Scope cache workaround to affected compiler and x64 builds

Author: lukewilliamboswell

CONTRIBUTING.md

@@ -170,12 +170,14 @@ with different arguments, stdin, environment, fixtures, helper servers,
 expected exit codes, or output assertions. `happy` is only a naming convention;
 an app can have any number of successful and failing cases.
 
-Example checks, tests and builds use `--no-cache` on every operating system so
-validation compiles the current platform sources against the current native ABI.
-The pinned nightly can produce invalid cached loop code after check/test
-warm-up; do not re-enable these caches until an upstream fix is verified.
-When upgrading a platform URL, rebuild your application with
-`roc build --no-cache your-app.roc` as well.
+The runner normally uses the compiler cache. A temporary workaround disables it
+for `x64musl` and `x64mac` builds with `nightly-2026-10-12-2664a27`: after
+`roc check` and `roc test` warm the cache, the command stress helper can SIGSEGV
+before its first write. The same build passes with `--no-cache`
+([controlled reproduction](https://github.com/roc-lang/basic-cli/actions/runs/39512913704)).
+TODO: remove this workaround once a compiler update passes that cached
+check/test/build sequence on both targets. Other Unix targets and check/test
+stages retain caching; the existing Windows workaround is unchanged.
 
 CI separates source validation, cross-target compilation, and native execution.
 Every example is compiled for each target declared in `platform/main.roc`:

docs/releases/0.26.0.md

@@ -260,7 +260,9 @@ The Windows x64 host keeps the MSVC ABI and now supplies the startup and runtime
 
 ## Upgrading from 0.25
 
-Update the platform URL and rebuild with `roc build --no-cache your-app.roc`: the native host interface changed, and mixing hosts and platform sources from different versions is not supported. Then work through the modules you use.
+Update the platform URL and rebuild: the native host interface changed, and mixing hosts and platform sources from different versions is not supported. Then work through the modules you use.
+
+With `nightly-2026-10-12-2664a27`, Linux x64 and Intel macOS builds can crash when reusing a cache warmed by `roc check`/`roc test`. For that compiler bug, use `roc build --no-cache your-app.roc` ([reproduction](https://github.com/roc-lang/basic-cli/actions/runs/39512913704)). This is a temporary compiler workaround, not a requirement of the platform upgrade.
 
 **Target discovery**
 

scripts/test.py

@@ -89,12 +89,24 @@ def binary_stem(source: Path) -> str:
     return "-".join(parts[1:])
 
 
-def roc_extra_args() -> tuple[str, ...]:
+@functools.cache
+def roc_version() -> str:
+    return subprocess.check_output([ROC, "version"], text=True).strip()
+
+
+def roc_extra_args(*, build_target: str | None = None) -> tuple[str, ...]:
     args = ("--max-transitive-mb=256",)
-    # Validate the current Roc sources against the current native ABI on every
-    # target. The pinned compiler can reuse invalid cached loop code after
-    # check/test warm-up (reproduced on Linux and Intel macOS).
-    return (*args, "--no-cache")
+    if platform.system() == "Windows":
+        return (*args, "--no-cache")
+    # TODO: Remove this workaround after a compiler update passes the cached
+    # check -> test -> build reproduction on x64musl and x64mac. With 2664a27,
+    # cmd-api's flood helper SIGSEGVs before its first write; --no-cache passes.
+    # Reproduction: https://github.com/roc-lang/basic-cli/actions/runs/39512913704
+    if build_target in ("x64musl", "x64mac") and roc_version() == (
+        "Roc compiler version nightly-2026-10-12-2664a27"
+    ):
+        return (*args, "--no-cache")
+    return args
 
 
 def load_spec() -> tuple[dict[str, bool], list[dict[str, object]]]:
@@ -619,7 +631,7 @@ def run_stage(
             binary.parent.mkdir(parents=True, exist_ok=True)
             command(
                 ROC, "build", source, f"--target={target}",
-                f"--output={binary}", *roc_extra_args(),
+                f"--output={binary}", *roc_extra_args(build_target=target),
             )
             binaries[path] = binary
         elif stage == "run":
@@ -793,7 +805,7 @@ def main() -> None:
 
     if shutil.which(ROC) is None:
         raise SystemExit(f"Roc executable '{ROC}' was not found")
-    print(f"Using roc version: {subprocess.check_output([ROC, 'version'], text=True).strip()}")
+    print(f"Using roc version: {roc_version()}")
     if not args.no_build and not args.bundle_path:
         print("\n=== Building platform ===")
         command(sys.executable, ROOT / "scripts" / "build.py", "--target", target)