tokio_postgres/

client.rs

#[cfg(feature = "runtime")]
use crate::Socket;
use crate::codec::{BackendMessages, FrontendMessage};
use crate::config::{SslMode, SslNegotiation};
use crate::connection::{Request, RequestMessages};
use crate::copy_out::CopyOutStream;
#[cfg(feature = "runtime")]
use crate::keepalive::KeepaliveConfig;
use crate::query::RowStream;
use crate::simple_query::SimpleQueryStream;
#[cfg(feature = "runtime")]
use crate::tls::MakeTlsConnect;
use crate::tls::TlsConnect;
use crate::types::{Oid, ToSql, Type};
use crate::{
    CancelToken, CopyInSink, Error, Row, SimpleQueryMessage, Statement, ToStatement, Transaction,
    TransactionBuilder, copy_in, copy_out, prepare, query, simple_query, slice_iter,
};
use bytes::{Buf, BytesMut};
use fallible_iterator::FallibleIterator;
use futures_channel::mpsc;
use futures_util::{StreamExt, TryStreamExt};
use parking_lot::Mutex;
use postgres_protocol::message::backend::Message;
use postgres_protocol::message::frontend;
use postgres_types::{BorrowToSql, FromSqlOwned};
use std::collections::HashMap;
use std::fmt;
use std::future;
#[cfg(feature = "runtime")]
use std::net::IpAddr;
#[cfg(feature = "runtime")]
use std::path::PathBuf;
use std::pin::pin;
use std::sync::Arc;
use std::task::{Context, Poll, ready};
#[cfg(feature = "runtime")]
use std::time::Duration;
use tokio::io::{AsyncRead, AsyncWrite};

pub struct Responses {
    receiver: mpsc::Receiver<BackendMessages>,
    cur: BackendMessages,
}

impl Responses {
    pub fn poll_next(&mut self, cx: &mut Context<'_>) -> Poll<Result<Message, Error>> {
        loop {
            match self.cur.next().map_err(Error::parse)? {
                Some(Message::ErrorResponse(body)) => return Poll::Ready(Err(Error::db(body))),
                Some(message) => return Poll::Ready(Ok(message)),
                None => {}
            }

            match ready!(self.receiver.poll_next_unpin(cx)) {
                Some(messages) => self.cur = messages,
                None => return Poll::Ready(Err(Error::closed())),
            }
        }
    }

    pub async fn next(&mut self) -> Result<Message, Error> {
        future::poll_fn(|cx| self.poll_next(cx)).await
    }
}

/// A cache of type info and prepared statements for fetching type info
/// (corresponding to the queries in the [prepare](prepare) module).
#[derive(Default)]
struct CachedTypeInfo {
    /// A statement for basic information for a type from its
    /// OID. Corresponds to [TYPEINFO_QUERY](prepare::TYPEINFO_QUERY) (or its
    /// fallback).
    typeinfo: Option<Statement>,
    /// A statement for getting information for a composite type from its OID.
    /// Corresponds to [TYPEINFO_QUERY](prepare::TYPEINFO_COMPOSITE_QUERY).
    typeinfo_composite: Option<Statement>,
    /// A statement for getting information for a composite type from its OID.
    /// Corresponds to [TYPEINFO_QUERY](prepare::TYPEINFO_COMPOSITE_QUERY) (or
    /// its fallback).
    typeinfo_enum: Option<Statement>,

    /// Cache of types already looked up.
    types: HashMap<Oid, Type>,
}

pub struct InnerClient {
    sender: mpsc::UnboundedSender<Request>,
    cached_typeinfo: Mutex<CachedTypeInfo>,

    /// A buffer to use when writing out postgres commands.
    buffer: Mutex<BytesMut>,
}

impl InnerClient {
    pub fn send(&self, messages: RequestMessages) -> Result<Responses, Error> {
        let (sender, receiver) = mpsc::channel(1);
        let request = Request { messages, sender };
        self.sender
            .unbounded_send(request)
            .map_err(|_| Error::closed())?;

