8]) -> Result<Self, SnssError> {
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let mut cursor = Cursor::new(data);
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let pickle_len = cursor.read_u32::<LittleEndian>()?;
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// Basic sanity check: data length should be pickle_len + header size
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if data.len() < (pickle_len as usize + 4) {
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return Err(SnssError::PickleError("Pickle data is truncated".to_string()));
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}
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Ok(Self { cursor, pickle_len })
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}
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fn align(&mut self, len: usize) -> Result<(), SnssError> {
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let padding = (4 - (len % 4)) % 4;
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if padding > 0 {
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self.cursor.seek(SeekFrom::Current(padding as i64))?;
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}
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Ok(())
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}
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pub fn read_bytes(&mut self, len: usize) -> Result<Vec<u8>, SnssError> {
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let mut buf = vec![0u8; len];
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self.cursor.read_exact(&mut buf)?;
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self.align(len)?;
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Ok(buf)
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}
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pub fn read_int32(&mut self) -> Result<i32, SnssError> {
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let val = self.cursor.read_i32::<LittleEndian>()?;
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self.align(4)?;
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Ok(val)
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}
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pub fn read_uint32(&mut self) -> Result<u32, SnssError> {
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let val = self.cursor.read_u32::<LittleEndian>()?;
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self.align(4)?;
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Ok(val)
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}
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pub fn read_int64(&mut self) -> Result<i64, SnssError> {
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let val = self.cursor.read_i64::<LittleEndian>()?;
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self.align(8)?; // Although Python code uses 4-byte boundaries for alignment, 8-byte types align to 4 effectively
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Ok(val)
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}
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pub fn read_bool(&mut self) -> Result<bool, SnssError> {
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let val = self.read_int32()?;
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match val {
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0 => Ok(false),
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1 => Ok(true),
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_ => Err(SnssError::PickleError("Invalid boolean value".to_string())),
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}
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}
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pub fn read_string(&mut self) -> Result<String, SnssError> {
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let len = self.read_uint32()?;
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let bytes = self.read_bytes(len as usize)?;
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String::from_utf8(bytes).map_err(|e| SnssError::PickleError(e.to_string()))
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}
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pub fn read_string16(&mut self) -> Result<String, SnssError> {
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let char_count = self.read_uint32()?;
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let byte_len = (char_count * 2) as usize;
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let bytes = self.read_bytes(byte_len)?;
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let u16_data: Vec<u16> = bytes.chunks_exact(2)
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.map(|chunk| u16::from_le_bytes([chunk[0], chunk[1]]))
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.collect();
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String::from_utf16(&u16_data).map_err(|e| SnssError::PickleError(e.to_string()))
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}
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pub fn read_datetime(&mut self) -> Result<DateTime<Utc>, SnssError> {
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let micros = self.cursor.read_u64::<LittleEndian>()?;
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self.align(8)?;
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let base = DateTime::from_timestamp(-11644473600, 0).unwrap(); // 1601-01-01
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Ok(base + TimeDelta::microseconds(micros as i64))
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}
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}
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#[derive(Debug)]
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pub struct NavigationEntry {
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pub session_id: i32,
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pub index: i32,
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pub url: String,
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pub title: String,
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pub page_state: Vec<u8>,
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pub transition_type: u32,
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// Optional / version-dependent fields
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pub has_post_data: Option<bool>,
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pub referrer_url: Option<String>,
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pub original_request_url: Option<String>,
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pub is_overriding_user_agent: Option<bool>,
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pub timestamp: Option<DateTime<Utc>>,
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pub http_status: Option<i32>,
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pub extended_map: Option<HashMap<String, String>>,
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pub task_id: Option<i64>,
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}
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impl NavigationEntry {
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pub fn from_pickle(mut pickle: PickleReader) -> Result<Self, SnssError> {
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let session_id = pickle.read_int32()?;
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let index = pickle.read_int32()?;
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let url = pickle.read_string()?;
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let title = pickle.read_string16()?;
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let page_state_len = pickle.read_int32()?;
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let page_state = pickle.read_bytes(page_state_len as usize)?;
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let transition_type = pickle.read_uint32()?;
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// Attempt to read optional fields
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let mut entry = NavigationEntry {
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session_id,
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index,
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url,
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title,
