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Decoder: implement zero_cache
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This change adds a zero_cache counter to the Decoder's state. The implementation uses this cache so that padding bytes aren't pushed into the buffer and need to be removed later.
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@felixwrt
felixwrt committed Jun 25, 2024
1 parent b5c6293 commit e4ab7b5
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Showing 3 changed files with 178 additions and 14 deletions.
185 changes: 173 additions & 12 deletions src/transport.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -286,6 +286,8 @@ pub enum DecodeErr {
end_esc_misaligned: bool,
/// the number of padding bytes.
num_padding_bytes: u8,
/// whether some padding bytes weren't equal to zero
invalid_padding_bytes: bool,
},
}

Expand Down Expand Up @@ -399,9 +401,13 @@ impl<B: Buffer> Decoder<B> {
}

pub(crate) struct NonOwningDecoder {
// the number of bytes that were read out of the byte source
raw_msg_len: usize,
crc: crc::Digest<'static, u16>,
state: DecodeState,
// the number of zero bytes that weren't written into the buffer
// immediately because they could be padding bytes
zero_cache: u8,
}

impl Default for NonOwningDecoder {
Expand All @@ -413,6 +419,7 @@ impl Default for NonOwningDecoder {
num_discarded_bytes: 0,
num_init_seq_bytes: 0,
},
zero_cache: 0,
}
}
}
Expand Down Expand Up @@ -445,7 +452,6 @@ impl NonOwningDecoder {
let num_discarded_bytes = *num_discarded_bytes;
self.state = ParsingNormal;
self.raw_msg_len = 8;
assert_eq!(buf.len(), 0);
self.crc = CRC_X25.digest();
self.crc
.update(&[0x1b, 0x1b, 0x1b, 0x1b, 0x01, 0x01, 0x01, 0x01]);
Expand Down Expand Up @@ -514,6 +520,7 @@ impl NonOwningDecoder {
// ignore everything that has previously been read and start reading a new transmission
let ignored_bytes = self.raw_msg_len - 8;
self.raw_msg_len = 8;
self.zero_cache = 0;
buf.clear();
self.crc = CRC_X25.digest();
self.crc
Expand All @@ -538,23 +545,36 @@ impl NonOwningDecoder {
};

// check alignment (end marker needs to have 4-byte alignment)
let misaligned = buf.len() % 4 != 0;
let misaligned = self.raw_msg_len % 4 != 0;

// check if padding is larger than the message length
let padding_too_large =
num_padding_bytes > 3 || (num_padding_bytes as usize) > buf.len();

if read_crc != calculated_crc || misaligned || padding_too_large {
let padding_too_large = num_padding_bytes > 3;
// hint: start esc + end esc = 16 bytes
let padding_larger_than_msg_size =
self.raw_msg_len < (num_padding_bytes as usize) + 16;

// check whether padding bytes are correct
let invalid_padding_bytes = num_padding_bytes > self.zero_cache;

if read_crc != calculated_crc
|| misaligned
|| padding_too_large
|| padding_larger_than_msg_size
|| invalid_padding_bytes
{
self.reset(buf);
return Err(DecodeErr::InvalidMessage {
checksum_mismatch: (read_crc, calculated_crc),
end_esc_misaligned: misaligned,
num_padding_bytes,
invalid_padding_bytes,
});
}

// subtract padding bytes and escape payload length from buffer length
buf.truncate(buf.len() - num_padding_bytes as usize);
// remove padding bytes
self.zero_cache -= num_padding_bytes;

self.flush(buf)?;

self.set_done();

Expand All @@ -580,7 +600,7 @@ impl NonOwningDecoder {
// If that's the case, simply reset the parser state by 1-3 steps. This
// will parse the 0x1b bytes in the message as regular bytes and check
// for the end escape code at the right position.
let bytes_until_alignment = (4 - (buf.len() % 4)) % 4;
let bytes_until_alignment = (4 - (self.raw_msg_len % 4)) % 4;
if bytes_until_alignment > 0
&& payload[..bytes_until_alignment].iter().all(|x| *x == 0x1b)
&& payload[bytes_until_alignment] == 0x1a
Expand Down Expand Up @@ -642,10 +662,37 @@ impl NonOwningDecoder {
};
buf.clear();
self.raw_msg_len = 0;
self.zero_cache = 0;
num_discarded
}