        Ok(Responses {
            receiver,
            cur: BackendMessages::empty(),
        })
    }

    pub fn typeinfo(&self) -> Option<Statement> {
        self.cached_typeinfo.lock().typeinfo.clone()
    }

    pub fn set_typeinfo(&self, statement: &Statement) {
        self.cached_typeinfo.lock().typeinfo = Some(statement.clone());
    }

    pub fn typeinfo_composite(&self) -> Option<Statement> {
        self.cached_typeinfo.lock().typeinfo_composite.clone()
    }

    pub fn set_typeinfo_composite(&self, statement: &Statement) {
        self.cached_typeinfo.lock().typeinfo_composite = Some(statement.clone());
    }

    pub fn typeinfo_enum(&self) -> Option<Statement> {
        self.cached_typeinfo.lock().typeinfo_enum.clone()
    }

    pub fn set_typeinfo_enum(&self, statement: &Statement) {
        self.cached_typeinfo.lock().typeinfo_enum = Some(statement.clone());
    }

    pub fn type_(&self, oid: Oid) -> Option<Type> {
        self.cached_typeinfo.lock().types.get(&oid).cloned()
    }

    pub fn set_type(&self, oid: Oid, type_: &Type) {
        self.cached_typeinfo.lock().types.insert(oid, type_.clone());
    }

    pub fn clear_type_cache(&self) {
        self.cached_typeinfo.lock().types.clear();
    }

    /// Call the given function with a buffer to be used when writing out
    /// postgres commands.
    pub fn with_buf<F, R>(&self, f: F) -> R
    where
        F: FnOnce(&mut BytesMut) -> R,
    {
        let mut buffer = self.buffer.lock();
        let r = f(&mut buffer);
        buffer.clear();
        r
    }
}

#[cfg(feature = "runtime")]
#[derive(Clone)]
pub(crate) struct SocketConfig {
    pub addr: Addr,
    pub hostname: Option<String>,
    pub port: u16,
    pub connect_timeout: Option<Duration>,
    pub tcp_user_timeout: Option<Duration>,
    pub keepalive: Option<KeepaliveConfig>,
}

#[cfg(feature = "runtime")]
#[derive(Clone)]
pub(crate) enum Addr {
    Tcp(IpAddr),
    #[cfg(unix)]
    Unix(PathBuf),
}

/// An asynchronous PostgreSQL client.
///
/// The client is one half of what is returned when a connection is established. Users interact with the database
/// through this client object.
pub struct Client {
    inner: Arc<InnerClient>,
    #[cfg(feature = "runtime")]
    socket_config: Option<SocketConfig>,
    ssl_mode: SslMode,
    ssl_negotiation: SslNegotiation,
    process_id: i32,
    secret_key: i32,
}

impl Client {
    pub(crate) fn new(
        sender: mpsc::UnboundedSender<Request>,
        ssl_mode: SslMode,
        ssl_negotiation: SslNegotiation,
        process_id: i32,
        secret_key: i32,
    ) -> Client {
        Client {
            inner: Arc::new(InnerClient {
                sender,
                cached_typeinfo: Default::default(),
                buffer: Default::default(),
            }),
            #[cfg(feature = "runtime")]
            socket_config: None,
            ssl_mode,
            ssl_negotiation,
            process_id,
            secret_key,
        }
    }

    pub(crate) fn inner(&self) -> &Arc<InnerClient> {
        &self.inner
    }

    #[cfg(feature = "runtime")]
    pub(crate) fn set_socket_config(&mut self, socket_config: SocketConfig) {
        self.socket_config = Some(socket_config);
    }

    /// Creates a new prepared statement.
    ///
    /// Prepared statements can be executed repeatedly, and may contain query parameters (indicated by `$1`, `$2`, etc),
    /// which are set when executed. Prepared statements can only be used with the connection that created them.
    pub async fn prepare(&self, query: &str) -> Result<Statement, Error> {
        self.prepare_typed(query, &[]).await
    }

    /// Like `prepare`, but allows the types of query parameters to be explicitly specified.
    ///
    /// The list of types may be smaller than the number of parameters - the types of the remaining parameters will be
    /// inferred. For example, `client.prepare_typed(query, &[])` is equivalent to `client.prepare(query)`.
    pub async fn prepare_typed(
        &self,
        query: &str,
        parameter_types: &[Type],
    ) -> Result<Statement, Error> {
        prepare::prepare(&self.inner, query, parameter_types).await
    }

    /// Executes a statement, returning a vector of the resulting rows.
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    pub async fn query<T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<Vec<Row>, Error>
    where
        T: ?Sized + ToStatement,
    {
        self.query_raw(statement, slice_iter(params))
            .await?
            .try_collect()
            .await
    }

