genmc/api: Use returned value from handleFree()

GenMC's handleFree() does return an optional<Error>, so we
may as well use that value.

Author: michaliskok

src/tools/miri/genmc-sys/cpp/include/MiriInterface.hpp

@@ -125,8 +125,11 @@ struct MiriGenmcShim : private GenMCDriver {
     void handle_fence(ThreadId thread_id, MemOrdering ord);
 
     /**** Memory (de)allocation ****/
+
     auto handle_malloc(ThreadId thread_id, uint64_t size, uint64_t alignment) -> uint64_t;
-    auto handle_free(ThreadId thread_id, uint64_t address) -> bool;
+
+    /** Returns null on success, or an error string if an error occurs. */
+    auto handle_free(ThreadId thread_id, uint64_t address) -> std::unique_ptr<std::string>;
 
     /**** Thread management ****/
     void handle_thread_create(ThreadId thread_id, ThreadId parent_id);

src/tools/miri/genmc-sys/cpp/src/MiriInterface/Exploration.cpp

@@ -288,11 +288,11 @@ auto MiriGenmcShim::handle_malloc(ThreadId thread_id, uint64_t size, uint64_t al
     return ret_val.get();
 }
 
-auto MiriGenmcShim::handle_free(ThreadId thread_id, uint64_t address) -> bool {
-    const auto pos = inc_pos(thread_id);
-    GenMCDriver::handleFree(nullptr, pos, SAddr(address), EventDeps());
-    // FIXME(genmc): use returned error from `handleFree` once implemented in GenMC.
-    return getResult().status.has_value();
+auto MiriGenmcShim::handle_free(ThreadId thread_id, uint64_t address)
+    -> std::unique_ptr<std::string> {
+    auto pos = inc_pos(thread_id);
+    auto ret = GenMCDriver::handleFree(nullptr, pos, SAddr(address), EventDeps());
+    return ret.has_value() ? format_error(*ret) : nullptr;
 }
 
 /**** Estimation mode result ****/

src/tools/miri/genmc-sys/src/lib.rs

@@ -438,7 +438,11 @@ mod ffi {
             alignment: u64,
         ) -> u64;
         /// Returns true if an error was found.
-        fn handle_free(self: Pin<&mut MiriGenmcShim>, thread_id: i32, address: u64) -> bool;
+        fn handle_free(
+            self: Pin<&mut MiriGenmcShim>,
+            thread_id: i32,
+            address: u64,
+        ) -> UniquePtr<CxxString>;
 
         /**** Thread management ****/
         fn handle_thread_create(self: Pin<&mut MiriGenmcShim>, thread_id: i32, parent_id: i32);

src/tools/miri/src/concurrency/genmc/mod.rs

@@ -622,15 +622,14 @@ impl GenmcCtx {
             !self.get_alloc_data_races(),
             "memory deallocation with data race checking disabled."
         );
-        if self
+        let free_result = self
             .handle
             .borrow_mut()
             .pin_mut()
-            .handle_free(self.active_thread_genmc_tid(machine), address.bytes())
-        {
+            .handle_free(self.active_thread_genmc_tid(machine), address.bytes());
+        if let Some(error) = free_result.as_ref() {
             // FIXME(genmc): improve error handling.
-            // An error was detected, so we get the error string from GenMC.
-            throw_ub_format!("{}", self.try_get_error().unwrap());
+            throw_ub_format!("{}", error.to_string_lossy());
         }
 
         interp_ok(())