Fix #1169.

It's a bit ad-hoc that we just lock here.  This should use the
criticalSection abstraction, but that's internal to GenericC.  This is
good enough for now, but if we ever do more complex entry/exit
operations, this will need refactoring.

CHANGELOG.md

@@ -43,6 +43,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   * Fixed a flattening issue for certain complex `map` nestings
     (#1168).
 
+  * Made API function `futhark_context_clear_caches()` thread safe
+    (#1169).
+
 ## [0.18.1]
 
 ### Added

src/Futhark/CodeGen/Backends/CCUDA/Boilerplate.hs

@@ -461,7 +461,9 @@ generateContextFuns cfg cost_centres kernels sizes failures = do
   GC.publicDef_ "context_clear_caches" GC.MiscDecl $ \s ->
     ( [C.cedecl|int $id:s(struct $id:ctx* ctx);|],
       [C.cedecl|int $id:s(struct $id:ctx* ctx) {
+                         lock_lock(&ctx->lock);
                          CUDA_SUCCEED(cuda_free_all(&ctx->cuda));
+                         lock_unlock(&ctx->lock);
                          return 0;
                        }|]
     )

src/Futhark/CodeGen/Backends/COpenCL/Boilerplate.hs

@@ -492,7 +492,9 @@ generateBoilerplate opencl_code opencl_prelude cost_centres kernels types sizes
   GC.publicDef_ "context_clear_caches" GC.MiscDecl $ \s ->
     ( [C.cedecl|int $id:s(struct $id:ctx* ctx);|],
       [C.cedecl|int $id:s(struct $id:ctx* ctx) {
+                         lock_lock(&ctx->lock);
                          ctx->error = OPENCL_SUCCEED_NONFATAL(opencl_free_all(&ctx->opencl));
+                         lock_unlock(&ctx->lock);
                          return ctx->error != NULL;
                        }|]
     )