    /// Returns a vector of scalars.
    pub async fn query_scalar<R: FromSqlOwned, T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<Vec<R>, Error>
    where
        T: ?Sized + ToStatement + fmt::Debug,
    {
        let rows: Vec<Row> = self
            .query_raw(statement, slice_iter(params))
            .await?
            .try_collect()
            .await?;

        if let Some(row) = rows.first() {
            if row.len() != 1 {
                return Err(Error::column_count());
            }
        };

        rows.into_iter().map(|r| r.try_get(0)).collect()
    }

    /// Executes a statement which returns a single row, returning it.
    ///
    /// Returns an error if the query does not return exactly one row.
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    pub async fn query_one<T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<Row, Error>
    where
        T: ?Sized + ToStatement,
    {
        self.query_opt(statement, params)
            .await
            .and_then(|res| res.ok_or_else(Error::row_count))
    }

    /// Like [`Client::query_one`] but returns one scalar.
    pub async fn query_one_scalar<R: FromSqlOwned, T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<R, Error>
    where
        T: ?Sized + ToStatement + fmt::Debug,
    {
        let row = self.query_one(statement, params).await?;

        if row.len() != 1 {
            return Err(Error::column_count());
        }

        row.try_get(0)
    }

    /// Executes a statements which returns zero or one rows, returning it.
    ///
    /// Returns an error if the query returns more than one row.
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    pub async fn query_opt<T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<Option<Row>, Error>
    where
        T: ?Sized + ToStatement,
    {
        let mut stream = pin!(self.query_raw(statement, slice_iter(params)).await?);

        let mut first = None;

        // Originally this was two calls to `try_next().await?`,
        // once for the first element, and second to error if more than one.
        //
        // However, this new form with only one .await in a loop generates
        // slightly smaller codegen/stack usage for the resulting future.
        while let Some(row) = stream.try_next().await? {
            if first.is_some() {
                return Err(Error::row_count());
            }

            first = Some(row);
        }

        Ok(first)
    }

    /// Like [`Client::query_opt`] but returns an optional scalar.
    pub async fn query_opt_scalar<R: FromSqlOwned, T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<Option<R>, Error>
    where
        T: ?Sized + ToStatement + fmt::Debug,
    {
        let row = self.query_opt(statement, params).await?;

        if let Some(row) = &row {
            if row.len() != 1 {
                return Err(Error::column_count());
            }
        }

        row.map(|x| x.try_get::<_, R>(0)).transpose()
    }

    /// The maximally flexible version of [`query`].
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    ///
    /// [`query`]: #method.query
    ///
    /// # Examples
    ///
    /// ```no_run
    /// # async fn async_main(client: &tokio_postgres::Client) -> Result<(), tokio_postgres::Error> {
    /// use std::pin::pin;
    /// use futures_util::TryStreamExt;
    ///
    /// let params: Vec<String> = vec![
    ///     "first param".into(),
    ///     "second param".into(),
    /// ];
    /// let mut it = pin!(client.query_raw(
    ///     "SELECT foo FROM bar WHERE biz = $1 AND baz = $2",
    ///     params,
    /// ).await?);
    ///
    /// while let Some(row) = it.try_next().await? {
    ///     let foo: i32 = row.get("foo");
    ///     println!("foo: {}", foo);
    /// }
    /// # Ok(())
    /// # }
    /// ```
    pub async fn query_raw<T, P, I>(&self, statement: &T, params: I) -> Result<RowStream, Error>
    where
        T: ?Sized + ToStatement,
        P: BorrowToSql,
        I: IntoIterator<Item = P>,
        I::IntoIter: ExactSizeIterator,
    {
        let statement = statement.__convert().into_statement(&self.inner).await?;
        query::query(&self.inner, statement, params).await
    }

    /// Like `query`, but requires the types of query parameters to be explicitly specified.
    ///
    /// Compared to `query`, this method allows performing queries without three round trips (for
    /// prepare, execute, and close) by requiring the caller to specify parameter values along with
    /// their Postgres type. Thus, this is suitable in environments where prepared statements aren't
    /// supported (such as Cloudflare Workers with Hyperdrive).
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the
    /// parameter of the list provided, 1-indexed.
    pub async fn query_typed(
        &self,
        query: &str,
        params: &[(&(dyn ToSql + Sync), Type)],
    ) -> Result<Vec<Row>, Error> {
        self.query_typed_raw(query, params.iter().map(|(v, t)| (*v, t.clone())))
            .await?
            .try_collect()
            .await
    }

