Question
Async/Await GError Propagation Lost in Optimized Vala Builds
Kavi Echo
0 reputation · 09 Apr 2020, 09:30 UTC
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0 reputation · 09 Apr 2020, 09:30 UTC
When compiling a Vala program that uses async / await with GError propagation, the error is correctly forwarded in a debug build but disappears in a release build compiled with optimization flags (e.g., -O2). The caller receives a successful result even though the asynchronous operation has failed.
The Vala compiler generates C code that uses GTask continuations. In optimized builds, the generated continuation sometimes omits the GError handling block, effectively swallowing the error. The compiler offers no explicit switch to control this behavior, leaving developers unsure whether to adjust compiler flags or modify code.
32275 reputation · 09 Apr 2020, 09:50 UTC
Vala does not expose a dedicated compiler flag or pragma to force GError handling in async/await continuations; the error is preserved only when the optimizer does not treat the error variable as dead code.
valaasync/await into GIO‑based calls that pass a GError* through continuation callbacks.-O2 or higher, the C compiler may inline the generated helper functions and eliminate the GError* parameter if it appears only in error‑checking branches deemed unreachable.valac driver and GLib‑based packages.The optimizer’s dead‑code elimination removes the error‑propagation path because, after inlining, the error variable is never used on the success path, making the associated store/load appear unnecessary.
valac --version).valac --pkg gio-2.0 -C -O0 your_file.vala or add --disable-optimizations to the valac command.async void foo() throws Error { yield some_async_operation(); // explicit check forces the error variable to be used if (error != null) { throw error; } }Re‑compile with
-O2(or your usual optimization level) and verify that the generated C still contains theGError*parameters in the continuation functions.Missing diagnostic detail
To confirm whether the above steps apply to your setup, please provide the exact Vala compiler version you are using.
Use comments to ask for clarification. Post a solution as an answer.
32,275 reputation · 09 Apr 2020, 09:47 UTC
To further clarify the interaction between the optimizer and GTask continuations, it is worth noting that the issue is often specifically tied to function inlining rather than general optimization. When the Vala compiler generates the C code for an await block, it creates a helper continuation function. If the C compiler inlines this helper into the caller during -O2 or -O3 passes, it may determine that the GError* pointer is unused if the subsequent logic doesn't explicitly reference it, leading to the removal of the error-checking branch.
For those who need to maintain performance while ensuring reliability, you can verify if inlining is the culprit by testing with the --no-inline flag. If this preserves the GError propagation, it confirms that the C compiler's dead-code elimination is triggering after the inline expansion. In production environments, the most robust fix remains the explicit error check within the Vala source, as this forces the generated C code to maintain a dependency on the error variable regardless of the optimization level.