Add GetExecutableDirectory to libbase

Tests will often want to get the executable directory in order to
find test data.

Test: out/host/linux-x86/nativetest64/libbase_tests/libbase_tests
Change-Id: Ica9d211bcd039fcf83a22fd494816abd01b97aa3

base/file.cpp

@@ -238,8 +238,11 @@ std::string GetExecutablePath() {
 #endif
 }
 
-std::string Basename(const std::string& path) {
+std::string GetExecutableDirectory() {
+  return Dirname(GetExecutablePath());
+}
 
+std::string Basename(const std::string& path) {
   // Copy path because basename may modify the string passed in.
   std::string result(path);
 

base/file_test.cpp

@@ -159,6 +159,14 @@ TEST(file, Readlink) {
 #endif
 }
 
+TEST(file, GetExecutableDirectory) {
+  std::string path = android::base::GetExecutableDirectory();
+  ASSERT_NE("", path);
+  ASSERT_NE(android::base::GetExecutablePath(), path);
+  ASSERT_EQ('/', path[0]);
+  ASSERT_NE('/', path[path.size() - 1]);
+}
+
 TEST(file, GetExecutablePath) {
   ASSERT_NE("", android::base::GetExecutablePath());
 }

base/include/android-base/file.h

@@ -51,6 +51,7 @@ bool Readlink(const std::string& path, std::string* result);
 #endif
 
 std::string GetExecutablePath();
+std::string GetExecutableDirectory();
 
 // Like the regular basename and dirname, but thread-safe on all
 // platforms and capable of correctly handling exotic Windows paths.