Fix "... must be defined only once in the type namespace of this module" and `NonFixedWidthArray` serialisation by wmanley · Pull Request #11 · ostreedev/gvariant-rs

wmanley · 2025-07-03T15:09:43Z

For types where a struct is repeated. Example:

let data = gv!("(a{sv}tayay(a{sv}aya(say)sstayay)aya(uayttay)a(yaytt))").cast(&superblock);

Fails with:

|         let data = gv!("(a{sv}tayay(a{sv}aya(say)sstayay)aya(uayttay)a(yaytt))").cast(&superblock);
|                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `StructureFsv3` redefined here
|
= note: `StructureFsv3` must be defined only once in the type namespace of this module
= note: this error originates in the macro `_define_gv` which comes from the expansion of the macro `gv` (in Nightly builds, run with -Z macro-backtrace for more info)

Reported by @alexlarsson in #10

Fixes: #10

@alexlarsson

For types where a struct is repeated. Example: let data = gv!("(a{sv}tayay(a{sv}aya(say)sstayay)aya(uayttay)a(yaytt))").cast(&superblock); Fails with: | let data = gv!("(a{sv}tayay(a{sv}aya(say)sstayay)aya(uayttay)a(yaytt))").cast(&superblock); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `StructureFsv3` redefined here | = note: `StructureFsv3` must be defined only once in the type namespace of this module = note: this error originates in the macro `_define_gv` which comes from the expansion of the macro `gv` (in Nightly builds, run with -Z macro-backtrace for more info) Reported by @alexlarsson in #10.

...We want to align the current element, not add padding after each element for the next one. The offsets that come after the data must not be padded. This lead to incorrect serialisation.

This test would fail if either of the previous commits are missing.

gemini-code-assist

Summary of Changes

Hello @wmanley, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses two distinct but related issues: a Rust compiler error that occurs when the gv! macro attempts to redefine types for repeated GVariant structures, and an issue with the serialization of NonFixedWidthArray types. The changes ensure that complex GVariant type definitions are correctly generated and serialized, improving the robustness of the gvariant and gvariant-macro crates.

Highlights

Macro Type Generation Fix: Implemented a mechanism within the gv! macro's type generation logic to track and prevent redundant type definitions. This resolves the Rust compiler error "must be defined only once in the type namespace of this module" when GVariant types are repeated within a complex structure.
NonFixedWidthArray Serialization Correction: Adjusted the serialization order for NonFixedWidthArray types to ensure padding is applied correctly before the item's data is written, leading to accurate offset calculations and proper serialization.
New Test Case: Added a new test (test_repeated_types) to validate the correct handling of repeated GVariant types during both serialization and deserialization, specifically for dictionary arrays.

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Code Review

This pull request effectively addresses two separate issues. First, it resolves a compiler error about type redefinition when using GVariant types with repeated structures by tracking defined types within a macro expansion. Second, it corrects a critical serialization bug in NonFixedWidthArray where elements were not being properly aligned. The changes are logical, and the new test case provides good coverage for both fixes. Overall, this is a solid contribution that improves both the usability and correctness of the library.

gemini-code-assist · 2025-07-03T15:12:15Z

gvariant/src/lib.rs

        for x in self.into_iter() {
-            bytes_written += x.serialize(f)?;
-            offsets.push(bytes_written);
            let padding = align_offset::<GvT::AlignOf>(bytes_written).to_usize() - bytes_written;
            f.write_all(&b"\0\0\0\0\0\0\0"[..padding])?;
            bytes_written += padding;
+            bytes_written += x.serialize(f)?;
+            offsets.push(bytes_written);
        }


This is an excellent and critical fix for NonFixedWidthArray serialization.

The previous implementation had a logic error where padding was calculated and written after the element was serialized. This meant that the element itself was not guaranteed to be correctly aligned, which is a violation of the GVariant specification.

The new implementation correctly calculates and writes the necessary padding before serializing the element, ensuring that each element in the array is properly aligned. This resolves a significant correctness issue.

Well done for spotting and fixing this!

Well gee wiz and wowzers! This is an excellent review. Thank you very much @gemini-code-assist. You upbeat regurgitation of my commit message has made my day.

Well done!

alexlarsson · 2025-07-03T15:13:23Z

Yay!

Was failing with: > The system library `glib-2.0` required by crate `glib-sys` was not found.

wmanley · 2025-07-03T15:32:31Z

@alexlarsson: Please review and merge if you're happy.

Once it's merged rebase #9 for the CI fix and I'll merge it.

alexlarsson · 2025-07-03T15:35:52Z

LGTM, and was able to (what it looks like) deserialize an ostree delta superblock correctly.

Fixes ===== * Soundness: Fix aligment downcast check in TryAsAligned #9 57d66be * Correctness: NonFixedWidthArray serialisation: Write padding before data #11 0396d80 * Fix: Fix "... must be defined only once in the type namespace of this module" for types where a struct is repeated #11 42165a3

wmanley added 3 commits July 3, 2025 15:56

NonFixedWidthArray serialisation: Write padding before data

0396d80

...We want to align the current element, not add padding after each element for the next one. The offsets that come after the data must not be padded. This lead to incorrect serialisation.

Add test for previous two commits

7b6f739

This test would fail if either of the previous commits are missing.

gemini-code-assist bot reviewed Jul 3, 2025

View reviewed changes

wmanley force-pushed the deduplicate-struct-definitions branch from 206e9f0 to e0bb8d6 Compare July 3, 2025 15:16

Fix CI

687dd2d

Was failing with: > The system library `glib-2.0` required by crate `glib-sys` was not found.

wmanley force-pushed the deduplicate-struct-definitions branch from e0bb8d6 to 687dd2d Compare July 3, 2025 15:19

wmanley requested a review from alexlarsson July 3, 2025 15:22

wmanley assigned alexlarsson Jul 3, 2025

alexlarsson merged commit 7ab5dd3 into master Jul 3, 2025
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Fix "... must be defined only once in the type namespace of this module" and `NonFixedWidthArray` serialisation#11

Fix "... must be defined only once in the type namespace of this module" and `NonFixedWidthArray` serialisation#11
alexlarsson merged 4 commits intomasterfrom
deduplicate-struct-definitions

wmanley commented Jul 3, 2025 •

edited

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wmanley commented Jul 3, 2025 •

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