Make sure we have at least kHeaderSize immutable pages in log (#284)

It seems that when there is less than kHeaderSize immutable pages in log,
we will not be able to dump the in-memory log into disk when the in-memory
log is full. Added a check to enforce this requirement.

A complication is compaction logic. In compaction logic, the logic makes
sure that nothing will be dumped to disk, so it provides no backing storage
path (the root_directory is "") and the requirement does not apply.
So we will only throw error if root_directory is not "".

cc/src/core/faster.h

@@ -101,7 +101,7 @@ class FasterKv {
            double log_mutable_fraction = 0.9, bool pre_allocate_log = false)
     : min_table_size_{ table_size }
     , disk{ filename, epoch_ }
-    , hlog{ log_size, epoch_, disk, disk.log(), log_mutable_fraction, pre_allocate_log }
+    , hlog{ filename.empty() /*hasNoBackingStorage*/, log_size, epoch_, disk, disk.log(), log_mutable_fraction, pre_allocate_log }
     , system_state_{ Action::None, Phase::REST, 1 }
     , num_pending_ios{ 0 } {
     if(!Utility::IsPowerOfTwo(table_size)) {

cc/src/core/persistent_memory_malloc.h

@@ -243,7 +243,7 @@ class PersistentMemoryMalloc {
   /// The first 4 HLOG pages should be below the head (i.e., being flushed to disk).
   static constexpr uint32_t kNumHeadPages = 4;
 
-  PersistentMemoryMalloc(uint64_t log_size, LightEpoch& epoch, disk_t& disk_, log_file_t& file_,
+  PersistentMemoryMalloc(bool has_no_backing_storage, uint64_t log_size, LightEpoch& epoch, disk_t& disk_, log_file_t& file_,
                          Address start_address, double log_mutable_fraction, bool pre_allocate_log)
     : sector_size{ static_cast<uint32_t>(file_.alignment()) }
     , epoch_{ &epoch }
@@ -282,6 +282,13 @@ class PersistentMemoryMalloc {
       // mutable page is full.
       throw std::invalid_argument{ "Must have at least 2 mutable pages" };
     }
+    // Make sure we have at least 'kNumHeadPages' immutable pages.
+    // Otherwise, we will not be able to dump log to disk when our in-memory log is full.
+    // If the user is certain that we will never need to dump anything to disk
+    // (this is the case in compaction), skip this check.
+    if(!has_no_backing_storage && buffer_size_ - num_mutable_pages_ < kNumHeadPages) {
+      throw std::invalid_argument{ "Must have at least 'kNumHeadPages' immutable pages" };
+    }
 
     page_status_ = new FullPageStatus[buffer_size_];
 
@@ -302,9 +309,9 @@ class PersistentMemoryMalloc {
     AllocatePage(tail_page_offset.page() + 1);
   }
 
-  PersistentMemoryMalloc(uint64_t log_size, LightEpoch& epoch, disk_t& disk_, log_file_t& file_,
+  PersistentMemoryMalloc(bool has_no_backing_storage, uint64_t log_size, LightEpoch& epoch, disk_t& disk_, log_file_t& file_,
                          double log_mutable_fraction, bool pre_allocate_log)
-    : PersistentMemoryMalloc(log_size, epoch, disk_, file_, Address{ 0 }, log_mutable_fraction, pre_allocate_log) {
+    : PersistentMemoryMalloc(has_no_backing_storage, log_size, epoch, disk_, file_, Address{ 0 }, log_mutable_fraction, pre_allocate_log) {
     /// Allocate the invalid page. Supports allocations aligned up to kCacheLineBytes.
     uint32_t discard;
     Allocate(Constants::kCacheLineBytes, discard);