json_escape: add fast-path for when we don't need to escape.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/json_escape/json_escape.c

@@ -19,6 +19,18 @@ bool json_escape_eq(const struct json_escape *a, const struct json_escape *b)
 	return streq(a->s, b->s);
 }
 
+bool json_escape_needed(const char *str, size_t len)
+{
+	for (size_t i = 0; i < len; i++) {
+		if ((unsigned)str[i] < ' '
+		    || str[i] == 127
+		    || str[i] == '"'
+		    || str[i] == '\\')
+			return true;
+	}
+	return false;
+}
+
 static struct json_escape *escape(const tal_t *ctx,
 				  const char *str TAKES,
 				  size_t len,
@@ -27,6 +39,16 @@ static struct json_escape *escape(const tal_t *ctx,
 	struct json_escape *esc;
 	size_t i, n;
 
+	/* Fast path: can steal, and nothing to escape. */
+	if (is_taken(str)
+	    && tal_count(str) > len
+	    && !json_escape_needed(str, len)) {
+		taken(str);
+		esc = (struct json_escape *)tal_steal(ctx, str);
+		esc->s[len] = '\0';
+		return esc;
+	}
+
 	/* Worst case: all \uXXXX */
 	esc = (struct json_escape *)tal_arr(ctx, char, len * 6 + 1);
 

ccan/json_escape/json_escape.h

@@ -27,7 +27,8 @@ struct json_escape *json_escape_len(const tal_t *ctx,
 struct json_escape *json_partial_escape(const tal_t *ctx,
 					 const char *str TAKES);
 
-/* Extract a JSON-escaped string. */
+/* Do we need to escape this str? */
+bool json_escape_needed(const char *str, size_t len);
 
 /* Are two escape json strings identical? */
 bool json_escape_eq(const struct json_escape *a,

ccan/json_escape/test/run-take.c

+#include <ccan/json_escape/json_escape.h>
+/* Include the C files directly. */
+#include <ccan/json_escape/json_escape.c>
+#include <ccan/tap/tap.h>
+
+int main(void)
+{
+	const tal_t *ctx = tal(NULL, char);
+	struct json_escape *e;
+	char *p;
+
+	/* This is how many tests you plan to run */
+	plan_tests(5);
+
+	/* This should simply be tal_steal */
+	p = tal_dup_arr(NULL, char, "Hello", 6, 0);
+	e = json_escape(ctx, take(p));
+	ok1(!strcmp(e->s, "Hello"));
+	ok1((void *)e == (void *)p);
+	ok1(tal_parent(e) == ctx);
+
+	/* This can't be tal_steal, but still should be freed. */
+	p = tal_dup_arr(NULL, char,
+			"\\\b\f\n\r\t\""
+			"\\\\\\b\\f\\n\\r\\t\\\"", 22, 0);
+	e = json_escape(ctx, take(p));
+	ok1(tal_parent(e) == ctx);
+	ok1(!strcmp(e->s,
+		    "\\\\\\b\\f\\n\\r\\t\\\""
+		    "\\\\\\\\\\\\b\\\\f\\\\n\\\\r\\\\t\\\\\\\""));
+	tal_free(ctx);
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}