Expose SRID from EWKB buffers when reading

[kylebarron]

Potentially we could even make this a separate standalone function, which would allow us to avoid exposing the concrete implementations on the reader side.

We already check whether the srid exists:

wkb/src/reader/util.rs

Lines 28 to 37 in 39bca1d

	pub(crate) fn has_srid(buf: &[u8], byte_order: Endianness, offset: u64) -> bool {
	// Read geometry code to see if an SRID exists.
	let mut reader = Cursor::new(buf);
	// Skip 1-byte byte order that we already know
	reader.set_position(1 + offset);
	let geometry_code = WKBGeometryCode::new(reader.read_u32(byte_order).unwrap());
	geometry_code.has_srid()
	}

[oreilles]

Ideally we'd have an fn srid(&self) -> Option<i32> in GeometryTrait to expose it wouldn't we ? That would also allow the writer to output EWKB, as a roundtrip isn't possible right now.

[kylebarron]

I agree that as part of this issue there should be a way to write an SRID to a WKB.

But I disagree that it should be part of GeometryTrait. SRID is a poor substitute for a full CRS, and only really makes sense in a database setting where you have a full lookup table to go from SRID integer to fully defined CRS. I don't think geo-traits should involve itself with CRS at all, in the same way that geo does planar operations without considering CRS.

In the case of WKB here, I think that we should add an options argument to the write APIs. So instead of the current signature

pub fn write_geometry(
    writer: &mut impl Write,
    geom: &impl GeometryTrait<T = f64>,
    endianness: Endianness,
) -> WKBResult<()>;

I think it would be better to have

#[derive(Copy)]
pub struct WkbOptions {
    pub endiannness: Endianness,
    // If not None, opts-in to EWKB and writes SRID.
    pub srid: Option<i32>,
}

pub fn write_geometry(
    writer: &mut impl Write,
    geom: &impl GeometryTrait<T = f64>,
    options: WkbOptions,
) -> WKBResult<()>;

[kylebarron]

I created #71 as the write-side counterpart to this issue.

I don't think I'll add this feature myself before the next release, but we can add this without a backwards-incompatible change as a new method on the Wkb struct.