    /// Like `query_one`, but requires the types of query parameters to be explicitly specified.
    ///
    /// Compared to `query_one`, this method allows performing queries without three round trips (for
    /// prepare, execute, and close) by requiring the caller to specify parameter values along with
    /// their Postgres type. Thus, this is suitable in environments where prepared statements aren't
    /// supported (such as Cloudflare Workers with Hyperdrive).
    ///
    /// Executes a statement which returns a single row, returning it.
    ///
    /// Returns an error if the query does not return exactly one row.
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    pub async fn query_typed_one(
        &self,
        statement: &str,
        params: &[(&(dyn ToSql + Sync), Type)],
    ) -> Result<Row, Error> {
        self.query_typed_opt(statement, params)
            .await
            .and_then(|res| res.ok_or_else(Error::row_count))
    }

    /// Like `query_one`, but requires the types of query parameters to be explicitly specified.
    ///
    /// Compared to `query_one`, this method allows performing queries without three round trips (for
    /// prepare, execute, and close) by requiring the caller to specify parameter values along with
    /// their Postgres type. Thus, this is suitable in environments where prepared statements aren't
    /// supported (such as Cloudflare Workers with Hyperdrive).
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the
    /// parameter of the list provided, 1-indexed.
    /// Executes a statements which returns zero or one rows, returning it.
    ///
    /// Returns an error if the query returns more than one row.
    pub async fn query_typed_opt(
        &self,
        statement: &str,
        params: &[(&(dyn ToSql + Sync), Type)],
    ) -> Result<Option<Row>, Error> {
        let mut stream = pin!(
            self.query_typed_raw(statement, params.iter().map(|(v, t)| (*v, t.clone())))
                .await?
        );

        let mut first = None;

        // Originally this was two calls to `try_next().await?`,
        // once for the first element, and second to error if more than one.
        //
        // However, this new form with only one .await in a loop generates
        // slightly smaller codegen/stack usage for the resulting future.
        while let Some(row) = stream.try_next().await? {
            if first.is_some() {
                return Err(Error::row_count());
            }

            first = Some(row);
        }

        Ok(first)
    }

    /// The maximally flexible version of [`query_typed`].
    ///
    /// Compared to `query`, this method allows performing queries without three round trips (for
    /// prepare, execute, and close) by requiring the caller to specify parameter values along with
    /// their Postgres type. Thus, this is suitable in environments where prepared statements aren't
    /// supported (such as Cloudflare Workers with Hyperdrive).
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the
    /// parameter of the list provided, 1-indexed.
    ///
    /// [`query_typed`]: #method.query_typed
    ///
    /// # Examples
    ///
    /// ```no_run
    /// # async fn async_main(client: &tokio_postgres::Client) -> Result<(), tokio_postgres::Error> {
    /// use std::pin::pin;
    /// use futures_util::{TryStreamExt};
    /// use tokio_postgres::types::Type;
    ///
    /// let params: Vec<(String, Type)> = vec![
    ///     ("first param".into(), Type::TEXT),
    ///     ("second param".into(), Type::TEXT),
    /// ];
    /// let mut it = pin!(client.query_typed_raw(
    ///     "SELECT foo FROM bar WHERE biz = $1 AND baz = $2",
    ///     params,
    /// ).await?);
    ///
    /// while let Some(row) = it.try_next().await? {
    ///     let foo: i32 = row.get("foo");
    ///     println!("foo: {}", foo);
    /// }
    /// # Ok(())
    /// # }
    /// ```
    pub async fn query_typed_raw<P, I>(&self, query: &str, params: I) -> Result<RowStream, Error>
    where
        P: BorrowToSql,
        I: IntoIterator<Item = (P, Type)>,
    {
        query::query_typed(&self.inner, query, params).await
    }

    /// Executes a statement, returning the number of rows modified.
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    ///
    /// If the statement does not modify any rows (e.g. `SELECT`), 0 is returned.
    pub async fn execute<T>(
        &self,
        statement: &T,
        params: &[&(dyn ToSql + Sync)],
    ) -> Result<u64, Error>
    where
        T: ?Sized + ToStatement,
    {
        self.execute_raw(statement, slice_iter(params)).await
    }

