Merge "Add Errorf and ErrnoErrorf" am: ebbe9e7391

am: a7c8b3c49f

Change-Id: Ie8b87bd13a324b9cc97b69a7a179e3e056919498

base/Android.bp

@@ -111,6 +111,9 @@ cc_library {
         "libbase_headers",
     ],
     export_header_lib_headers: ["libbase_headers"],
+    static_libs: ["fmtlib"],
+    whole_static_libs: ["fmtlib"],
+    export_static_lib_headers: ["fmtlib"],
 }
 
 cc_library_static {
@@ -119,6 +122,9 @@ cc_library_static {
     sdk_version: "current",
     stl: "c++_static",
     export_include_dirs: ["include"],
+    static_libs: ["fmtlib_ndk"],
+    whole_static_libs: ["fmtlib_ndk"],
+    export_static_lib_headers: ["fmtlib_ndk"],
 }
 
 // Tests

base/include/android-base/result.h

@@ -42,10 +42,15 @@
 // to the end of the failure string to aid in interacting with C APIs.  Alternatively, an errno
 // value can be directly specified via the Error() constructor.
 //
-// ResultError can be used in the ostream when using Error to construct a Result<T>.  In this case,
-// the string that the ResultError takes is passed through the stream normally, but the errno is
-// passed to the Result<T>.  This can be used to pass errno from a failing C function up multiple
-// callers.
+// Errorf and ErrnoErrorf accept the format string syntax of the fmblib (https://fmt.dev).
+// Errorf("{} errors", num) is equivalent to Error() << num << " errors".
+//
+// ResultError can be used in the ostream and when using Error/Errorf to construct a Result<T>.
+// In this case, the string that the ResultError takes is passed through the stream normally, but
+// the errno is passed to the Result<T>. This can be used to pass errno from a failing C function up
+// multiple callers. Note that when the outer Result<T> is created with ErrnoError/ErrnoErrorf then
+// the errno from the inner ResultError is not passed. Also when multiple ResultError objects are
+// used, the errno of the last one is respected.
 //
 // ResultError can also directly construct a Result<T>.  This is particularly useful if you have a
 // function that return Result<T> but you have a Result<U> and want to return its error.  In this
@@ -55,10 +60,10 @@
 // Result<U> CalculateResult(const T& input) {
 //   U output;
 //   if (!SomeOtherCppFunction(input, &output)) {
-//     return Error() << "SomeOtherCppFunction(" << input << ") failed";
+//     return Errorf("SomeOtherCppFunction {} failed", input);
 //   }
 //   if (!c_api_function(output)) {
-//     return ErrnoError() << "c_api_function(" << output << ") failed";
+//     return ErrnoErrorf("c_api_function {} failed", output);
 //   }
 //   return output;
 // }
@@ -74,6 +79,8 @@
 #include <sstream>
 #include <string>
 
+#include <fmt/ostream.h>
+
 #include "android-base/expected.h"
 
 namespace android {
@@ -147,16 +154,51 @@ class Error {
   Error& operator=(const Error&) = delete;
   Error& operator=(Error&&) = delete;
 
+  template <typename... Args>
+  friend Error Errorf(const char* fmt, const Args&... args);
+
+  template <typename... Args>
+  friend Error ErrnoErrorf(const char* fmt, const Args&... args);
+
  private:
+  Error(bool append_errno, int errno_to_append, const std::string& message)
+      : errno_(errno_to_append), append_errno_(append_errno) {
+    (*this) << message;
+  }
+
   std::stringstream ss_;
   int errno_;
-  bool append_errno_;
+  const bool append_errno_;
 };
 
 inline Error ErrnoError() {
   return Error(errno);
 }
 
+inline int ErrorCode(int code) {
+  return code;
+}
+
+// Return the error code of the last ResultError object, if any.
+// Otherwise, return `code` as it is.
+template <typename T, typename... Args>
+inline int ErrorCode(int code, T&& t, const Args&... args) {
+  if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<T>>, ResultError>) {
+    return ErrorCode(t.code(), args...);
+  }
+  return ErrorCode(code, args...);
+}
+
+template <typename... Args>
+inline Error Errorf(const char* fmt, const Args&... args) {
+  return Error(false, ErrorCode(0, args...), fmt::format(fmt, args...));
+}
+
+template <typename... Args>
+inline Error ErrnoErrorf(const char* fmt, const Args&... args) {
+  return Error(true, errno, fmt::format(fmt, args...));
+}
+
 template <typename T>
 using Result = android::base::expected<T, ResultError>;
 

base/result_test.cpp

@@ -355,5 +355,68 @@ TEST(result, preserve_errno) {
   EXPECT_EQ(old_errno, result2.error().code());
 }
 
+TEST(result, error_with_fmt) {
+  Result<int> result = Errorf("{} {}!", "hello", "world");
+  EXPECT_EQ("hello world!", result.error().message());
+
+  result = Errorf("{} {}!", std::string("hello"), std::string("world"));
+  EXPECT_EQ("hello world!", result.error().message());
+
+  result = Errorf("{h} {w}!", fmt::arg("w", "world"), fmt::arg("h", "hello"));
+  EXPECT_EQ("hello world!", result.error().message());
+
+  result = Errorf("hello world!");
+  EXPECT_EQ("hello world!", result.error().message());
+
+  Result<int> result2 = Errorf("error occurred with {}", result.error());
+  EXPECT_EQ("error occurred with hello world!", result2.error().message());
+
+  constexpr int test_errno = 6;
+  errno = test_errno;
+  result = ErrnoErrorf("{} {}!", "hello", "world");
+  EXPECT_EQ(test_errno, result.error().code());
+  EXPECT_EQ("hello world!: "s + strerror(test_errno), result.error().message());
+}
+
+TEST(result, error_with_fmt_carries_errno) {
+  constexpr int inner_errno = 6;
+  errno = inner_errno;
+  Result<int> inner_result = ErrnoErrorf("inner failure");
+  errno = 0;
+  EXPECT_EQ(inner_errno, inner_result.error().code());
+
+  // outer_result is created with Errorf, but its error code is got from inner_result.
+  Result<int> outer_result = Errorf("outer failure caused by {}", inner_result.error());
+  EXPECT_EQ(inner_errno, outer_result.error().code());
+  EXPECT_EQ("outer failure caused by inner failure: "s + strerror(inner_errno),
+            outer_result.error().message());
+
+  // now both result objects are created with ErrnoErrorf. errno from the inner_result
+  // is not passed to outer_result.
+  constexpr int outer_errno = 10;
+  errno = outer_errno;
+  outer_result = ErrnoErrorf("outer failure caused by {}", inner_result.error());
+  EXPECT_EQ(outer_errno, outer_result.error().code());
+  EXPECT_EQ("outer failure caused by inner failure: "s + strerror(inner_errno) + ": "s +
+                strerror(outer_errno),
+            outer_result.error().message());
+}
+
+TEST(result, errno_chaining_multiple) {
+  constexpr int errno1 = 6;
+  errno = errno1;
+  Result<int> inner1 = ErrnoErrorf("error1");
+
+  constexpr int errno2 = 10;
+  errno = errno2;
+  Result<int> inner2 = ErrnoErrorf("error2");
+
+  // takes the error code of inner2 since its the last one.
+  Result<int> outer = Errorf("two errors: {}, {}", inner1.error(), inner2.error());
+  EXPECT_EQ(errno2, outer.error().code());
+  EXPECT_EQ("two errors: error1: "s + strerror(errno1) + ", error2: "s + strerror(errno2),
+            outer.error().message());
+}
+
 }  // namespace base
 }  // namespace android