Fix racy assignment of rowset indexes

Fixes a Jenkins failure seen in build #749:

I0617 22:40:14.869637 25346 tablet.cc:416] Compaction: entering phase 1 (flushing snapshot)
I0617 22:40:14.870381 25345 tablet.cc:416] Compaction: entering phase 1 (flushing snapshot)
I0617 22:40:14.870650 25345 diskrowset.cc:110] Opened CFile writer for column key[type='uint32'] at path /tmp/kudutest-1106/MultiThreadedTabletTest_1.DeleteAndReinsert.1371534007/tablet/rowset_0000000954.compact-tmp/col_0
I0617 22:40:14.870782 25345 diskrowset.cc:110] Opened CFile writer for column val[type='uint32'] at path /tmp/kudutest-1106/MultiThreadedTabletTest_1.DeleteAndReinsert.1371534007/tablet/rowset_0000000954.compact-tmp/col_1
I0617 22:40:14.870944 25345 diskrowset.cc:110] Opened CFile writer for column update_count[type='uint32'] at path /tmp/kudutest-1106/MultiThreadedTabletTest_1.DeleteAndReinsert.1371534007/tablet/rowset_0000000954.compact-tmp/col_2
F0617 22:40:14.870992 25346 mt-tablet-test.cc:242] Check failed: _s.ok() Bad status: IO error: /tmp/kudutest-1106/MultiThreadedTabletTest_1.DeleteAndReinsert.1371534007/tablet/rowset_0000000954.compact-tmp: File exists (error 17)

In this case, a concurrent Flush and Compact ended up getting assigned rowset #954 due to
a non-atomic and non-locked increment of next_rowset_idx. This commit changes it to be
an Atomic variable using atomic increment.

No new unit tests since this race is hard to trigger and an existing test caught it.

Review: http://review.sf.cloudera.com/r/29324/

src/tablet/tablet.cc

@@ -12,6 +12,7 @@
 #include "common/iterator.h"
 #include "common/scan_spec.h"
 #include "common/schema.h"
+#include "gutil/atomicops.h"
 #include "gutil/strings/numbers.h"
 #include "gutil/strings/strip.h"
 #include "tablet/compaction.h"
@@ -30,7 +31,7 @@ namespace kudu { namespace tablet {
 using std::string;
 using std::vector;
 using std::tr1::shared_ptr;
-
+using base::subtle::Barrier_AtomicIncrement;
 
 const char *kRowSetPrefix = "rowset_";
 
@@ -78,8 +79,8 @@ Status Tablet::Open() {
     if (TryStripPrefixString(child, kRowSetPrefix, &suffix)) {
       // The file should be named 'rowset_<N>'. N here is the index
       // of the rowset (indicating the order in which it was flushed).
-      uint32_t rowset_idx;
-      if (!safe_strtou32(suffix.c_str(), &rowset_idx)) {
+      int32_t rowset_idx;
+      if (!safe_strto32(suffix.c_str(), &rowset_idx)) {
         return Status::IOError(string("Bad rowset file: ") + absolute_path);
       }
 
@@ -92,8 +93,7 @@ Status Tablet::Open() {
       }
       rowsets_.push_back(rowset);
 
-      next_rowset_idx_ = std::max(next_rowset_idx_,
-                                 (size_t)rowset_idx + 1);
+      next_rowset_idx_ = std::max(next_rowset_idx_, rowset_idx + 1);
     } else {
       LOG(WARNING) << "ignoring unknown file: " << absolute_path;
     }
@@ -410,7 +410,8 @@ Status Tablet::PickRowSetsToCompact(RowSetsInCompaction *picked) const
 }
 
 Status Tablet::DoCompactionOrFlush(const RowSetsInCompaction &input) {
-  string new_rowset_dir = GetRowSetPath(dir_, next_rowset_idx_++);
+  AtomicWord my_index = Barrier_AtomicIncrement(&next_rowset_idx_, 1) - 1;
+  string new_rowset_dir = GetRowSetPath(dir_, my_index);
   string tmp_rowset_dir = new_rowset_dir + ".compact-tmp";
 
   LOG(INFO) << "Compaction: entering phase 1 (flushing snapshot)";

src/tablet/tablet.h

@@ -8,6 +8,7 @@
 #include "common/generic_iterators.h"
 #include "common/iterator.h"
 #include "common/schema.h"
+#include "gutil/atomicops.h"
 #include "gutil/gscoped_ptr.h"
 #include "tablet/diskrowset.h"
 #include "tablet/memrowset.h"
@@ -155,7 +156,7 @@ class Tablet {
   // and an RCU-style quiesce phase, but not worth it for now.
   mutable percpu_rwlock component_lock_;
 
-  size_t next_rowset_idx_;
+  Atomic32 next_rowset_idx_;
 
   Env *env_;