tal: add TAL_TAKE.

TAL_TAKE provides a magic context meaning "consume my args and return
a replacement".  This is useful for writing convenience functions,
though not so useful in the standard routines here.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/tal.c

@@ -706,7 +706,12 @@ char *tal_strndup(const tal_t *ctx, const char *p, size_t n)
 
 void *tal_memdup(const tal_t *ctx, const void *p, size_t n)
 {
-	void *ret = tal_arr(ctx, char, n);
+	void *ret;
+
+	if (ctx == TAL_TAKE)
+		return (void *)p;
+
+	ret = tal_arr(ctx, char, n);
 	if (ret)
 		memcpy(ret, p, n);
 	return ret;
@@ -727,9 +732,14 @@ char *tal_asprintf(const tal_t *ctx, const char *fmt, ...)
 char *tal_vasprintf(const tal_t *ctx, const char *fmt, va_list ap)
 {
 	size_t max = strlen(fmt) * 2;
-	char *buf = tal_arr(ctx, char, max);
+	char *buf;
 	int ret;
 
+	if (ctx == TAL_TAKE)
+		buf = tal_arr(tal_parent(fmt), char, max);
+	else
+		buf = tal_arr(ctx, char, max);
+
 	while (buf) {
 		va_list ap2;
 
@@ -741,6 +751,8 @@ char *tal_vasprintf(const tal_t *ctx, const char *fmt, va_list ap)
 			break;
 		buf = tal_resize(buf, max *= 2);
 	}
+	if (ctx == TAL_TAKE)
+		tal_free(fmt);
 	return buf;
 }
 

ccan/tal/tal.h

@@ -17,6 +17,18 @@
  */
 typedef void tal_t;
 
+/**
+ * TAL_TAKE - fake tal_t to indicate function will own arguments.
+ *
+ * Various functions take a context on which to allocate: if you use
+ * TAL_TAKE there instead, it means that the argument(s) are actually
+ * tal objects.  The returned value will share the same parent; it may
+ * even be the same pointer as the arguments.  The arguments themselves
+ * will be reused, freed, or made a child of the return value: they are
+ * no longer valid for external use.
+ */
+#define TAL_TAKE ((tal_t *)-2L)
+
 /**
  * tal - basic allocator function
  * @ctx: NULL, or tal allocated object to be parent.
@@ -146,22 +158,23 @@ tal_t *tal_parent(const tal_t *ctx);
 
 /**
  * tal_memdup - duplicate memory.
- * @ctx: NULL, or tal allocated object to be parent.
+ * @ctx: NULL, or tal allocated object to be parent (or TAL_TAKE).
  * @p: the memory to copy
  * @n: the number of bytes.
+ *
  */
 void *tal_memdup(const tal_t *ctx, const void *p, size_t n);
 
 /**
- * tal_strdup - duplicate a string.
- * @ctx: NULL, or tal allocated object to be parent.
+ * tal_strdup - duplicate a string
+ * @ctx: NULL, or tal allocated object to be parent (or TAL_TAKE).
  * @p: the string to copy
  */
 char *tal_strdup(const tal_t *ctx, const char *p);
 
 /**
  * tal_strndup - duplicate a limited amount of a string.
- * @ctx: NULL, or tal allocated object to be parent.
+ * @ctx: NULL, or tal allocated object to be parent (or TAL_TAKE).
  * @p: the string to copy
  * @n: the maximum length to copy.
  *
@@ -171,16 +184,22 @@ char *tal_strndup(const tal_t *ctx, const char *p, size_t n);
 
 /**
  * tal_asprintf - allocate a formatted string
- * @ctx: NULL, or tal allocated object to be parent.
+ * @ctx: NULL, or tal allocated object to be parent (or TAL_TAKE).
  * @fmt: the printf-style format.
+ *
+ * If @ctx is TAL_TAKE, @fmt is freed and its parent will be the parent
+ * of the return value.
  */
 char *tal_asprintf(const tal_t *ctx, const char *fmt, ...) PRINTF_FMT(2,3);
 
 /**
  * tal_vasprintf - allocate a formatted string (va_list version)
- * @ctx: NULL, or tal allocated object to be parent.
+ * @ctx: NULL, or tal allocated object to be parent (or TAL_TAKE).
  * @fmt: the printf-style format.
  * @va: the va_list containing the format args.
+ *
+ * If @ctx is TAL_TAKE, @fmt is freed and its parent will be the parent
+ * of the return value.
  */
 char *tal_vasprintf(const tal_t *ctx, const char *fmt, va_list ap)
 	PRINTF_FMT(2,0);

ccan/tal/test/run-take.c

+#include <ccan/tal/tal.h>
+#include <ccan/tal/tal.c>
+#include <ccan/tap/tap.h>
+
+int main(void)
+{
+	char *parent, *c;
+
+	plan_tests(13);
+
+	parent = tal(NULL, char);
+	ok1(parent);
+
+	c = tal_strdup(parent, "hello");
+
+	c = tal_strdup(TAL_TAKE, c);
+	ok1(strcmp(c, "hello") == 0);
+	ok1(tal_parent(c) == parent);
+
+	c = tal_strndup(TAL_TAKE, c, 5);
+	ok1(strcmp(c, "hello") == 0);
+	ok1(tal_parent(c) == parent);
+
+	c = tal_strndup(TAL_TAKE, c, 3);
+	ok1(strcmp(c, "hel") == 0);
+	ok1(tal_parent(c) == parent);
+
+	c = tal_memdup(TAL_TAKE, c, 1);
+	ok1(c[0] == 'h');
+	ok1(tal_parent(c) == parent);
+
+	/* No leftover allocations. */
+	tal_free(c);
+	ok1(tal_first(parent) == NULL);
+
+	c = tal_strdup(parent, "hello %s");
+	c = tal_asprintf(TAL_TAKE, c, "there");
+	ok1(strcmp(c, "hello there") == 0);
+	ok1(tal_parent(c) == parent);
+	/* No leftover allocations. */
+	tal_free(c);
+	ok1(tal_first(parent) == NULL);
+
+	tal_free(parent);
+
+	return exit_status();
+}