Remove ssize_t from code that is not POSIX-specific.

ssize_t is not standard C++. It is a POSIX extension. Therefore, it does
not belong in generic code.

This change tweaks the logic in DBIter to remove the need for signed
integers, so ssize_t can be replaced with size_t. The impacted method
and private member are renamed to better express their purpose.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211471606

db/db_iter.cc

@@ -57,7 +57,7 @@ class DBIter: public Iterator {
         direction_(kForward),
         valid_(false),
         rnd_(seed),
-        bytes_counter_(RandomPeriod()) {
+        bytes_until_read_sampling_(RandomCompactionPeriod()) {
   }
   virtual ~DBIter() {
     delete iter_;
@@ -103,8 +103,8 @@ class DBIter: public Iterator {
     }
   }
 
-  // Pick next gap with average value of config::kReadBytesPeriod.
-  ssize_t RandomPeriod() {
+  // Picks the number of bytes that can be read until a compaction is scheduled.
+  size_t RandomCompactionPeriod() {
     return rnd_.Uniform(2*config::kReadBytesPeriod);
   }
 
@@ -120,7 +120,7 @@ class DBIter: public Iterator {
   bool valid_;
 
   Random rnd_;
-  ssize_t bytes_counter_;
+  size_t bytes_until_read_sampling_;
 
   // No copying allowed
   DBIter(const DBIter&);
@@ -129,12 +129,15 @@ class DBIter: public Iterator {
 
 inline bool DBIter::ParseKey(ParsedInternalKey* ikey) {
   Slice k = iter_->key();
-  ssize_t n = k.size() + iter_->value().size();
-  bytes_counter_ -= n;
-  while (bytes_counter_ < 0) {
-    bytes_counter_ += RandomPeriod();
+
+  size_t bytes_read = k.size() + iter_->value().size();
+  while (bytes_until_read_sampling_ < bytes_read) {
+    bytes_until_read_sampling_ += RandomCompactionPeriod();
     db_->RecordReadSample(k);
   }
+  assert(bytes_until_read_sampling_ >= bytes_read);
+  bytes_until_read_sampling_ -= bytes_read;
+
   if (!ParseInternalKey(k, ikey)) {
     status_ = Status::Corruption("corrupted internal key in DBIter");
     return false;

db/fault_injection_test.cc

@@ -85,9 +85,9 @@ Status Truncate(const std::string& filename, uint64_t length) {
 
 struct FileState {
   std::string filename_;
-  ssize_t pos_;
-  ssize_t pos_at_last_sync_;
-  ssize_t pos_at_last_flush_;
+  int64_t pos_;
+  int64_t pos_at_last_sync_;
+  int64_t pos_at_last_flush_;
 
   FileState(const std::string& filename)
       : filename_(filename),
@@ -360,7 +360,7 @@ void FaultInjectionTestEnv::WritableFileClosed(const FileState& state) {
 }
 
 Status FileState::DropUnsyncedData() const {
-  ssize_t sync_pos = pos_at_last_sync_ == -1 ? 0 : pos_at_last_sync_;
+  int64_t sync_pos = pos_at_last_sync_ == -1 ? 0 : pos_at_last_sync_;
   return Truncate(filename_, sync_pos);
 }