[Tokio-Postgres] Error handling

[JojiiOfficial]

How to check against a certain error::Kind given a result eg coming from query_one(...)?
The kind enum is private but I need to check somehow against Kind::RowCount in order to check whether its a 'real' error or if simply no entry was found.

[sfackler]

If you want to programmatically interact with the number of rows returned, I would recommend just using client.query instead.

[JojiiOfficial]

So I have to unnecessarily allocate a vector, load all entries from the database, even though I always want just one single item? Why is the error type almost entirely closed?

[sfackler]

You are going to be loading all entries from the database regardless of if you consume them or not in the client. If you have carefully benchmarked your application and found that the time spent allocating a one element vector for this process is significant, then you can take the approach in the implementation of query_one: https://github.com/sfackler/rust-postgres/blob/master/tokio-postgres/src/client.rs.

The intended use case for query_one is if any number of rows other than exactly 1 is a programming error.

[JojiiOfficial]

Wouldn't be an own implementation of query_one() a much bigger effort than simply making the error type and Kind public and providing some sort of get_kind() function? It needs to be maintained and its unnecessarily duplicated code. What is the reason for the error type being private?

It is not necessarily about the performance impact of allocating a new vector but I don't see any reason to do so in this chase since simply providing a more public error type would fix this issue and probably more.

Would you agree with a PR which improves this error handling by making the error type and the Kind enum more accessible?

[sfackler]

Can you explain more concretely what you want to do when you encounter a row count error? The kind is made opaque to provide more flexibility in the future.

[l-7-l]

Can you explain more concretely what you want to do when you encounter a row count error? The kind is made opaque to provide more flexibility in the future.

I have some immature opinions, database errors may be divided into two types:

1. Infrastructure errors: such as timeout, connection closed
2. Operation error: not found and some other similar

In my project, the format of data model and data table is inconsistent, i need to manually convert it and map tokio-postgre Error to Custom Error in order to return the corresponding service status code instead of all returning 500 or 404 or similar

   // model
   struct Base<ID> {
     id: ID,
     created_at: DateTime<Utc>
   }
   type ModelID = SomeStringID;
    struct Model {
        base: Base<ModelId>,
        name: String
    }

  -- table
  id: text,
   name: text
  created_at: timestamptz

so I think it is necessary to make ErrorKind public or make query_one result optional?

[sfackler]

or make query_one result optional?

https://docs.rs/tokio-postgres/0.7.2/tokio_postgres/struct.Client.html#method.query_opt

[johnmave126]

Encountered similar issue.
I am just wondering whether there is a easy way to differentiate between "too many rows returned" and "database connection is closed"? It seems that both these 2 errors only have kind but not cause.

[sfackler]

Count the rows yourself.

[stanislav-tkach]

I also find this inconvenient. In my case I'm dealing with a library that returns a custom error with tokio_postgres::Error inside one of its variants. Sure it would be better to return Result<Option<T>, tokio_postgres::Error> from the library itself, but I will be able to work this around in my code if the error kind in the tokio_postgres is public.

[beowulf1416]

i'm trying to handle this error by comparing the Error result in an if statement which doesn't work.
if e == Error::row_count()
doesn't work because the row_count function is private. can you turn make it public or turn it into an enum? is there another way to do this?

[ehrktia]

am having a similar issue as above, using query_one to check if a record exists in the table.
if not it returns the tokio_postgres::Errror. This error covers both infra and operation errors.Would be helpful to
have a kind() operation on an error as the public interface.
As a work around am trying to use Row.is_empty() from the result to determine the error kind.

[BaptisteRoseau]

I believe it would be better to enable users to match on the enum kind to give them more control for error conversion.

This can simply be done by making the Kind enum public and adding a getter en the error struct.

• rust-postgres/tokio-postgres/src/error/mod.rs

  Line 340 in 98f5a11

  enum Kind {

• rust-postgres/tokio-postgres/src/error/mod.rs

  Line 409 in 98f5a11

  impl error::Error for Error {

For example, as a user library I want to be able to convert the error for specific cases:

#[derive(thiserror::Error)]
pub(crate) enum MyDatabaseError {
    #[error("'{0}' does not exist")]
    NotFound(String),
    #[error("Cannot add this resource as it already exists")]
    AlreadyExists,
    #[error("Unable to authenticate database")]
    Authentication,
    #[error(transparent)]
    PostgresError(tokio_postgres::Error),
}

impl From<tokio_postgres::Error> for MyDatabaseError {
    fn from(err: tokio_postgres::Error) -> Self {
        match err.kind() {
            Kind::RowCount => DatabaseError::AlreadyExists,
            Kind::Authentication => DatabaseError::Authentication,
            Kind::RowCount => DatabaseError::NotFound("Maybe extract more info from 'err' here to specify what was not found".to_string()),
            _ => DatabaseError::PostgresError(err),
        }
    }
}

It is not convenient to manually count the rows when we know only one is expected. Rust offers such a rich way to deal with errors, we definitely should give more read-only control to the user.

Edit: I made a pull request with the modification, see #1138

[DiegoTavares]

Any plan to merge the PR adding this feature on the next release? Is there a plan to merge @BaptisteRoseau 's PR and release this soon?