Bug 2018944 - glean-sym - a Rust API built on top of the Glean UniFFI C FFI by badboy · Pull Request #3426 · mozilla/glean

badboy · 2026-03-30T11:20:22Z

This small patch on a-s makes use of the new glean-sys crate: mozilla/application-services@main...badboy:application-services:glean-dylib

Todo:

~~Naming! Not sure I like glean-sys.~~ it was decided: glean-sym (as in glean-symbols)
Testing. Not clear how we will do that. Probably some sample crate that uses glean-sys and we inject a libxul.so into it at test time
glean-sys/src/metrics.rs is generated. Currently by a test. That has a bit of a problem that you need to compile the code to generate the code, which breaks if the code doesn't compile. Might want to have a non-test way to generate that code.
libxul.so is hard-coded. Ok for manual testing in Fenix, not sure we want to ship that as-is
No way to ensure we're not the first ones loading libxul.
Should we have the UniFFI-alike runtime checks? (like the version check)
Extend glean_parser (and glean-build) to generate the correct code usable with glean-sys
- Bug 2018944 - Add support to target glean_sym glean_parser#841
Dependency vetting
Rust minimum version bump probably necessary (or we need to exclude glean-sys)

…ated code over FFI This will be used later by glean-sys to build a Rust API via C FFI.

…C FFI This loads `libxul.so` (currently hard-coded), looks up the required symbols and wraps that in a nice Rust API. Other crates can depend on `glean-sym`, without needing to have Glean in the same library.

not done yet

…vokes functions in them

badboy · 2026-04-23T16:15:34Z

I would like to land this (experimentally) sometime soon. The plan is to test it out with a metric in appservices, see https://bugzilla.mozilla.org/show_bug.cgi?id=2034534
PoC: badboy/application-services@2c0f79a

Given this is an experiment and not a released crate we can tackle the open questions as we go along, instead of landing the perfectly finished PR.

travis79

Okay, a decent first pass with several questions. Nothing really big aside from the comments in the metrics.rs about the mismatched Double type being bool

travis79 · 2026-04-29T18:33:49Z

+            std::mem::transmute(
+                (crate::GLEAN
+                    .uniffi_glean_core_fn_clone_countermetric)(
+                    std::mem::transmute(self.handle),
+                    &mut call_status,
+                ),


We call transmute twice in each of the clone_handle methods in this file, is that needed? Won't the underlying type be u64 on both sides?

travis79 · 2026-04-29T18:41:28Z

+    }
+
+    fn resolve_ffi(&self) -> TokenStream {
+        quote! { bool }


Should this be bool here? That doesn't seem correct

travis79 · 2026-04-29T18:43:03Z

+
+impl TypeResolver for weedle::term::Boolean {
+    fn resolve(&self) -> TokenStream {
+        quote! { i8 }


Does this mean consumers will need to cast this to a Rust bool?

travis79 · 2026-04-29T18:54:21Z

@@ -0,0 +1,119 @@
+// This Source Code Form is subject to the terms of the Mozilla Public


I take it this is pretty much duplication of types from glean-core? And, if we make changes there we need to make changes here too?

Be sure and document anything like that, preferably in a comment close to where these live so we don't forget to update things in both places.

travis79 · 2026-04-29T18:55:05Z

+
+impl CloneFfiArg<RustBuffer> for RustBuffer {
+    fn clone_for_ffi(&self) -> RustBuffer {
+        // SAFETY: trust me, bro.


travis79 · 2026-04-29T18:56:04Z

+use xshell::{Shell, cmd};
+
+#[test]
+fn generated_metrics_code_up_to_date() {


Is this test what keeps things in sync? I see you writing to a committed file and then checking for modifications, and that's what I assume. Could you add some documentation comment(s) on what this does?

travis79 · 2026-04-29T19:01:32Z

 version = "0.3.7"
 criteria = "safe-to-run"

+[[exemptions.prettyplease]]


Just out of curiosity, should we document these exemptions anywhere?

travis79 · 2026-04-29T19:04:23Z

          command: |
            cd samples/glean-sym-test
+            python3 -m venv ${PWD}/venv
+            venv/bin/pip install "git+ssh://git@github.com/mozilla/glean_parser@push-lskqopuplvts#egg=glean-parser"


Might be worth filing a bug to update this once it's no longer experimental? I expect this will eventually be pinned to a proper version

travis79 · 2026-04-29T19:05:35Z

+edition = "2024"
+
+[dependencies]
+bytes = "1.11.1"


I don't see this being useed anywhere

badboy force-pushed the glean-sys-generated branch 2 times, most recently from dab03d7 to 23adcc4 Compare March 30, 2026 11:31

badboy commented Mar 30, 2026

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Comment thread glean-core/glean-sym/src/lib.rs

badboy force-pushed the glean-sys-generated branch from 23adcc4 to a6d1ef4 Compare March 30, 2026 12:54

badboy mentioned this pull request Apr 16, 2026

Update to MSRV 1.90 #3439

Merged

badboy force-pushed the glean-sys-generated branch 5 times, most recently from e1571f1 to 6a7603a Compare April 17, 2026 17:34

badboy changed the title ~~Bug 2018944 - glean-sys - a Rust API built on top of the Glean UniFFI C FFI~~ Bug 2018944 - glean-sym - a Rust API built on top of the Glean UniFFI C FFI Apr 22, 2026

badboy mentioned this pull request Apr 22, 2026

glean-build: 2 improvements: Allow to overwrite venv dir in code & allow to change format #3442

Merged

badboy force-pushed the glean-sys-generated branch 4 times, most recently from 48eeb4d to 9b3dd70 Compare April 22, 2026 17:47

badboy added 6 commits April 23, 2026 11:01

A parser & compiler to generate the bare minimum to call UniFFI-gener…

f68d9c4

…ated code over FFI This will be used later by glean-sys to build a Rust API via C FFI.

A new crate: glean-sym - a Rust API built on top of the Glean UniFFI …

1a1a7a5

…C FFI This loads `libxul.so` (currently hard-coded), looks up the required symbols and wraps that in a nice Rust API. Other crates can depend on `glean-sym`, without needing to have Glean in the same library.

Update dependencies to avoid duplications

3d8407c

Dependency vetting

a866734

not done yet

A test for glean-sym: A Python sample that loads the libraries and in…

58c1c90

…vokes functions in them

[testing] get glean_parser from an upstream branch for testing

85d08d6

badboy force-pushed the glean-sys-generated branch from 9b3dd70 to 85d08d6 Compare April 23, 2026 15:59

badboy marked this pull request as ready for review April 23, 2026 15:59

badboy requested a review from a team as a code owner April 23, 2026 15:59

badboy requested review from travis79 and removed request for a team April 23, 2026 15:59

badboy mentioned this pull request Apr 23, 2026

Bug 2034534 - Use glean-sym to record a metric from places Rust implementation mozilla/application-services#7336

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travis79 reviewed Apr 29, 2026

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Bug 2018944 - glean-sym - a Rust API built on top of the Glean UniFFI C FFI#3426

Bug 2018944 - glean-sym - a Rust API built on top of the Glean UniFFI C FFI#3426
badboy wants to merge 6 commits intomainfrom
glean-sys-generated

badboy commented Mar 30, 2026 •

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		@@ -0,0 +1,119 @@
		// This Source Code Form is subject to the terms of the Mozilla Public

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badboy commented Mar 30, 2026 •

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