Export of internal Abseil changes

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cca8a0c0d709803ce3413861ccbdb1b47e8fdcef by Abseil Team <absl-team@google.com>:

Add AssertHeld() to SpinLock.

PiperOrigin-RevId: 418034654

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9e95a0e614b4cd7929d8db4324ca69d7be5351b2 by Evan Brown <ezb@google.com>:

Make btree use an unsigned size_type (size_t).

This brings btree in line with std::set, et al., which are required to use an unsigned size type - see https://en.cppreference.com/w/cpp/container/set.

Also avoid some warnings about comparing integers of different signs.

PiperOrigin-RevId: 418014466
GitOrigin-RevId: cca8a0c0d709803ce3413861ccbdb1b47e8fdcef
Change-Id: I2b951cf1d69a3bb9c8dc236c69ab4a06b45047ea

absl/base/internal/spinlock.h

@@ -120,6 +120,14 @@ class ABSL_LOCKABLE SpinLock {
     return (lockword_.load(std::memory_order_relaxed) & kSpinLockHeld) != 0;
   }
 
+  // Return immediately if this thread holds the SpinLock exclusively.
+  // Otherwise, report an error by crashing with a diagnostic.
+  inline void AssertHeld() const ABSL_ASSERT_EXCLUSIVE_LOCK() {
+    if (!IsHeld()) {
+      ABSL_RAW_LOG(FATAL, "thread should hold the lock on SpinLock");
+    }
+  }
+
  protected:
   // These should not be exported except for testing.
 

absl/container/internal/btree.h

@@ -238,7 +238,7 @@ struct common_params {
 
   using allocator_type = Alloc;
   using key_type = Key;
-  using size_type = std::make_signed<size_t>::type;
+  using size_type = size_t;
   using difference_type = ptrdiff_t;
 
   using slot_policy = SlotPolicy;
@@ -1497,7 +1497,7 @@ inline void btree_node<P>::emplace_value(const size_type i,
   set_finish(finish() + 1);
 
   if (!leaf() && finish() > i + 1) {
-    for (int j = finish(); j > i + 1; --j) {
+    for (field_type j = finish(); j > i + 1; --j) {
       set_child(j, child(j - 1));
     }
     clear_child(i + 1);
@@ -2124,7 +2124,7 @@ auto btree<P>::erase_range(iterator begin, iterator end)
     return {0, begin};
   }
 
-  if (count == size_) {
+  if (static_cast<size_type>(count) == size_) {
     clear();
     return {count, this->end()};
   }