// pushes bytes from the `zero_cache` into the output buffer
fn flush(&mut self, buf: &mut impl Buffer) -> Result<(), DecodeErr> {
for _ in 0..self.zero_cache {
self.push_inner(buf, 0)?;
}
self.zero_cache = 0;
Ok(())
}

fn push(&mut self, buf: &mut impl Buffer, b: u8) -> Result<(), DecodeErr> {
if b == 0 {
if self.zero_cache <= 3 {
self.zero_cache += 1;
} else {
// directly push into the output buffer if there are already 3
// zero bytes in the cache. Padding cannot be larger than three
// and this makes sure that the zero_cache cannot grow infinitely.
self.push_inner(buf, b)?;
}
} else {
self.flush(buf)?;
self.push_inner(buf, b)?;
}
Ok(())
}

fn push_inner(&mut self, buf: &mut impl Buffer, b: u8) -> Result<(), DecodeErr> {
if buf.push(b).is_err() {
self.reset(buf);
return Err(DecodeErr::OutOfMemory);
Expand Down Expand Up @@ -921,6 +968,7 @@ mod decode_tests {
checksum_mismatch: (0xFFb8, 0x7bb8),
end_esc_misaligned: false,
num_padding_bytes: 0,
invalid_padding_bytes: false,
})];

test_parse_input::<ArrayBuf<4>>(&bytes, exp);
Expand All @@ -933,6 +981,7 @@ mod decode_tests {
checksum_mismatch: (0xb613, 0xb613),
end_esc_misaligned: true,
num_padding_bytes: 0,
invalid_padding_bytes: false,
})];

test_parse_input::<ArrayBuf<16>>(&bytes, exp);
Expand All @@ -945,6 +994,7 @@ mod decode_tests {
checksum_mismatch: (0x50f9, 0x50f9),
end_esc_misaligned: false,
num_padding_bytes: 4,
invalid_padding_bytes: true,
})];

test_parse_input::<ArrayBuf<16>>(&bytes, exp);
Expand All @@ -957,6 +1007,7 @@ mod decode_tests {
checksum_mismatch: (0xf44f, 0xf44f),
end_esc_misaligned: false,
num_padding_bytes: 1,
invalid_padding_bytes: true,
})];

test_parse_input::<ArrayBuf<16>>(&bytes, exp);
Expand Down Expand Up @@ -995,8 +1046,8 @@ mod decode_tests {

#[test]
fn incomplete_esc_sequence() {
let bytes = hex!("1b1b1b1b 01010101 12345678 1b1b1b00 12345678 1b1b1b1b 1a030A07");
let exp = &[Ok(hex!("12345678 1b1b1b00 12").as_slice())];
let bytes = hex!("1b1b1b1b 01010101 12345678 1b1b1b00 12345678 1b1b1b1b 1a009135");
let exp = &[Ok(hex!("12345678 1b1b1b00 12345678").as_slice())];

test_parse_input::<ArrayBuf<128>>(&bytes, exp);
}
Expand Down Expand Up @@ -1064,12 +1115,122 @@ mod decode_tests {
}

#[test]
#[ignore = "not supported yet"]
fn padding_exceeding_buffer_size() {
let bytes = hex!("1b1b1b1b 01010101 12345678 12345600 1b1b1b1b 1a01f4c8");
let exp_bytes = hex!("12345678 123456");
let exp = &[Ok(exp_bytes.as_slice())];

