Update realloc(p,0) semantics

This commit changes the behaviour of realloc(p,0) to be free(p) if p!=nullptr, and malloc(0) if p== nullptr.

src/snmalloc/global/libc.h

@@ -52,6 +52,25 @@ namespace snmalloc::libc
 
   SNMALLOC_FAST_PATH_INLINE void* realloc(void* ptr, size_t size)
   {
+    // Glibc treats
+    //   realloc(p, 0) as free(p)
+    //   realloc(nullptr, s) as malloc(s)
+    // and for the overlap
+    //   realloc(nullptr, 0) as malloc(1)
+    // Without the first two we don't pass the glibc tests.
+    // The last one is required by various gnu utilities such as grep, sed.
+    if (SNMALLOC_UNLIKELY(!is_small_sizeclass(size)))
+    {
+      if (SNMALLOC_UNLIKELY(size == 0))
+      {
+        if (SNMALLOC_UNLIKELY(ptr == nullptr))
+          return malloc(1);
+
+        dealloc(ptr);
+        return nullptr;
+      }
+    }
+
     size_t sz = alloc_size(ptr);
     // Keep the current allocation if the given size is in the same sizeclass.
     if (sz == round_size(size))
@@ -72,13 +91,10 @@ namespace snmalloc::libc
       }
       dealloc(ptr);
     }
-    else if (SNMALLOC_LIKELY(size == 0))
-    {
-      dealloc(ptr);
-    }
     else
     {
-      return set_error();
+      // Error should be set by alloc on this path already.
+      SNMALLOC_ASSERT(errno == ENOMEM);
     }
     return p;
   }

src/test/func/malloc/malloc.cc

@@ -265,6 +265,9 @@ int main(int argc, char** argv)
     test_calloc(0, size, SUCCESS, false);
   }
 
+  // Check realloc(nullptr,0) behaves like malloc(1)
+  test_realloc(nullptr, 0, SUCCESS, false);
+
   for (smallsizeclass_t sc = 0; sc < NUM_SMALL_SIZECLASSES; sc++)
   {
     const size_t size = sizeclass_to_size(sc);
@@ -277,6 +280,8 @@ int main(int argc, char** argv)
       test_realloc(our_malloc(size), size2, SUCCESS, false);
       test_realloc(our_malloc(size + 1), size2, SUCCESS, false);
     }
+    // Check realloc(p,0), behaves like free(p), if p != nullptr
+    test_realloc(our_malloc(size), 0, SUCCESS, true);
   }
 
   for (smallsizeclass_t sc = 0; sc < (MAX_SMALL_SIZECLASS_BITS + 4); sc++)