From 9bb7971ae794ecfc6dcaab28d13d3ade8ab18517 Mon Sep 17 00:00:00 2001
From: Elliott Hughes <enh@google.com>
Date: Mon, 20 Mar 2017 19:16:18 -0700
Subject: [PATCH] Keep the ReadFileToString/ReadFdToString overhead down.

Bug: https://code.google.com/p/android/issues/detail?id=258500
Bug: http://b/36046324
Test: ran tests
Change-Id: I40e76a6dd164ea9a5e8e18159f543e1bb221dcba
---
 base/file.cpp      |  8 ++++++++
 base/file_test.cpp | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/base/file.cpp b/base/file.cpp
index 378a405bc..d4e58942c 100644
--- a/base/file.cpp
+++ b/base/file.cpp
@@ -49,6 +49,14 @@ using namespace android::base::utf8;
 bool ReadFdToString(int fd, std::string* content) {
   content->clear();
 
+  // Although original we had small files in mind, this code gets used for
+  // very large files too, where the std::string growth heuristics might not
+  // be suitable. https://code.google.com/p/android/issues/detail?id=258500.
+  struct stat sb;
+  if (fstat(fd, &sb) != -1 && sb.st_size > 0) {
+    content->reserve(sb.st_size);
+  }
+
   char buf[BUFSIZ];
   ssize_t n;
   while ((n = TEMP_FAILURE_RETRY(read(fd, &buf[0], sizeof(buf)))) > 0) {
diff --git a/base/file_test.cpp b/base/file_test.cpp
index 266131ecf..02b431de7 100644
--- a/base/file_test.cpp
+++ b/base/file_test.cpp
@@ -214,3 +214,46 @@ TEST(file, Dirname) {
   EXPECT_EQ(".", android::base::Dirname("sh"));
   EXPECT_EQ("/system/bin", android::base::Dirname("/system/bin/sh/"));
 }
+
+TEST(file, ReadFileToString_capacity) {
+  TemporaryFile tf;
+  ASSERT_TRUE(tf.fd != -1);
+
+  // For a huge file, the overhead should still be small.
+  std::string s;
+  size_t size = 16 * 1024 * 1024;
+  ASSERT_TRUE(android::base::WriteStringToFile(std::string(size, 'x'), tf.path));
+  ASSERT_TRUE(android::base::ReadFileToString(tf.path, &s));
+  EXPECT_EQ(size, s.size());
+  EXPECT_LT(s.capacity(), size + 16);
+
+  // Even for weird badly-aligned sizes.
+  size += 12345;
+  ASSERT_TRUE(android::base::WriteStringToFile(std::string(size, 'x'), tf.path));
+  ASSERT_TRUE(android::base::ReadFileToString(tf.path, &s));
+  EXPECT_EQ(size, s.size());
+  EXPECT_LT(s.capacity(), size + 16);
+
+  // We'll shrink an enormous string if you read a small file into it.
+  size = 64;
+  ASSERT_TRUE(android::base::WriteStringToFile(std::string(size, 'x'), tf.path));
+  ASSERT_TRUE(android::base::ReadFileToString(tf.path, &s));
+  EXPECT_EQ(size, s.size());
+  EXPECT_LT(s.capacity(), size + 16);
+}
+
+TEST(file, ReadFileToString_capacity_0) {
+  TemporaryFile tf;
+  ASSERT_TRUE(tf.fd != -1);
+
+  // Because /proc reports its files as zero-length, we don't actually trust
+  // any file that claims to be zero-length. Rather than add increasingly
+  // complex heuristics for shrinking the passed-in string in that case, we
+  // currently leave it alone.
+  std::string s;
+  size_t initial_capacity = s.capacity();
+  ASSERT_TRUE(android::base::WriteStringToFile("", tf.path));
+  ASSERT_TRUE(android::base::ReadFileToString(tf.path, &s));
+  EXPECT_EQ(0U, s.size());
+  EXPECT_EQ(initial_capacity, s.capacity());
+}