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//! The header block decoder (RFC 7541 §3, §6).
use crate::table::{DynTable, Name, STATIC_TABLE};
use crate::{decode_integer, field_size, read_string, Error};
/// A stateful header block decoder.
#[derive(Debug, Clone)]
pub struct Decoder {
table: DynTable,
/// Upper bound for dynamic table size updates: our SETTINGS_HEADER_TABLE_SIZE.
max_allowed: usize,
/// Cap on the decoded list (name + value + 32 per field).
max_list: usize,
name_buf: Vec<u8>,
value_buf: Vec<u8>,
}
impl Decoder {
/// A decoder whose dynamic table may grow to `max_table_size` (the value advertised in
/// SETTINGS_HEADER_TABLE_SIZE) and whose decoded header lists are capped at `max_list_size`.
pub fn new(max_table_size: usize, max_list_size: usize) -> Self {
Decoder {
table: DynTable::new(max_table_size),
max_allowed: max_table_size,
max_list: max_list_size,
name_buf: Vec::new(),
value_buf: Vec::new(),
}
}
/// Changes the largest table size the peer may select (after our new SETTINGS were
/// acknowledged). A smaller value shrinks the table at once.
pub fn set_max_allowed(&mut self, max: usize) {
self.max_allowed = max;
if self.table.max() > max {
self.table.set_max(max);
}
}
/// Changes the header list cap.
pub fn set_max_list_size(&mut self, max: usize) {
self.max_list = max;
}
/// Current dynamic table size in octets (RFC 7541 §4.1).
pub fn table_size(&self) -> usize {
self.table.size()
}
/// Current dynamic table maximum (the last size update).
pub fn table_max_size(&self) -> usize {
self.table.max()
}
/// Number of dynamic table entries.
pub fn table_len(&self) -> usize {
self.table.len()
}
/// Dynamic table entry `i` (0 = newest).
pub fn table_entry(&self, i: usize) -> Option<(&[u8], &[u8])> {
self.table.get(i)
}
#[inline]
fn lookup(&self, idx: usize) -> Result<(&[u8], &[u8]), Error> {
if idx == 0 {
Err(Error::InvalidIndex)
} else if idx <= STATIC_TABLE.len() {
Ok(STATIC_TABLE[idx - 1])
} else {
self.table.get(idx - STATIC_TABLE.len() - 1).ok_or(Error::InvalidIndex)
}
}
/// Decodes one complete header block, passing each field (name, value) to `emit` in order.
///
/// When the list grows past the cap the rest of the block is still decoded, to keep the
/// dynamic table in step, but not emitted, and [`Error::HeaderListTooLarge`] is returned.
pub fn decode(&mut self, block: &[u8], emit: &mut dyn FnMut(&[u8], &[u8])) -> Result<(), Error> {
let mut pos = 0;
let mut size = 0usize;
let mut over = false;
let mut seen_field = false;
while pos < block.len() {
let b = block[pos];
if b & 0x80 != 0 {
// indexed field (§6.1)
let idx = decode_integer(block, &mut pos, 7)?;
let (n, v) = self.lookup(idx)?;
seen_field = true;
size += field_size(n, v);
if size > self.max_list {
over = true;
}
if !over {
emit(n, v);
}
continue;
}
if b & 0xe0 == 0x20 {
// dynamic table size update (§6.3): only before the first field
if seen_field {
return Err(Error::InvalidTableSizeUpdate);
}
let s = decode_integer(block, &mut pos, 5)?;
if s > self.max_allowed {
return Err(Error::InvalidTableSizeUpdate);
}
self.table.set_max(s);
continue;
}
// literals: with incremental indexing (01), without (0000), never indexed (0001)
let indexing = b & 0x40 != 0;
let idx = decode_integer(block, &mut pos, if indexing { 6 } else { 4 })?;
seen_field = true;
let dyn_name = idx > STATIC_TABLE.len();
if dyn_name && idx - STATIC_TABLE.len() - 1 >= self.table.len() {
return Err(Error::InvalidIndex);
}
let lit_name: &[u8] = if idx == 0 { read_string(block, &mut pos, &mut self.name_buf)? } else { &[] };
let value = read_string(block, &mut pos, &mut self.value_buf)?;
let name: &[u8] = if idx == 0 {
lit_name
} else if dyn_name {
self.table.get(idx - STATIC_TABLE.len() - 1).map_or(&[][..], |e| e.0)
} else {
STATIC_TABLE[idx - 1].0
};
size += field_size(name, value);
if size > self.max_list {
over = true;
}
if !indexing || field_size(name, value) > self.table.max() {
// emit from the sources; an oversized entry then just empties the table
if !over {
emit(name, value);
}
if indexing {
self.table.clear(); // RFC 7541 §4.4
}
continue;
}
let src = if idx == 0 {
Name::Bytes(lit_name)
} else if dyn_name {
Name::Dyn(idx - STATIC_TABLE.len() - 1)
} else {
Name::Bytes(STATIC_TABLE[idx - 1].0)
};
self.table.insert(src, value);
if !over {
if let Some((n, v)) = self.table.get(0) {
emit(n, v);
}
}
}
if over {
Err(Error::HeaderListTooLarge)
} else {
Ok(())
}
}
}