Implement path canonicalization function

To search for config files, we need to resolve symbolic links and
simplify paths. Create a function, canonicalize_path, which computes an
absolute canonical path.

A future commit will use the new canonicalize_path function.

Author: strager

src/file.cpp

@@ -16,12 +16,17 @@
 #include <quick-lint-js/math-overflow.h>
 #include <quick-lint-js/narrow-cast.h>
 #include <quick-lint-js/utf-16.h>
+#include <stdlib.h>
 #include <string>
 
 #if QLJS_HAVE_FCNTL_H
 #include <fcntl.h>
 #endif
 
+#if QLJS_HAVE_STD_FILESYSTEM
+#include <filesystem>
+#endif
+
 #if QLJS_HAVE_SYS_STAT_H
 #include <sys/stat.h>
 #endif
@@ -271,6 +276,50 @@ void write_file(const char *path, string8_view content) {
 
   std::fclose(file);
 }
+
+const char *canonical_path_result::c_str() const noexcept {
+  QLJS_ASSERT(this->ok());
+  return this->path.c_str();
+}
+
+canonical_path_result canonical_path_result::failure(std::string &&error) {
+  canonical_path_result result;
+  result.error = std::move(error);
+  QLJS_ASSERT(!result.ok());
+  return result;
+}
+
+canonical_path_result canonicalize_path(const char *path) {
+#if QLJS_HAVE_STD_FILESYSTEM
+  std::error_code error;
+  std::filesystem::path canonical = std::filesystem::canonical(path, error);
+  if (error) {
+    return canonical_path_result::failure(
+        std::string("failed to canonicalize path ") + path + ": " +
+        error.message());
+  }
+  canonical_path_result result;
+  result.path = canonical.string();
+  return result;
+#elif QLJS_HAVE_REALPATH
+  char *allocated_canonical = ::realpath(path, nullptr);
+  if (!allocated_canonical) {
+    return canonical_path_result::failure(
+        std::string("failed to canonicalize path ") + path + ": " +
+        std::strerror(errno));
+  }
+  canonical_path_result result;
+  result.path = allocated_canonical;
+  std::free(allocated_canonical);
+  return result;
+#else
+#error "Unsupported platform"
+#endif
+}
+
+canonical_path_result canonicalize_path(const std::string &path) {
+  return canonicalize_path(path.c_str());
+}
 }
 
 // quick-lint-js finds bugs in JavaScript programs.

src/quick-lint-js/file.h

@@ -29,6 +29,20 @@ read_file_result read_stdin(void);
 
 void write_file(const std::string &path, string8_view content);
 void write_file(const char *path, string8_view content);
+
+struct canonical_path_result {
+  std::string path;
+  std::string error;
+
+  bool ok() const noexcept { return this->error.empty(); }
+
+  const char *c_str() const noexcept;
+
+  static canonical_path_result failure(std::string &&error);
+};
+
+canonical_path_result canonicalize_path(const char *path);
+canonical_path_result canonicalize_path(const std::string &path);
 }
 
 #endif

src/quick-lint-js/have.h

@@ -136,6 +136,16 @@
 #endif
 #endif
 
+#if !defined(QLJS_HAVE_REALPATH)
+#if (defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L) || \
+    (defined(__APPLE__) && defined(_POSIX_VERSION) &&         \
+     _POSIX_VERSION >= 200112L)
+#define QLJS_HAVE_REALPATH 1
+#else
+#define QLJS_HAVE_REALPATH 0
+#endif
+#endif
+
 #if !defined(QLJS_HAVE_SETRLIMIT)
 #if (defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L) || \
     (defined(__APPLE__) && defined(_POSIX_VERSION) &&         \

test/test-file.cpp

@@ -200,6 +200,29 @@ TEST_F(test_file, read_pipe_empty_writes) {
 
   writer_thread.join();
 }
+
+TEST_F(test_file, canonical_path_to_regular_file) {
+  std::string temp_file_path = this->make_temporary_directory() + "/temp.js";
+  write_file(temp_file_path, u8"hello\nworld!\n");
+
+  canonical_path_result canonical = canonicalize_path(temp_file_path);
+  ASSERT_TRUE(canonical.ok()) << canonical.error;
+
+  read_file_result file_content = read_file(canonical.c_str());
+  ASSERT_TRUE(file_content.ok()) << file_content.error;
+  EXPECT_EQ(file_content.content, string8_view(u8"hello\nworld!\n"));
+}
+
+TEST_F(test_file, canonical_path_to_non_existing_file) {
+  std::string temp_file_path =
+      this->make_temporary_directory() + "/does-not-exist.js";
+
+  canonical_path_result canonical = canonicalize_path(temp_file_path);
+  EXPECT_FALSE(canonical.ok());
+  EXPECT_THAT(canonical.error, HasSubstr("does-not-exist.js"));
+  EXPECT_THAT(canonical.error,
+              AnyOf(HasSubstr("No such file"), HasSubstr("cannot find")));
+}
 }
 }