don't do atomic overrides with failed files

We deploy atomic renames for some files, but these renames also happen
if something about the file failed which isn't really the point of the
exercise…

Closes: 828908

apt-pkg/contrib/fileutl.cc

@@ -2623,7 +2623,7 @@ bool FileFd::Close()
    }
 
    if ((Flags & Replace) == Replace) {
-      if (rename(TemporaryFileName.c_str(), FileName.c_str()) != 0)
+      if (Failed() == false && rename(TemporaryFileName.c_str(), FileName.c_str()) != 0)
 	 Res &= _error->Errno("rename",_("Problem renaming the file %s to %s"), TemporaryFileName.c_str(), FileName.c_str());
 
       FileName = TemporaryFileName; // for the unlink() below.

test/libapt/fileutl_test.cc

@@ -327,6 +327,7 @@ TEST(FileUtlTest, flAbsPath)
 
 static void TestDevNullFileFd(unsigned int const filemode)
 {
+   SCOPED_TRACE(filemode);
    FileFd f("/dev/null", filemode);
    EXPECT_FALSE(f.Failed());
    EXPECT_TRUE(f.IsOpen());
@@ -354,3 +355,37 @@ TEST(FileUtlTest, WorkingWithDevNull)
    TestDevNullFileFd(FileFd::WriteTemp);
    TestDevNullFileFd(FileFd::WriteAtomic);
 }
+constexpr char const * const TESTSTRING = "This is a test";
+static void TestFailingAtomicKeepsFile(char const * const label, std::string const &filename)
+{
+   SCOPED_TRACE(label);
+   EXPECT_TRUE(FileExists(filename));
+   FileFd fd;
+   EXPECT_TRUE(fd.Open(filename, FileFd::ReadOnly));
+   char buffer[50];
+   EXPECT_NE(nullptr, fd.ReadLine(buffer, sizeof(buffer)));
+   EXPECT_STREQ(TESTSTRING, buffer);
+}
+TEST(FileUtlTest, FailingAtomic)
+{
+   FileFd fd;
+   std::string filename;
+   createTemporaryFile("failingatomic", fd, &filename, TESTSTRING);
+   TestFailingAtomicKeepsFile("init", filename);
+
+   FileFd f;
+   EXPECT_TRUE(f.Open(filename, FileFd::ReadWrite | FileFd::Atomic));
+   f.EraseOnFailure();
+   EXPECT_FALSE(f.Failed());
+   EXPECT_TRUE(f.IsOpen());
+   TestFailingAtomicKeepsFile("before-fail", filename);
+   EXPECT_TRUE(f.Write("Bad file write", 10));
+   f.OpFail();
+   EXPECT_TRUE(f.Failed());
+   TestFailingAtomicKeepsFile("after-fail", filename);
+   EXPECT_TRUE(f.Close());
+   TestFailingAtomicKeepsFile("closed", filename);
+
+   if (filename.empty() == false)
+      unlink(filename.c_str());
+}