Fix TransactionLogIterator EOF caching

Summary:
When TransactionLogIterator comes to EOF, it calls UnmarkEOF and continues reading. However, if glibc cached the EOF status of the file, it will get EOF again, even though the new data might have been written to it.

This has been causing errors in Mac OS.

Test Plan: test passes, was failing before

Reviewers: dhruba, haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18381

db/db_test.cc

@@ -5555,9 +5555,6 @@ TEST(DBTest, TransactionLogIteratorMoveOverZeroFiles) {
   } while (ChangeCompactOptions());
 }
 
-// TODO(kailiu) disable the in non-linux platforms to temporarily solve
-// // the unit test failure.
-#ifdef OS_LINUX
 TEST(DBTest, TransactionLogIteratorStallAtLastRecord) {
   do {
     Options options = OptionsForLogIterTest();
@@ -5575,7 +5572,6 @@ TEST(DBTest, TransactionLogIteratorStallAtLastRecord) {
     ASSERT_TRUE(iter->Valid());
   } while (ChangeCompactOptions());
 }
-#endif
 
 TEST(DBTest, TransactionLogIteratorJustEmptyFile) {
   do {

util/env_posix.cc

@@ -182,6 +182,9 @@ class PosixSequentialFile: public SequentialFile {
     if (r < n) {
       if (feof(file_)) {
         // We leave status as ok if we hit the end of the file
+        // We also clear the error so that the reads can continue
+        // if a new data is written to the file
+        clearerr(file_);
       } else {
         // A partial read with an error: return a non-ok status
         s = IOError(filename_, errno);