util/HugeAllocator: move MADV_DONTFORK setting to HugeForkCow()

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

src/util/HugeAllocator.cxx

@@ -73,12 +73,6 @@ HugeAllocate(size_t size)
 	madvise(p, size, MADV_HUGEPAGE);
 #endif
 
-#ifdef MADV_DONTFORK
-	/* just in case MPD needs to fork, don't copy this allocation
-	   to the child process, to reduce overhead */
-	madvise(p, size, MADV_DONTFORK);
-#endif
-
 	return p;
 }
 
@@ -88,6 +82,15 @@ HugeFree(void *p, size_t size) noexcept
 	munmap(p, AlignToPageSize(size));
 }
 
+void
+HugeForkCow(void *p, size_t size, bool enable) noexcept
+{
+#ifdef MADV_DONTFORK
+	madvise(p, AlignToPageSize(size),
+		enable ? MADV_DOFORK : MADV_DONTFORK);
+#endif
+}
+
 void
 HugeDiscard(void *p, size_t size) noexcept
 {

src/util/HugeAllocator.hxx

@@ -56,6 +56,13 @@ HugeAllocate(size_t size);
 void
 HugeFree(void *p, size_t size) noexcept;
 
+/**
+ * Control whether this allocation is copied to newly forked child
+ * processes.  Disabling that makes forking a little bit cheaper.
+ */
+void
+HugeForkCow(void *p, size_t size, bool enable) noexcept;
+
 /**
  * Discard any data stored in the allocation and give the memory back
  * to the kernel.  After returning, the allocation still exists and
@@ -80,6 +87,11 @@ HugeFree(void *p, gcc_unused size_t size) noexcept
 	VirtualFree(p, 0, MEM_RELEASE);
 }
 
+static inline void
+HugeForkCow(void *, size_t, bool) noexcept
+{
+}
+
 static inline void
 HugeDiscard(void *p, size_t size) noexcept
 {
@@ -106,6 +118,11 @@ HugeFree(void *_p, size_t) noexcept
 	delete[] p;
 }
 
+static inline void
+HugeForkCow(void *, size_t, bool) noexcept
+{
+}
+
 static inline void
 HugeDiscard(void *, size_t) noexcept
 {
@@ -140,6 +157,10 @@ public:
 		return *this;
 	}
 
+	void ForkCow(bool enable) noexcept {
+		HugeForkCow(data, size, enable);
+	}
+
 	void Discard() noexcept {
 		HugeDiscard(data, size);
 	}

src/util/SliceBuffer.hxx

@@ -74,6 +74,8 @@ public:
 		:n_max(_count),
 		 data((Slice *)HugeAllocate(CalcAllocationSize())) {
 		assert(n_max > 0);
+
+		HugeForkCow(data, CalcAllocationSize(), false);
 	}
 
 	~SliceBuffer() {