Fixes possible crash in SessionBackend. I believe what's happening

here is we fail to create the file, and so current_session_file_.get()
is NULL and we crash. current_session_file_.get() is NULL if
OpenAndWriteHeader returns NULL (which is does if the full header
isn't written correctly.

Here's how I'm changing the code:
. The file is now truncated instead of closed/reopened. Hopefully this avoids the possibility of a scanner locking the file and the delete failing.
. Added a unit test for coverage of truncation.
. The file is opened in exclusive access. There is no reason why a scanner should open this file.
. Added null checks.

BUG=8476
TEST=none

Review URL: http://codereview.chromium.org/39275

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@11262 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 29 insertions, 0 deletions

diff --git a/chrome/browser/sessions/session_backend_unittest.cc b/chrome/browser/sessions/session_backend_unittest.cc
index fb424f2..e5ba17d4 100644
--- a/chrome/browser/sessions/session_backend_unittest.cc
+++ b/chrome/browser/sessions/session_backend_unittest.cc
@@ -180,3 +180,32 @@ TEST_F(SessionBackendTest, EmptyCommand) {
   AssertCommandEqualsData(empty_command, commands[0]);
   STLDeleteElements(&commands);
 }
+
+// Writes a command, appends another command with reset to true, then reads
+// making sure we only get back the second command.
+TEST_F(SessionBackendTest, Truncate) {
+  scoped_refptr<SessionBackend> backend(
+      new SessionBackend(BaseSessionService::SESSION_RESTORE, path_));
+  struct TestData first_data = { 1,  "a" };
+  std::vector<SessionCommand*> commands;
+  commands.push_back(CreateCommandFromData(first_data));
+  backend->AppendCommands(new SessionCommands(commands), false);
+  commands.clear();
+
+  // Write another command, this time resetting the file when appending.
+  struct TestData second_data = { 2,  "b" };
+  commands.push_back(CreateCommandFromData(second_data));
+  backend->AppendCommands(new SessionCommands(commands), true);
+  commands.clear();
+
+  // Read it back in.
+  backend = NULL;
+  backend = new SessionBackend(BaseSessionService::SESSION_RESTORE, path_);
+  backend->ReadLastSessionCommandsImpl(&commands);
+
+  // And make sure we get back the expected data.
+  ASSERT_EQ(1U, commands.size());
+  AssertCommandEqualsData(second_data, commands[0]);
+
+  STLDeleteElements(&commands);
+}