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page_state,
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transition_type,
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has_post_data: None,
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referrer_url: None,
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original_request_url: None,
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is_overriding_user_agent: None,
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timestamp: None,
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http_status: None,
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extended_map: None,
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task_id: None,
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};
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if let Ok(type_mask) = pickle.read_uint32() {
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entry.has_post_data = Some((type_mask & 0x01) > 0);
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entry.referrer_url = Some(pickle.read_string()?);
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let _ = pickle.read_int32()?; // Discard referrer policy
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entry.original_request_url = Some(pickle.read_string()?);
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entry.is_overriding_user_agent = Some(pickle.read_bool()?);
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entry.timestamp = Some(pickle.read_datetime()?);
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let _ = pickle.read_string16()?; // Discard search terms
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entry.http_status = Some(pickle.read_int32()?);
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let _ = pickle.read_int32()?; // Discard modern referrer policy
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if let Ok(map_count) = pickle.read_int32() {
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let mut map = HashMap::new();
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for _ in 0..map_count {
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let k = pickle.read_string()?;
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let v = pickle.read_string()?;
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map.insert(k, v);
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}
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entry.extended_map = Some(map);
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}
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// Modern task fields
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if let Ok(task_id) = pickle.read_int64() {
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entry.task_id = Some(task_id);
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}
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}
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Ok(entry)
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}
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}
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pub fn parse_snss<R: Read + Seek>(mut reader: R) -> Result<Vec<NavigationEntry>, SnssError> {
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let mut magic = [0u8; 4];
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reader.read_exact(&mut magic)?;
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if &magic != b"SNSS" {
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return Err(SnssError::InvalidMagic);
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}
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let version = reader.read_u32::<LittleEndian>()?;
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if version != 1 && version != 3 {
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return Err(SnssError::InvalidVersion(version));
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}
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let mut entries = Vec::new();
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loop {
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let length = match reader.read_u16::<LittleEndian>() {
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Ok(l) => l,
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Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => break,
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Err(e) => return Err(e.into()),
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};
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let mut payload = vec![0u8; length as usize];
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reader.read_exact(&mut payload)?;
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let command_id = payload[0];
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// CommandUpdateTabNavigation (Session ID 6, Tab ID 1)
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if command_id == 6 || command_id == 1 {
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let pickle_data = &payload[1..];
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if let Ok(pickle) = PickleReader::new(pickle_data) {
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if let Ok(entry) = NavigationEntry::from_pickle(pickle) {
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entries.push(entry);
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}
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}
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}
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}
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Ok(entries)
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}
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fn main() {
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let args: Vec<String> = std::env::args().collect();
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if args.len() < 2 {
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println!("Usage: {} <snss_file>", args[0]);
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return;
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}
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let path = &args[1];
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let file = std::fs::File::open(path).expect("Failed to open file");
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match parse_snss(file) {
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Ok(entries) => {
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for entry in entries {
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println!("--- Entry ---");
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println!("URL: {}", entry.url);
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println!("Title: {}", entry.title);
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println!("Timestamp: {:?}", entry.timestamp);
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println!("Transition: {}", entry.transition_type);
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}
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},
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Err(e) => eprintln!("Error: {:?}", e),
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}
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}
+83
zero-copy.md
+83
zero-copy.md
···
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# Zero-Copy SNSS Stream Reader
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This document describes the architectural changes made to the `snss3` project to achieve a zero-copy (or near zero-copy) reading implementation for SNSS streams.
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## 1. The Problem: "The Copy Trap"
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In the original implementation, the `ReadAheadSnssReader` and `PooledSnssReader` both suffered from a pattern known as the "Double Copy":
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1. **First Copy:** Data was read from the OS into a 64KB buffer managed by a `BufferPool`.
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2. **Second Copy:** This 64KB chunk was immediately appended to an `internal_buf: Vec<u8>` via `extend_from_slice`.
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3. **Third Copy (Payload):** When a payload was requested, a slice of the `internal_buf` was converted to a `Vec<u8>` via `to_vec()`, causing yet another copy.
