add helpers for allocating flex-array structs

Allocating a struct with a flex array is pretty simple in
practice: you over-allocate the struct, then copy some data
into the over-allocation. But it can be a slight pain to
make sure you're allocating and copying the right amounts.

This patch adds a few helpers to turn simple cases of
flex-array struct allocation into a one-liner that properly
checks for overflow. See the embedded documentation for
details.

Ideally we could provide a more flexible version that could
handle multiple strings, like:

  FLEX_ALLOC_FMT(ref, name, "%s%s", prefix, name);

But we have to implement this as a macro (because of the
offset calculation of the flex member), which means we would
need all compilers to support variadic macros.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -782,6 +782,68 @@ extern FILE *fopen_for_writing(const char *path);
 #define ALLOC_ARRAY(x, alloc) (x) = xmalloc(st_mult(sizeof(*(x)), (alloc)))
 #define REALLOC_ARRAY(x, alloc) (x) = xrealloc((x), st_mult(sizeof(*(x)), (alloc)))
 
+/*
+ * These functions help you allocate structs with flex arrays, and copy
+ * the data directly into the array. For example, if you had:
+ *
+ *   struct foo {
+ *     int bar;
+ *     char name[FLEX_ARRAY];
+ *   };
+ *
+ * you can do:
+ *
+ *   struct foo *f;
+ *   FLEX_ALLOC_MEM(f, name, src, len);
+ *
+ * to allocate a "foo" with the contents of "src" in the "name" field.
+ * The resulting struct is automatically zero'd, and the flex-array field
+ * is NUL-terminated (whether the incoming src buffer was or not).
+ *
+ * The FLEXPTR_* variants operate on structs that don't use flex-arrays,
+ * but do want to store a pointer to some extra data in the same allocated
+ * block. For example, if you have:
+ *
+ *   struct foo {
+ *     char *name;
+ *     int bar;
+ *   };
+ *
+ * you can do:
+ *
+ *   struct foo *f;
+ *   FLEX_ALLOC_STR(f, name, src);
+ *
+ * and "name" will point to a block of memory after the struct, which will be
+ * freed along with the struct (but the pointer can be repointed anywhere).
+ *
+ * The *_STR variants accept a string parameter rather than a ptr/len
+ * combination.
+ *
+ * Note that these macros will evaluate the first parameter multiple
+ * times, and it must be assignable as an lvalue.
+ */
+#define FLEX_ALLOC_MEM(x, flexname, buf, len) do { \
+	(x) = NULL; /* silence -Wuninitialized for offset calculation */ \
+	(x) = xalloc_flex(sizeof(*(x)), (char *)(&((x)->flexname)) - (char *)(x), (buf), (len)); \
+} while (0)
+#define FLEXPTR_ALLOC_MEM(x, ptrname, buf, len) do { \
+	(x) = xalloc_flex(sizeof(*(x)), sizeof(*(x)), (buf), (len)); \
+	(x)->ptrname = (void *)((x)+1); \
+} while(0)
+#define FLEX_ALLOC_STR(x, flexname, str) \
+	FLEX_ALLOC_MEM((x), flexname, (str), strlen(str))
+#define FLEXPTR_ALLOC_STR(x, ptrname, str) \
+	FLEXPTR_ALLOC_MEM((x), ptrname, (str), strlen(str))
+
+static inline void *xalloc_flex(size_t base_len, size_t offset,
+				const void *src, size_t src_len)
+{
+	unsigned char *ret = xcalloc(1, st_add3(base_len, src_len, 1));
+	memcpy(ret + offset, src, src_len);
+	return ret;
+}
+
 static inline char *xstrdup_or_null(const char *str)
 {
 	return str ? xstrdup(str) : NULL;