    /// Executes a statement, returning the number of rows modified.
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    ///
    /// If the statement does not modify any rows (e.g. `SELECT`), 0 is returned.
    pub async fn execute_typed(
        &self,
        statement: &str,
        params: &[(&(dyn ToSql + Sync), Type)],
    ) -> Result<u64, Error> {
        query::execute_typed(
            &self.inner,
            statement,
            params.iter().map(|(v, t)| (*v, t.clone())),
        )
        .await
    }

    /// The maximally flexible version of [`execute`].
    ///
    /// A statement may contain parameters, specified by `$n`, where `n` is the index of the parameter of the list
    /// provided, 1-indexed.
    ///
    /// The `statement` argument can either be a `Statement`, or a raw query string. If the same statement will be
    /// repeatedly executed (perhaps with different query parameters), consider preparing the statement up front
    /// with the `prepare` method.
    ///
    /// [`execute`]: #method.execute
    pub async fn execute_raw<T, P, I>(&self, statement: &T, params: I) -> Result<u64, Error>
    where
        T: ?Sized + ToStatement,
        P: BorrowToSql,
        I: IntoIterator<Item = P>,
        I::IntoIter: ExactSizeIterator,
    {
        let statement = statement.__convert().into_statement(&self.inner).await?;
        query::execute(self.inner(), statement, params).await
    }

    /// Executes a `COPY FROM STDIN` statement, returning a sink used to write the copy data.
    ///
    /// PostgreSQL does not support parameters in `COPY` statements, so this method does not take any. The copy *must*
    /// be explicitly completed via the `Sink::close` or `finish` methods. If it is not, the copy will be aborted.
    pub async fn copy_in<T, U>(&self, statement: &T) -> Result<CopyInSink<U>, Error>
    where
        T: ?Sized + ToStatement,
        U: Buf + 'static + Send,
    {
        let statement = statement.__convert().into_statement(&self.inner).await?;
        copy_in::copy_in(self.inner(), statement).await
    }

    /// Executes a `COPY TO STDOUT` statement, returning a stream of the resulting data.
    ///
    /// PostgreSQL does not support parameters in `COPY` statements, so this method does not take any.
    pub async fn copy_out<T>(&self, statement: &T) -> Result<CopyOutStream, Error>
    where
        T: ?Sized + ToStatement,
    {
        let statement = statement.__convert().into_statement(&self.inner).await?;
        copy_out::copy_out(self.inner(), statement).await
    }

    /// Executes a sequence of SQL statements using the simple query protocol, returning the resulting rows.
    ///
    /// Statements should be separated by semicolons. If an error occurs, execution of the sequence will stop at that
    /// point. The simple query protocol returns the values in rows as strings rather than in their binary encodings,
    /// so the associated row type doesn't work with the `FromSql` trait. Rather than simply returning a list of the
    /// rows, this method returns a list of an enum which indicates either the completion of one of the commands,
    /// or a row of data. This preserves the framing between the separate statements in the request.
    ///
    /// # Warning
    ///
    /// Prepared statements should be use for any query which contains user-specified data, as they provided the
    /// functionality to safely embed that data in the request. Do not form statements via string concatenation and pass
    /// them to this method!
    pub async fn simple_query(&self, query: &str) -> Result<Vec<SimpleQueryMessage>, Error> {
        self.simple_query_raw(query).await?.try_collect().await
    }

    /// Executes a sequence of SQL statements using the simple query protocol, returning the resulting rows as a stream.
    ///
    /// Statements should be separated by semicolons. If an error occurs, execution of the sequence will stop at that
    /// point. The simple query protocol returns the values in rows as strings rather than in their binary encodings,
    /// so the associated row type doesn't work with the `FromSql` trait. Rather than simply returning a list of the
    /// rows, this method returns a list of an enum which indicates either the completion of one of the commands,
    /// or a row of data. This preserves the framing between the separate statements in the request.
    ///
    /// # Warning
    ///
    /// Prepared statements should be use for any query which contains user-specified data, as they provided the
    /// functionality to safely embed that data in the request. Do not form statements via string concatenation and pass
    /// them to this method!
    pub async fn simple_query_raw(&self, query: &str) -> Result<SimpleQueryStream, Error> {
        simple_query::simple_query(self.inner(), query).await
    }

