Decode internal "char" type (OID 18) natively

Fixes #165. Any query touching PostgreSQL system
catalogs (pg_type, pg_class, pg_attribute, pg_proc, ...) raised
RustToPyValueMappingError because the internal "char" type — distinct
from character(n)/BPCHAR — had no native decoder and fell through to
other_postgres_bytes_to_py.

Add an InternalChar(u8) wrapper next to the existing InternalUuid /
InnerDecimal / InnerInterval helpers and wire it into postgres_bytes_to_py
via two new match arms (Type::CHAR, Type::CHAR_ARRAY). The byte is read
through tokio-postgres' i8 FromSql impl, cast back to u8, and mapped to a
one-character Python str through char::from(u8) — i.e. Unicode code
points 0..=255 (Latin-1 round-trip), matching psycopg2/psycopg3.

The custom_decoders dispatch is intentionally unchanged: it stays keyed
by column name per the existing documented contract.

Tests:
- python/tests/test_value_converter.py:
  * test_char_internal_type_pg_type_reproduction — exact snippet from #165
  * test_char_internal_type_byte_spectrum — reachable ASCII bytes 0x20,
    0x41, 0x61, 0x7E plus NULL (SQL chr() rejects NUL and re-encodes
    >=0x80 as multi-byte UTF-8)
  * test_char_internal_type_array — "char"[] decoded to list[str]
- src/value_converter/models/internal_char.rs:
  * from_sql_round_trips_full_byte_range — full 0..=255 byte mapping the
    SQL test cannot reach
  * accepts_only_char_type — type guard rejects TEXT/VARCHAR/BPCHAR

Author: Dev-iL

python/tests/test_value_converter.py

@@ -625,6 +625,85 @@ class TestStrEnum(str, Enum):
     assert qs_result.result()[0]["test_mood2"] == TestStrEnum.OK
 
 
+async def test_char_internal_type_pg_type_reproduction(
+    psql_pool: ConnectionPool,
+) -> None:
+    """Regression for issue #165.
+
+    The original repro queried system catalog columns of the internal
+    ``"char"`` type (OID 18). Prior to the fix this raised
+    ``RustToPyValueMappingError`` even when ``custom_decoders`` was supplied,
+    because the type had no native decoder.
+    """
+    async with psql_pool.acquire() as conn:
+        result = await conn.execute(
+            "SELECT typname, typtype FROM pg_type LIMIT 5",
+        )
+        rows = result.result()
+        assert len(rows) == 5
+        for row in rows:
+            assert isinstance(row["typname"], str)
+            assert isinstance(row["typtype"], str)
+            assert len(row["typtype"]) == 1
+
+
+async def test_char_internal_type_byte_spectrum(
+    psql_pool: ConnectionPool,
+) -> None:
+    """Round-trip representative ASCII bytes through a ``"char"`` column.
+
+    The internal ``"char"`` type holds a single byte. SQL ``chr(N)`` rejects
+    NUL (0x00) with "null character not permitted", and ``chr(N)`` for N >= 128
+    produces multi-byte UTF-8 whose cast to ``"char"`` keeps only the first byte
+    (e.g. chr(128)::"char" stores 0xC2, not 0x80). So this integration test
+    covers the reachable ASCII slice. The full 0..=255 byte mapping is verified
+    by the Rust unit test in models/internal_char.rs.
+    """
+    bytes_under_test = [0x20, 0x41, 0x61, 0x7E]
+
+    async with psql_pool.acquire() as conn:
+        await conn.execute('DROP TABLE IF EXISTS for_char_test')
+        await conn.execute(
+            'CREATE TABLE for_char_test (id INT, c "char")',
+        )
+        for i, b in enumerate(bytes_under_test):
+            await conn.execute(
+                'INSERT INTO for_char_test (id, c) VALUES ($1, chr($2)::"char")',
+                [i, b],
+            )
+        await conn.execute(
+            'INSERT INTO for_char_test (id, c) VALUES ($1, NULL)',
+            [len(bytes_under_test)],
+        )
+
+        result = await conn.execute(
+            "SELECT id, c FROM for_char_test ORDER BY id",
+        )
+        rows = result.result()
+
+    decoded = {row["id"]: row["c"] for row in rows}
+    for i, b in enumerate(bytes_under_test):
+        value = decoded[i]
+        assert isinstance(value, str)
+        assert len(value) == 1
+        assert ord(value) == b, (
+            f"byte 0x{b:02x} round-tripped to ord(value)=0x{ord(value):02x}"
+        )
+    assert decoded[len(bytes_under_test)] is None
+
+
+async def test_char_internal_type_array(
+    psql_pool: ConnectionPool,
+) -> None:
+    """Decode an array of ``"char"`` (OID 1002) into a list of one-character strs."""
+    async with psql_pool.acquire() as conn:
+        result = await conn.execute(
+            "SELECT ARRAY['a'::\"char\", 'b'::\"char\", 'c'::\"char\"] AS chars",
+        )
+        rows = result.result()
+        assert rows[0]["chars"] == ["a", "b", "c"]
+
+
 async def test_custom_type_as_parameter(
     psql_pool: ConnectionPool,
 ) -> None:

src/value_converter/models/internal_char.rs

@@ -0,0 +1,72 @@
+use postgres_types::FromSql;
+use pyo3::{types::PyString, Bound, IntoPyObject, Python};
+use tokio_postgres::types::Type;
+
+use crate::exceptions::rust_errors::RustPSQLDriverError;
+
+/// Wrapper around the single-byte payload of PostgreSQL's internal `"char"`
+/// type (OID 18, distinct from `character(n)`/BPCHAR). Bytes 0..=255 map to
+/// Unicode code points 0..=255 (Latin-1 round-trip), matching psycopg2/psycopg3.
+#[derive(Clone, Copy)]
+pub struct InternalChar(u8);
+
+impl<'py> IntoPyObject<'py> for InternalChar {
+    type Target = PyString;
+    type Output = Bound<'py, Self::Target>;
+    type Error = RustPSQLDriverError;
+
+    fn into_pyobject(self, py: Python<'py>) -> Result<Self::Output, Self::Error> {
+        let mut tmp = [0u8; 4];
+        let s = char::from(self.0).encode_utf8(&mut tmp);
+        Ok(PyString::new(py, s))
+    }
+}
+
+impl<'a> FromSql<'a> for InternalChar {
+    fn from_sql(
+        ty: &Type,
+        raw: &'a [u8],
+    ) -> Result<Self, Box<dyn std::error::Error + Sync + Send>> {
+        let byte = <i8 as FromSql>::from_sql(ty, raw)?;
+        Ok(InternalChar(byte as u8))
+    }
+
+    fn accepts(ty: &Type) -> bool {
+        *ty == Type::CHAR
+    }
+}
+
+#[cfg(test)]
+impl InternalChar {
+    pub(crate) fn byte(self) -> u8 {
+        self.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::InternalChar;
+    use postgres_types::{FromSql, Type};
+
+    #[test]
+    fn from_sql_round_trips_full_byte_range() {
+        // The signed-byte cast (i8 -> u8) inside from_sql must preserve every
+        // raw byte. Cover all 256 values so a sign-extension or normalization
+        // regression cannot slip through.
+        for b in 0u16..=255 {
+            let byte = b as u8;
+            let buf = [byte];
+            let decoded = <InternalChar as FromSql>::from_sql(&Type::CHAR, &buf)
+                .expect("char decode");
+            assert_eq!(decoded.byte(), byte, "byte 0x{byte:02x} not preserved");
+        }
+    }
+
+    #[test]
+    fn accepts_only_char_type() {
+        assert!(<InternalChar as FromSql>::accepts(&Type::CHAR));
+        assert!(!<InternalChar as FromSql>::accepts(&Type::TEXT));
+        assert!(!<InternalChar as FromSql>::accepts(&Type::VARCHAR));
+        assert!(!<InternalChar as FromSql>::accepts(&Type::BPCHAR));
+    }
+}

src/value_converter/models/mod.rs

@@ -1,4 +1,5 @@
 pub mod decimal;
+pub mod internal_char;
 pub mod interval;
 pub mod serde_value;
 pub mod uuid;

src/value_converter/to_python.rs

@@ -21,8 +21,8 @@ use crate::{
             RustRect,
         },
         models::{
-            decimal::InnerDecimal, interval::InnerInterval, serde_value::InternalSerdeValue,
-            uuid::InternalUuid,
+            decimal::InnerDecimal, internal_char::InternalChar, interval::InnerInterval,
+            serde_value::InternalSerdeValue, uuid::InternalUuid,
         },
     },
 };
@@ -191,6 +191,13 @@ fn postgres_bytes_to_py(
             composite_field_postgres_to_py::<Option<String>>(type_, buf, is_simple)?
                 .into_py_any(py)?,
         ),
+        // Convert internal "char" (OID 18, single byte) into a one-character str.
+        Type::CHAR => {
+            match composite_field_postgres_to_py::<Option<InternalChar>>(type_, buf, is_simple)? {
+                Some(ic) => Ok(ic.into_pyobject(py)?.unbind().into_any()),
+                None => Ok(py.None()),
+            }
+        }
         // ---------- Boolean Types ----------
         // Convert BOOL type into bool
         Type::BOOL => Ok(
@@ -367,6 +374,12 @@ fn postgres_bytes_to_py(
             composite_field_postgres_to_py::<Option<Array<String>>>(type_, buf, is_simple)?,
         )
         .into_py_any(py)?),
+        // Convert ARRAY of internal "char" into list[str] (each element is one byte).
+        Type::CHAR_ARRAY => Ok(postgres_array_to_py(
+            py,
+            composite_field_postgres_to_py::<Option<Array<InternalChar>>>(type_, buf, is_simple)?,
+        )
+        .into_py_any(py)?),
         // ---------- Array Integer Types ----------
         // Convert ARRAY of SmallInt into Vec<i16>, then into list[int]
         Type::INT2_ARRAY => Ok(postgres_array_to_py(