test_parse_input::<ArrayBuf<7>>(&bytes, exp);
}

#[test]
fn invalid_padding_bytes_1() {
let bytes = hex!("1b1b1b1b 01010101 12345678 12345601 1b1b1b1b 1a012157");
let exp = &[Err(DecodeErr::InvalidMessage {
checksum_mismatch: (0x5721, 0x5721),
end_esc_misaligned: false,
num_padding_bytes: 1,
invalid_padding_bytes: true,
})];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn invalid_padding_bytes_2() {
let bytes = hex!("1b1b1b1b 01010101 12345678 12000100 1b1b1b1b 1a03297e");
let exp = &[Err(DecodeErr::InvalidMessage {
checksum_mismatch: (0x7e29, 0x7e29),
end_esc_misaligned: false,
num_padding_bytes: 3,
invalid_padding_bytes: true,
})];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn invalid_padding_bytes_3() {
let bytes = hex!("1b1b1b1b 01010101 12345678 12ff0000 1b1b1b1b 1a03a743");
let exp = &[Err(DecodeErr::InvalidMessage {
checksum_mismatch: (0x43a7, 0x43a7),
end_esc_misaligned: false,
num_padding_bytes: 3,
invalid_padding_bytes: true,
})];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn another_msg_start_after_padding_1() {
let bytes = hex!("1b1b1b1b 01010101 120000 1b1b1b1b 01010101 87654321 1b1b1b1b 1a00423c");
let exp = &[
Err(DecodeErr::DiscardedBytes(11)),
Ok(hex!("87654321").as_slice()),
];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn another_msg_start_after_padding_2() {
let bytes = hex!("1b1b1b1b 01010101 120000 1b1b1b1b 01010101 1b1b1b1b 1a00c6e5");
let exp = &[Err(DecodeErr::DiscardedBytes(11)), Ok(hex!("").as_slice())];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn another_msg_start_after_padding_3() {
let bytes = hex!("1b1b1b1b 01010101 120000 1b1b1b1b 01010101 1b1b1b1b 1a014ff4");
let exp = &[
Err(DecodeErr::DiscardedBytes(11)),
Err(InvalidMessage {
checksum_mismatch: (0xf44f, 0xf44f),
end_esc_misaligned: false,
num_padding_bytes: 1,
invalid_padding_bytes: true,
}),
];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn msg_end_with_zeroes_and_padding() {
let bytes = hex!("1b1b1b1b 01010101 12000000 1b1b1b1b 1a01e1b1");
let exp = &[Ok(hex!("120000").as_slice())];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}

#[test]
fn many_zeroes_in_msg() {
let bytes = hex!(
"1b1b1b1b 01010101 12345678"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"1b1b1b1b 1a00f14a"
);
let exp_bytes = hex!(
"12345678"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
"00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000"
);
let exp = &[Ok(exp_bytes.as_slice())];
test_parse_input::<ArrayBuf<1024>>(&bytes, exp);
}

#[test]
fn eof_after_zero() {
let bytes = hex!("1b1b1b1b 01010101 12340000");
let exp = &[Err(DecodeErr::DiscardedBytes(12))];
test_parse_input::<ArrayBuf<12>>(&bytes, exp);
}
}
4 changes: 3 additions & 1 deletion tests/snapshots/libsml_testing__files@EasyMeter_Q3A_A1064V1009.bin.snap
View file
Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ Err(
),
end_esc_misaligned: false,
num_padding_bytes: 3,
invalid_padding_bytes: false,
},
)
Ok(
Expand Down Expand Up @@ -242,6 +243,7 @@ Err(
),
end_esc_misaligned: true,
num_padding_bytes: 3,
invalid_padding_bytes: false,
},
)
Err(
Expand All @@ -252,6 +254,7 @@ Err(
),
end_esc_misaligned: true,
num_padding_bytes: 3,
invalid_padding_bytes: false,
},
)
Ok(
Expand Down Expand Up @@ -475,4 +478,3 @@ Err(
146,
),
)

3 changes: 2 additions & 1 deletion tests/snapshots/libsml_testing__files@dzg_dwsb20_2th_3byte.bin.snap
View file
Original file line number Diff line number Diff line change
Expand Up @@ -147,6 +147,7 @@ Err(
),
end_esc_misaligned: true,
num_padding_bytes: 1,
invalid_padding_bytes: false,
},
)
Ok(
Expand Down Expand Up @@ -769,6 +770,7 @@ Err(
),
end_esc_misaligned: true,
num_padding_bytes: 1,
invalid_padding_bytes: false,
},
)
Ok(
Expand Down Expand Up @@ -980,4 +982,3 @@ Err(
68,
),
)

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