std: ArenaAllocator tries to resize before allocating

Closes #5116

lib/std/heap.zig

@@ -919,6 +919,13 @@ pub fn testAllocator(base_allocator: *mem.Allocator) !void {
     const zero_bit_ptr = try allocator.create(u0);
     zero_bit_ptr.* = 0;
     allocator.destroy(zero_bit_ptr);
+
+    const oversize = try allocator.allocAdvanced(u32, null, 5, .at_least);
+    testing.expect(oversize.len >= 5);
+    for (oversize) |*item| {
+        item.* = 0xDEADBEEF;
+    }
+    allocator.free(oversize);
 }
 
 pub fn testAllocatorAligned(base_allocator: *mem.Allocator, comptime alignment: u29) !void {

lib/std/heap/arena_allocator.zig

@@ -75,14 +75,23 @@ pub const ArenaAllocator = struct {
             const adjusted_addr = mem.alignForward(addr, ptr_align);
             const adjusted_index = self.state.end_index + (adjusted_addr - addr);
             const new_end_index = adjusted_index + n;
-            if (new_end_index > cur_buf.len) {
+
-                cur_node = try self.createNode(cur_buf.len, n + ptr_align);
+            if (new_end_index <= cur_buf.len) {
-                continue;
-            }
                 const result = cur_buf[adjusted_index..new_end_index];
                 self.state.end_index = new_end_index;
                 return result;
             }
+
+            const bigger_buf_size = @sizeOf(BufNode) + new_end_index;
+            // Try to grow the buffer in-place
+            cur_node.data = self.child_allocator.resize(cur_node.data, bigger_buf_size) catch |err| switch (err) {
+                error.OutOfMemory => {
+                    // Allocate a new node if that's not possible
+                    cur_node = try self.createNode(cur_buf.len, n + ptr_align);
+                    continue;
+                },
+            };
+        }
     }
 
     fn resize(allocator: *Allocator, buf: []u8, buf_align: u29, new_len: usize, len_align: u29, ret_addr: usize) Allocator.Error!usize {