usvg writing trips over defs referencing other defs

[flxzt]

When writing an usvg tree to string with the following Svg content, it seems to be failing this debug assertion while writing the group that is located in <defs>.

Reproducing the issue can be done by executing the usvg cli (built with the dev profile):

usvg minimal.svg out.svg

with this file (it is generated through cairo's svg surface in my application Rnote):

minimal

its content:

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="793.701px" height="1122.52px" viewBox="0 0 793.701 1122.52">
    <defs>
        <clipPath id="clip-0">
            <path clip-rule="nonzero" d="M 0 0 L 793.699219 0 L 793.699219 1122.519531 L 0 1122.519531 Z M 0 0 " />
        </clipPath>
        <clipPath id="clip-1">
            <rect x="0" y="0" width="37" height="37" />
        </clipPath>
        <g id="source-20076" clip-path="url(#clip-1)">
            <path fill-rule="nonzero" fill="rgb(70.196078%, 77.647059%, 81.568627%)" fill-opacity="1" d="M 0.488281 0 L 0.980469 0 C 1.25 0 1.46875 0.21875 1.46875 0.488281 L 1.46875 0.980469 C 1.46875 1.25 1.25 1.46875 0.980469 1.46875 L 0.488281 1.46875 C 0.21875 1.46875 0 1.25 0 0.980469 L 0 0.488281 C 0 0.21875 0.21875 0 0.488281 0 Z M 0.488281 0 " />
        </g>
        <pattern id="pattern-0" patternUnits="userSpaceOnUse" x="0" y="0" width="37" height="37" viewBox="0 0 37 37" patternTransform="matrix(1.021486, 0, 0, 1.021486, 0, 0)">
            <use xlink:href="#source-20076" />
        </pattern>
    </defs>
    <g clip-path="url(#clip-0)">
        <path fill-rule="nonzero" fill="rgb(100%, 100%, 100%)" fill-opacity="1" d="M 0 0 L 793.699219 0 L 793.699219 1122.519531 L 0 1122.519531 Z M 0 0 " />
        <path fill-rule="nonzero" fill="url(#pattern-0)" d="M 0 0 L 793.699219 0 L 793.699219 1122.519531 L 0 1122.519531 Z M 0 0 " />
    </g>
    <path fill="none" stroke-width="2" stroke-linecap="butt" stroke-linejoin="miter" stroke="rgb(10.196078%, 37.254902%, 70.196078%)" stroke-opacity="1" stroke-miterlimit="10" d="M 299 165 L 478 301 " />
</svg>

the stacktrace:

thread 'main' panicked at crates/usvg/src/writer.rs:148:9:
assertion failed: self.id_map.contains_key(&key)
stack backtrace:
   0: rust_begin_unwind
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:72:14
   2: core::panicking::panic
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:127:5
   3: usvg::writer::WriterContext::get_defs_id
             at ./crates/usvg/src/writer.rs:148:9
   4: usvg::writer::write_group_element
             at ./crates/usvg/src/writer.rs:1005:27
   5: usvg::writer::write_element
             at ./crates/usvg/src/writer.rs:834:13
   6: usvg::writer::write_elements
             at ./crates/usvg/src/writer.rs:800:9
   7: usvg::writer::write_group_element
             at ./crates/usvg/src/writer.rs:1062:5
   8: usvg::writer::write_element
             at ./crates/usvg/src/writer.rs:834:13
   9: usvg::writer::write_elements
             at ./crates/usvg/src/writer.rs:800:9
  10: usvg::writer::write_defs
             at ./crates/usvg/src/writer.rs:696:13
  11: usvg::writer::convert
             at ./crates/usvg/src/writer.rs:207:5
  12: <usvg_tree::Tree as usvg::TreeWriting>::to_string
             at ./crates/usvg/src/lib.rs:72:9
  13: usvg::process
             at ./crates/usvg/src/main.rs:447:13
  14: usvg::main
             at ./crates/usvg/src/main.rs:314:21
  15: core::ops::function::FnOnce::call_once
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[LaurenzV]

Yeah, it's somewhat of a known bug (see second example here), the problem is that we are currently not handling def's referencing other def's correctly.

[RazrFalcon]

Yes, this is a know issue. usvg CLI is beta quality. It's more like a demo at this point.

[RazrFalcon]

@LaurenzV if you have more tests for this issue - feel free to add them.

[LaurenzV]

I don't have any other ones currently, but will do if I encounter them.