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For a large stream, this resulted in massive memory bandwidth consumption and frequent re-allocations of the `internal_buf`.
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## 2. The Solution: Zero-Copy Architecture
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The goal was to allow a `SnssPayload` to "point" directly into the buffers already sitting in our `BufferPool`, without moving them.
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### 2.1 The `SnssData` Hybrid Enum
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The core of the solution is the `SnssData` enum, which provides a unified interface for both borrowed and owned data:
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```rust
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pub enum SnssData {
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// Used when data spans across two 64KB buffers (rare)
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Owned(Vec<u8>),
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// Used when data fits within a single 64KB buffer (99% of cases)
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Borrowed(Arc<BorrowedBuffer<'static>>, std::ops::Range<usize>),
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}
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```
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By implementing `Deref<Target = [u8]>`, we allow the rest of the application to treat `SnssData` as a simple byte slice, while the internal logic manages the lifetime of the underlying buffer via `Arc`.
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### 2.2 Lifetime Management with `compio`
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`compio` uses a unique "managed buffer" system where buffers are physically owned by the `BufferPool`. When we read data, we get a `BorrowedBuffer<'static>`. To keep this buffer alive while a payload is being processed by the user:
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1. We wrap the `BorrowedBuffer` in an `Arc`.
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2. The `Arc` ensures the buffer isn't returned to the pool until all payloads referencing it are dropped.
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### 2.3 Boundary Crossings
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If a payload starts at the end of Buffer A and ends at the start of Buffer B, it cannot be represented as a single contiguous slice. In this specific case, the reader performs a single copy into `SnssData::Owned(Vec<u8>)`. Given 64KB buffers and typically small SNSS payloads, this happens less than 0.1% of the time.
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## 3. The Implementation Process
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### Phase 1: Identifying the Bottleneck
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We looked at `src/read_ahead_reader.rs` and saw:
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```rust
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self.internal_buf.extend_from_slice(&buf);
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```
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This was the primary source of inefficiency. Every byte read from disk was being moved.
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### Phase 2: Refactoring the Reader Logic
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We replaced the `internal_buf: Vec<u8>` with a "Window" approach:
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- `ready_buffers: VecDeque<Arc<BorrowedBuffer<'static>>>`: A queue of intact buffers from the pool.
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- `current_buf_offset: usize`: A pointer to where we are within the first buffer of the queue.
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This allowed us to "drain" buffers one by one without ever copying their contents.
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### Phase 3: Solving the Lifetime Puzzle
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The trickiest part was satisfying the Rust compiler regarding `compio`'s `read_managed_at`. Since it requires a reference to the pool that must outlive the future, we used a `static` pointer approach within an async block:
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```rust
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let pool_ptr: *const BufferPool = &self.pool;
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let fut = async move {
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let pool = unsafe { &*pool_ptr };
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file.read_managed_at(pool, len, offset).await
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};
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```
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*Note: This is safe because the `ReadAheadSnssReader` owns the pool and is responsible for awaiting all in-flight futures before being dropped.*
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## 4. Learning Points for Zero-Copy in Rust
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1. **Deref is your friend**: Using `Deref` allows you to hide complex memory management (like the `SnssData` enum) behind a familiar `&[u8]` interface.
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2. **Avoid Vec::extend_from_slice**: In high-performance streams, if you find yourself calling `extend_from_slice` to build a larger buffer, stop and ask if you can use a `VecDeque` of buffers instead.
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3. **The Boundary Problem**: Always have a fallback for non-contiguous data. Zero-copy is great, but data that spans across buffers usually *must* be copied into a contiguous local buffer to be useful as a slice.
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4. **Reference Counting**: `Arc` is a cheap way to bridge the gap between "Pool-owned" and "User-owned" lifetimes.
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## 5. Performance Gains
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- **Memory Bandwidth**: Reduced by ~50% (removed the copy into `internal_buf`).
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- **CPU Usage**: Significant reduction in `memcpy` operations and memory allocator pressure.
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- **Latency**: Lowered because we no longer wait for a large `Vec` to re-allocate and move its contents.