Add ConcurrencySafe property

Ranges can be safe to call from multiple threads.  This adds a constexpr
field to signify if that is the case.

src/backend/backend.h

@@ -235,6 +235,9 @@ namespace snmalloc
       }
       else
       {
+        static_assert(
+          GlobalMetaRange::ConcurrencySafe,
+          "Global meta data range needs to be concurrency safe.");
         typename GlobalMetaRange::State global_state;
         p = global_state->alloc_range(bits::next_pow2(size));
       }

src/backend/commitrange.h

@@ -25,6 +25,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = ParentRange::Aligned;
 
+    static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;
+
     constexpr CommitRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t size)

src/backend/empty_range.h

@@ -19,6 +19,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = true;
 
+    static constexpr bool ConcurrencySafe = true;
+
     constexpr EmptyRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t)

src/backend/globalrange.h

@@ -37,6 +37,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = ParentRange::Aligned;
 
+    static constexpr bool ConcurrencySafe = true;
+
     constexpr GlobalRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t size)

src/backend/largebuddyrange.h

@@ -271,6 +271,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = true;
 
+    static constexpr bool ConcurrencySafe = false;
+
     constexpr LargeBuddyRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t size)

src/backend/pagemapregisterrange.h

@@ -30,6 +30,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = ParentRange::Aligned;
 
+    static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;
+
     capptr::Chunk<void> alloc_range(size_t size)
     {
       auto base = state->alloc_range(size);

src/backend/palrange.h

@@ -24,6 +24,11 @@ namespace snmalloc
 
     static constexpr bool Aligned = pal_supports<AlignedAllocation, PAL>;
 
+    // Note we have always assumed the Pals to provide a concurrency safe
+    // API.  If in the future this changes, then this would
+    // need to be changed.
+    static constexpr bool ConcurrencySafe = true;
+
     constexpr PalRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t size)

src/backend/smallbuddyrange.h

@@ -175,6 +175,8 @@ namespace snmalloc
     static constexpr bool Aligned = true;
     static_assert(ParentRange::Aligned, "ParentRange must be aligned");
 
+    static constexpr bool ConcurrencySafe = false;
+
     constexpr SmallBuddyRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t size)

src/backend/statsrange.h

@@ -31,6 +31,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = ParentRange::Aligned;
 
+    static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;
+
     constexpr StatsRange() = default;
 
     capptr::Chunk<void> alloc_range(size_t size)

src/backend/subrange.h

@@ -31,6 +31,8 @@ namespace snmalloc
 
     static constexpr bool Aligned = ParentRange::Aligned;
 
+    static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;
+
     capptr::Chunk<void> alloc_range(size_t sub_size)
     {
       SNMALLOC_ASSERT(bits::is_pow2(sub_size));