PSQLDecodingError improvements

[fabianfett]

Split out to #209.

Motivation

Currently we hand a large PSQLDecodingContext into the PSQLDecodable's decode method. Lets look at it, as it is today:

struct PSQLDecodingContext {
    let jsonDecoder: PSQLJSONDecoder
    
    let columnIndex: Int
    let columnName: String
    
    let file: String
    let line: Int
    
    init(jsonDecoder: PSQLJSONDecoder, columnName: String, columnIndex: Int, file: String, line: Int) {
        self.jsonDecoder = jsonDecoder
        self.columnName = columnName
        self.columnIndex = columnIndex
        
        self.file = file
        self.line = line
    }
}

We use most of the PSQLDecodingContext's properties to create good error messages for users. Let's look at an implementation example:

    static func decode(from buffer: inout ByteBuffer, type: PSQLDataType, format: PSQLFormat, context: PSQLDecodingContext) throws -> Bool {
        guard type == .bool else {
            throw PSQLCastingError.failure(targetType: Self.self, type: type, postgresData: buffer, context: context)
        }
        
        switch format {
        case .binary:
            guard buffer.readableBytes == 1 else {
                throw PSQLCastingError.failure(targetType: Self.self, type: type, postgresData: buffer, context: context)
            }
            
            switch buffer.readInteger(as: UInt8.self) {
            case .some(0):
                return false
            case .some(1):
                return true
            default:
                throw PSQLCastingError.failure(targetType: Self.self, type: type, postgresData: buffer, context: context)
            }
        case .text:
            guard buffer.readableBytes == 1 else {
                throw PSQLCastingError.failure(targetType: Self.self, type: type, postgresData: buffer, context: context)
            }
            
            switch buffer.readInteger(as: UInt8.self) {
            case .some(UInt8(ascii: "f")):
                return false
            case .some(UInt8(ascii: "t")):
                return true
            default:
                throw PSQLCastingError.failure(targetType: Self.self, type: type, postgresData: buffer, context: context)
            }
        }
    }

As we can see the context is only used to create errors. If library adopters want to implement decoding for their own types, they need to forward the context as well, if they want to create good errors.

However we, as the Postgres library authors, don't need to pass the decoding info to the users and expect them to return it to us.

All decoding metadata can be kept in our code. For this reason users should return a PSQLDecodingError.Code from the decode method and we can create a full PSQLDecodingError based on this information. This also reduces code size quite a bit.

Changes

• Only return PSQLDecodingError.Code from PSQLDecodable decode function.
• Remove decode methods from PSQLData as those don't work anyway.
• Change Optional decoding to something actually working.

Result

• We can remove unused properties from PSQLDecodingContext
• Easier code for adopters to write
• Less code to maintain

[codecov-commenter]

Codecov Report

Merging #211 (99ba2c9) into main (7660f79) will increase coverage by 3.10%.
The diff coverage is 46.46%.

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   40.70%   43.81%   +3.10%     
==========================================
  Files         118      118              
  Lines        7706     7700       -6     
==========================================
+ Hits         3137     3374     +237     
+ Misses       4569     4326     -243     

Flag	Coverage Δ
unittests	43.81% <46.46%> (+3.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Sources/PostgresNIO/New/PSQLCodable.swift	77.77% <0.00%> (-22.23%)	⬇️
Sources/PostgresNIO/New/PSQLData.swift	0.00% <ø> (-100.00%)	⬇️
Sources/PostgresNIO/New/Data/Int+PSQLCodable.swift	20.26% <5.26%> (+2.61%)	⬆️
...ces/PostgresNIO/New/Data/Decimal+PSQLCodable.swift	74.19% <33.33%> (+6.45%)	⬆️
Sources/PostgresNIO/New/PSQLError.swift	57.74% <35.71%> (-26.04%)	⬇️
...ources/PostgresNIO/New/Data/Bool+PSQLCodable.swift	95.34% <60.00%> (+4.65%)	⬆️
...es/PostgresNIO/New/Data/Optional+PSQLCodable.swift	63.15% <70.00%> (+16.09%)	⬆️
...urces/PostgresNIO/New/Data/Float+PSQLCodable.swift	76.66% <75.00%> (+3.33%)	⬆️
...ources/PostgresNIO/New/Data/UUID+PSQLCodable.swift	98.30% <75.00%> (ø)
...urces/PostgresNIO/New/Data/Array+PSQLCodable.swift	94.68% <80.00%> (+1.06%)	⬆️
... and 34 more

[gwynne]

No actual serious changes need making, just want to be sure I'm clear on what's going on in a couple of spots before moving ahead.

[gwynne]

Looks g2g! 👍 ❤️