    /// Executes a sequence of SQL statements using the simple query protocol.
    ///
    /// Statements should be separated by semicolons. If an error occurs, execution of the sequence will stop at that
    /// point. This is intended for use when, for example, initializing a database schema.
    ///
    /// # Warning
    ///
    /// Prepared statements should be use for any query which contains user-specified data, as they provided the
    /// functionality to safely embed that data in the request. Do not form statements via string concatenation and pass
    /// them to this method!
    pub async fn batch_execute(&self, query: &str) -> Result<(), Error> {
        simple_query::batch_execute(self.inner(), query).await
    }

    /// Check that the connection is alive and wait for the confirmation.
    pub async fn check_connection(&self) -> Result<(), Error> {
        // sync is a very quick message to test the connection health.
        query::sync(self.inner()).await
    }

    /// Begins a new database transaction.
    ///
    /// The transaction will roll back by default - use the `commit` method to commit it.
    pub async fn transaction(&mut self) -> Result<Transaction<'_>, Error> {
        self.build_transaction().start().await
    }

    /// Returns a builder for a transaction with custom settings.
    ///
    /// Unlike the `transaction` method, the builder can be used to control the transaction's isolation level and other
    /// attributes.
    pub fn build_transaction(&mut self) -> TransactionBuilder<'_> {
        TransactionBuilder::new(self)
    }

    /// Constructs a cancellation token that can later be used to request cancellation of a query running on the
    /// connection associated with this client.
    pub fn cancel_token(&self) -> CancelToken {
        CancelToken {
            #[cfg(feature = "runtime")]
            socket_config: self.socket_config.clone(),
            ssl_mode: self.ssl_mode,
            ssl_negotiation: self.ssl_negotiation,
            process_id: self.process_id,
            secret_key: self.secret_key,
        }
    }

    /// Attempts to cancel an in-progress query.
    ///
    /// The server provides no information about whether a cancellation attempt was successful or not. An error will
    /// only be returned if the client was unable to connect to the database.
    ///
    /// Requires the `runtime` Cargo feature (enabled by default).
    #[cfg(feature = "runtime")]
    #[deprecated(since = "0.6.0", note = "use Client::cancel_token() instead")]
    pub async fn cancel_query<T>(&self, tls: T) -> Result<(), Error>
    where
        T: MakeTlsConnect<Socket>,
    {
        self.cancel_token().cancel_query(tls).await
    }

    /// Like `cancel_query`, but uses a stream which is already connected to the server rather than opening a new
    /// connection itself.
    #[deprecated(since = "0.6.0", note = "use Client::cancel_token() instead")]
    pub async fn cancel_query_raw<S, T>(&self, stream: S, tls: T) -> Result<(), Error>
    where
        S: AsyncRead + AsyncWrite + Unpin,
        T: TlsConnect<S>,
    {
        self.cancel_token().cancel_query_raw(stream, tls).await
    }

    /// Clears the client's type information cache.
    ///
    /// When user-defined types are used in a query, the client loads their definitions from the database and caches
    /// them for the lifetime of the client. If those definitions are changed in the database, this method can be used
    /// to flush the local cache and allow the new, updated definitions to be loaded.
    pub fn clear_type_cache(&self) {
        self.inner().clear_type_cache();
    }

    /// Determines if the connection to the server has already closed.
    ///
    /// In that case, all future queries will fail.
    pub fn is_closed(&self) -> bool {
        self.inner.sender.is_closed()
    }

    #[doc(hidden)]
    pub fn __private_api_rollback(&self, name: Option<&str>) {
        let buf = self.inner().with_buf(|buf| {
            if let Some(name) = name {
                frontend::query(&format!("ROLLBACK TO {}", name), buf).unwrap();
            } else {
                frontend::query("ROLLBACK", buf).unwrap();
            }
            buf.split().freeze()
        });
        let _ = self
            .inner()
            .send(RequestMessages::Single(FrontendMessage::Raw(buf)));
    }

    #[doc(hidden)]
    pub fn __private_api_close(&mut self) {
        self.inner.sender.close_channel()
    }
}

impl fmt::Debug for Client {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Client").finish()
    }
}