common: avoid locale dependent strtod(3)

Replace `json_to_double()` (which uses `strtod(3)`) with our own
floating-point parsing function `json_to_millionths()` that
specifically expects to receive such a number that can fit in a
64 bit integer after being multiplied by 1 million.

The main piece of the code in this patch comes from
https://github.com/ElementsProject/lightning/pull/3535#discussion_r381041419

Changelog-None

common/json.c

@@ -13,8 +13,10 @@
 #include <common/json.h>
 #include <common/json_stream.h>
 #include <common/node_id.h>
+#include <common/overflows.h>
 #include <common/utils.h>
 #include <common/wireaddr.h>
+#include <ctype.h>
 #include <errno.h>
 #include <inttypes.h>
 #include <stdarg.h>
@@ -97,13 +99,45 @@ bool json_to_s64(const char *buffer, const jsmntok_t *tok, s64 *num)
 	return true;
 }
 
-bool json_to_double(const char *buffer, const jsmntok_t *tok, double *num)
-{
-	char *end;
+bool json_to_millionths(const char *buffer, const jsmntok_t *tok,
+			u64 *millionths)
+{
+	int decimal_places = -1;
+	bool has_digits = 0;
+
+	*millionths = 0;
+	for (int i = tok->start; i < tok->end; i++) {
+		if (isdigit(buffer[i])) {
+			has_digits = true;
+			/* Ignore too much precision */
+			if (decimal_places >= 0 && ++decimal_places > 6)
+				continue;
+			if (mul_overflows_u64(*millionths, 10))
+				return false;
+			*millionths *= 10;
+			if (add_overflows_u64(*millionths, buffer[i] - '0'))
+				return false;
+			*millionths += buffer[i] - '0';
+		} else if (buffer[i] == '.') {
+			if (decimal_places != -1)
+				return false;
+			decimal_places = 0;
+		} else
+			return false;
+	}
 
-	*num = strtod(buffer + tok->start, &end);
-	if (end != buffer + tok->end)
+	if (!has_digits)
 		return false;
+
+	if (decimal_places == -1)
+		decimal_places = 0;
+
+	while (decimal_places < 6) {
+		if (mul_overflows_u64(*millionths, 10))
+			return false;
+		*millionths *= 10;
+		decimal_places++;
+	}
 	return true;
 }
 

common/json.h

@@ -69,8 +69,13 @@ bool json_to_u32(const char *buffer, const jsmntok_t *tok,
 bool json_to_u16(const char *buffer, const jsmntok_t *tok,
                  uint16_t *num);
 
-/* Extract double from this (must be a number literal) */
-bool json_to_double(const char *buffer, const jsmntok_t *tok, double *num);
+/*
+ * Extract a non-negative (either 0 or positive) floating-point number from this
+ * (must be a number literal), multiply it by 1 million and return it as an
+ * integer. Any fraction smaller than 0.000001 is ignored.
+ */
+bool json_to_millionths(const char *buffer, const jsmntok_t *tok,
+			u64 *millionths);
 
 /* Extract signed integer from this (may be a string, or a number literal) */
 bool json_to_int(const char *buffer, const jsmntok_t *tok, int *num);

common/json_tok.c

@@ -39,12 +39,9 @@ struct command_result *param_millionths(struct command *cmd, const char *name,
 					const char *buffer,
 					const jsmntok_t *tok, uint64_t **num)
 {
-	double d;
-	if (json_to_double(buffer, tok, &d) && d >= 0.0) {
-		*num = tal(cmd, uint64_t);
-		**num = (uint64_t)(d * 1000000);
+	*num = tal(cmd, uint64_t);
+	if (json_to_millionths(buffer, tok, *num))
 		return NULL;
-	}
 
 	return command_fail(
 	    cmd, JSONRPC2_INVALID_PARAMS,

common/test/run-json.c

@@ -46,6 +46,62 @@ static int test_json_tok_bitcoin_amount(void)
 	return 0;
 }
 
+static void do_json_tok_millionths(const char *val, bool ok, uint64_t expected)
+{
+	uint64_t amount;
+	jsmntok_t tok;
+
+	tok.start = 0;
+	tok.end = strlen(val);
+
+	assert(json_to_millionths(val, &tok, &amount) == ok);
+	if (ok)
+		assert(amount == expected);
+}
+
+static int test_json_tok_millionths(void)
+{
+	do_json_tok_millionths("", false, 0);
+	do_json_tok_millionths("0..0", false, 0);
+	do_json_tok_millionths("0.0.", false, 0);
+	do_json_tok_millionths(".", false, 0);
+	do_json_tok_millionths("..", false, 0);
+
+	do_json_tok_millionths("0", true, 0);
+	do_json_tok_millionths(".0", true, 0);
+	do_json_tok_millionths("0.", true, 0);
+	do_json_tok_millionths("100", true, 100 * 1000000);
+	do_json_tok_millionths("100.0", true, 100 * 1000000);
+	do_json_tok_millionths("100.", true, 100 * 1000000);
+	do_json_tok_millionths("100.000001", true, 100 * 1000000 + 1);
+	do_json_tok_millionths("100.0000001", true, 100 * 1000000);
+	do_json_tok_millionths(".000009", true, 9);
+	do_json_tok_millionths(".0000099", true, 9);
+	do_json_tok_millionths("18446744073709.551615", true,
+			       18446744073709551615ULL);
+	do_json_tok_millionths("18446744073709.551616", false, 0);
+	do_json_tok_millionths("18446744073709.551625", false, 0);
+	do_json_tok_millionths("18446744073709.551715", false, 0);
+	do_json_tok_millionths("18446744073709.552615", false, 0);
+	do_json_tok_millionths("18446744073709.561615", false, 0);
+	do_json_tok_millionths("18446744073709.651615", false, 0);
+	do_json_tok_millionths("18446744073710.551615", false, 0);
+	do_json_tok_millionths("18446744073809.551615", false, 0);
+	do_json_tok_millionths("18446744074709.551615", false, 0);
+	do_json_tok_millionths("18446744083709.551615", false, 0);
+	do_json_tok_millionths("18446744173709.551615", false, 0);
+	do_json_tok_millionths("18446745073709.551615", false, 0);
+	do_json_tok_millionths("18446754073709.551615", false, 0);
+	do_json_tok_millionths("18446844073709.551615", false, 0);
+	do_json_tok_millionths("18447744073709.551615", false, 0);
+	do_json_tok_millionths("18456744073709.551615", false, 0);
+	do_json_tok_millionths("18546744073709.551615", false, 0);
+	do_json_tok_millionths("19446744073709.551615", false, 0);
+	do_json_tok_millionths("28446744073709.551615", false, 0);
+
+	return 0;
+}
+
 static void test_json_tok_size(void)
 {
 	const jsmntok_t *toks;
@@ -190,6 +246,7 @@ int main(void)
 
 	test_json_tok_size();
 	test_json_tok_bitcoin_amount();
+	test_json_tok_millionths();
 	test_json_delve();
 	assert(!taken_any());
 	take_cleanup();