From d3ff6e5231f53a95518532ff74a7256aabbf32e5 Mon Sep 17 00:00:00 2001
From: Elliott Hughes <enh@google.com>
Date: Tue, 23 Aug 2016 15:53:45 -0700
Subject: [PATCH] Add android::base::Readlink.

Bug: http://b/30988271
Change-Id: Ib844d7c9e33465dabf7aee5e24dc3d1d8d799abd
---
 base/file.cpp                    | 26 ++++++++++++++++++++++++++
 base/file_test.cpp               | 26 ++++++++++++++++++++++++++
 base/include/android-base/file.h |  4 ++++
 3 files changed, 56 insertions(+)

diff --git a/base/file.cpp b/base/file.cpp
index 4e7ac82d1..39630815c 100644
--- a/base/file.cpp
+++ b/base/file.cpp
@@ -20,8 +20,10 @@
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <unistd.h>
 
 #include <string>
+#include <vector>
 
 #include "android-base/macros.h"  // For TEMP_FAILURE_RETRY on Darwin.
 #include "android-base/logging.h"
@@ -171,5 +173,29 @@ bool RemoveFileIfExists(const std::string& path, std::string* err) {
   return true;
 }
 
+#if !defined(_WIN32)
+bool Readlink(const std::string& path, std::string* result) {
+  result->clear();
+
+  // Most Linux file systems (ext2 and ext4, say) limit symbolic links to
+  // 4095 bytes. Since we'll copy out into the string anyway, it doesn't
+  // waste memory to just start there. We add 1 so that we can recognize
+  // whether it actually fit (rather than being truncated to 4095).
+  std::vector<char> buf(4095 + 1);
+  while (true) {
+    ssize_t size = readlink(path.c_str(), &buf[0], buf.size());
+    // Unrecoverable error?
+    if (size == -1) return false;
+    // It fit! (If size == buf.size(), it may have been truncated.)
+    if (static_cast<size_t>(size) < buf.size()) {
+      result->assign(&buf[0], size);
+      return true;
+    }
+    // Double our buffer and try again.
+    buf.resize(buf.size() * 2);
+  }
+}
+#endif
+
 }  // namespace base
 }  // namespace android
diff --git a/base/file_test.cpp b/base/file_test.cpp
index 17755bfed..ca01ee88a 100644
--- a/base/file_test.cpp
+++ b/base/file_test.cpp
@@ -110,3 +110,29 @@ TEST(file, RemoveFileIfExist) {
   ASSERT_FALSE(android::base::RemoveFileIfExists(td.path, &err));
   ASSERT_EQ("is not a regular or symbol link file", err);
 }
+
+TEST(file, Readlink) {
+#if !defined(_WIN32)
+  // Linux doesn't allow empty symbolic links.
+  std::string min("x");
+  // ext2 and ext4 both have PAGE_SIZE limits.
+  std::string max(static_cast<size_t>(4096 - 1), 'x');
+
+  TemporaryDir td;
+  std::string min_path{std::string(td.path) + "/" + "min"};
+  std::string max_path{std::string(td.path) + "/" + "max"};
+
+  ASSERT_EQ(0, symlink(min.c_str(), min_path.c_str()));
+  ASSERT_EQ(0, symlink(max.c_str(), max_path.c_str()));
+
+  std::string result;
+
+  result = "wrong";
+  ASSERT_TRUE(android::base::Readlink(min_path, &result));
+  ASSERT_EQ(min, result);
+
+  result = "wrong";
+  ASSERT_TRUE(android::base::Readlink(max_path, &result));
+  ASSERT_EQ(max, result);
+#endif
+}
diff --git a/base/include/android-base/file.h b/base/include/android-base/file.h
index aa18ea796..2726a623d 100644
--- a/base/include/android-base/file.h
+++ b/base/include/android-base/file.h
@@ -43,6 +43,10 @@ bool WriteFully(int fd, const void* data, size_t byte_count);
 
 bool RemoveFileIfExists(const std::string& path, std::string* err = nullptr);
 
+#if !defined(_WIN32)
+bool Readlink(const std::string& path, std::string* result);
+#endif
+
 }  // namespace base
 }  // namespace android