Added GetRunfilesDir() to be able to reliably determine prelude location under various invocation scenarios

common/BUILD

@@ -69,6 +69,24 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "runfiles_util",
+    srcs = ["runfiles_util.cpp"],
+    hdrs = ["runfiles_util.h"],
+    deps = [
+        ":check",
+    ],
+)
+
+cc_test(
+    name = "runfiles_util_test",
+    srcs = ["runfiles_util_test.cpp"],
+    deps = [
+        ":runfiles_util",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "string_helpers",
     srcs = ["string_helpers.cpp"],

common/runfiles_util.cpp

@@ -0,0 +1,46 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/runfiles_util.h"
+
+#include <filesystem>
+
+#include "common/check.h"
+
+#if defined(__APPLE__)
+#include <mach-o/dyld.h>
+#include <sys/param.h>
+#include <sys/stat.h>
+#endif
+
+namespace Carbon {
+
+static auto GetProgramPath() -> std::string {
+  std::string program_name;
+#if defined(__APPLE__)
+  char ch = '\0';
+  uint32_t buf_size = 1;
+  // First call is to determine required buffer size.
+  CHECK(_NSGetExecutablePath(&ch, &buf_size) < 0);
+  std::unique_ptr<char[]> buf(new char[buf_size]);
+  CHECK(_NSGetExecutablePath(buf.get(), &buf_size) == 0);
+  program_name = buf.get();
+#else
+  program_name = "/proc/self/exe";
+#endif
+  std::error_code error;
+  program_name = std::filesystem::canonical(program_name, error);
+  CHECK(error.value() == 0);
+  llvm::errs() << "### program name=" << program_name << "\n";
+  return program_name;
+}
+
+auto GetRunfilesDir() -> std::string {
+  std::string runfiles_dir = GetProgramPath() + ".runfiles";
+  llvm::errs() << "### runfiles dir=" << runfiles_dir << "\n";
+  CHECK(std::filesystem::exists(runfiles_dir));
+  return runfiles_dir;
+}
+
+}  // namespace Carbon

common/runfiles_util.h

@@ -0,0 +1,16 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef COMMON_RUNFILES_UTIL_H_
+#define COMMON_RUNFILES_UTIL_H_
+
+#include <string>
+
+namespace Carbon {
+
+// Returns bazel's runfiles directory.
+auto GetRunfilesDir() -> std::string;
+
+}  // namespace Carbon
+#endif  // COMMON_RUNFILES_UTIL_H_

common/runfiles_util_test.cpp

@@ -0,0 +1,21 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/runfiles_util.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <string>
+
+namespace Carbon::Testing {
+namespace {
+
+TEST(RunfilesUtilTest, GetRunfilesDir) {
+  EXPECT_THAT(GetRunfilesDir(),
+              testing::EndsWith("/common/runfiles_util_test.runfiles"));
+}
+
+}  // namespace
+}  // namespace Carbon::Testing

executable_semantics/fuzzing/BUILD

@@ -100,6 +100,7 @@ cc_fuzz_test(
     tags = ["manual"],
     deps = [
         ":fuzzer_util",
+        "//common:runfiles_util",
         "//common/fuzzing:carbon_cc_proto",
         "//executable_semantics/interpreter:exec_program",
         "//executable_semantics/syntax",

executable_semantics/fuzzing/executable_semantics_fuzzer.cpp

@@ -6,6 +6,7 @@
 #include <libprotobuf_mutator/src/libfuzzer/libfuzzer_macro.h>
 
 #include "common/fuzzing/carbon.pb.h"
+#include "common/runfiles_util.h"
 #include "executable_semantics/fuzzing/fuzzer_util.h"
 #include "executable_semantics/interpreter/exec_program.h"
 #include "executable_semantics/syntax/parse.h"
@@ -25,10 +26,11 @@ void ParseAndExecute(const Fuzzing::CompilationUnit& compilation_unit) {
     llvm::errs() << "Parsing failed: " << ast.error().message() << "\n";
     return;
   }
-  AddPrelude("executable_semantics/data/prelude.carbon", &arena,
-             &ast->declarations);
+  AddPrelude(
+      GetRunfilesDir() + "/carbon/executable_semantics/data/prelude.carbon",
+      &arena, &ast->declarations);
   const ErrorOr<int> result =
-      ExecProgram(&arena, *ast, /*trace_stream=*/nullptr);
+      ExecProgram(&arena, *ast, /*trace_stream=*/std::nullopt);
   if (!result.ok()) {
     llvm::errs() << "Execution failed: " << result.error().message() << "\n";
     return;