libstdc++: handle small max_blocks_per_chunk in pool resources [PR 94160]

When a pool resource is constructed with max_blocks_per_chunk=1 it ends
up creating a pool with blocks_per_chunk=0 which means it never
allocates anything. Instead it returns null pointers, which should be
impossible.

To avoid this problem, round the max_blocks_per_chunk value to a
multiple of four, so it's never smaller than four.

libstdc++-v3/ChangeLog:

	PR libstdc++/94160
	* src/c++17/memory_resource.cc (munge_options): Round
	max_blocks_per_chunk to a multiple of four.
	(__pool_resource::_M_alloc_pools()): Simplify slightly.
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc:
	Check that valid pointers are returned when small values are
	used for max_blocks_per_chunk.

libstdc++-v3/src/c++17/memory_resource.cc

@@ -898,7 +898,18 @@ namespace pmr
       }
     else
       {
-	// TODO round to preferred granularity ?
+	// Round to preferred granularity.
+	if (opts.max_blocks_per_chunk < size_t(-4))
+	  {
+	    // round up
+	    opts.max_blocks_per_chunk
+	      = aligned_ceil(opts.max_blocks_per_chunk, 4);
+	  }
+	else
+	  {
+	    // round down
+	    opts.max_blocks_per_chunk &= ~size_t(3);
+	  }
       }
 
     if (opts.max_blocks_per_chunk > chunk::max_blocks_per_chunk())
@@ -1039,11 +1050,9 @@ namespace pmr
 	  : pool_sizes[i];
 
 	// Decide on initial number of blocks per chunk.
-	// Always have at least 16 blocks per chunk:
-	const size_t min_blocks_per_chunk = 16;
-	// But for smaller blocks, use a larger initial size:
-	size_t blocks_per_chunk
-	  = std::max(1024 / block_size, min_blocks_per_chunk);
+	// At least 16 blocks per chunk seems reasonable,
+	// more for smaller blocks:
+	size_t blocks_per_chunk = std::max(size_t(16), 1024 / block_size);
 	// But don't exceed the requested max_blocks_per_chunk:
 	blocks_per_chunk
 	  = std::min(blocks_per_chunk, _M_opts.max_blocks_per_chunk);

libstdc++-v3/testsuite/20_util/unsynchronized_pool_resource/allocate.cc

@@ -300,6 +300,25 @@ test07()
   }
 }
 
+void
+test08()
+{
+  std::pmr::pool_options opts;
+  opts.largest_required_pool_block = 64;
+
+  // PR libstdc++/94160
+  // max_blocks_per_chunk=1 causes pool resources to return null pointers
+  for (int i = 0; i < 8; ++i)
+  {
+    opts.max_blocks_per_chunk = i;
+    std::pmr::unsynchronized_pool_resource upr(opts);
+    auto* p = (int*)upr.allocate(4);
+    VERIFY( p != nullptr );
+    *p = i;
+    upr.deallocate(p, 4);
+  }
+}
+
 int
 main()
 {
@@ -310,4 +329,5 @@ main()
   test05();
   test06();
   test07();
+  test08();
 }