i#6778: Gracefully handle too few drcachesim cpus

Adds a check with an error message for too few simulated drcachesim
cpus with a core-sharded-on-disk trace.

Adds a unit test which crashes without the fix.

Also tested manually:
Before:
  $ bin64/drrun -t drcachesim -indir ../src/clients/drcachesim/tests/drmemtrace.threadsig-core-sharded.x64.tracedir/ -cores 3
  Segmentation fault
After:
  $ bin64/drrun -t drcachesim -indir ../src/clients/drcachesim/tests/drmemtrace.threadsig-core-sharded.x64.tracedir/ -cores 3
  ERROR: failed to run analyzer: Too-small core count 3 for trace core #3

Fixes #6778

clients/drcachesim/simulator/cache_simulator.cpp

@@ -469,6 +469,11 @@ cache_simulator_t::process_memref(const memref_t &memref)
         }
     } else
         core_index = core_for_thread(memref.data.tid);
+    if (core_index >= static_cast<int>(knobs_.num_cores)) {
+        error_string_ = "Too-small core count " + std::to_string(knobs_.num_cores) +
+            " for trace core #" + std::to_string(core_index);
+        return false;
+    }
 
     // To support swapping to physical addresses without modifying the passed-in
     // memref (which is also passed to other tools run at the same time) we use

clients/drcachesim/tests/drcachesim_unit_tests.cpp

@@ -814,6 +814,23 @@ Core #1 \(traced CPU\(s\): #567800\)
 )DELIM")));
 #endif
     }
+    {
+        // Test graceful handling of too-few cpus.
+        cache_simulator_knobs_t knobs = make_test_knobs();
+        knobs.num_cores = 2;
+        cache_simulator_t sim(knobs);
+        default_memtrace_stream_t stream;
+        sim.initialize_stream(&stream);
+        std::string error = sim.initialize_shard_type(SHARD_BY_CORE);
+        assert(error.empty());
+        memref_t ref = make_memref(42);
+        stream.set_shard_index(2); // Too large for knobs.num_cores.
+        stream.set_output_cpuid(1);
+        bool res = sim.process_memref(ref);
+        // We should see graceful failure and not a crash.
+        assert(!res);
+        assert(!sim.get_error_string().empty());
+    }
 }
 
 int