common: implement tal_arr_append and tal_arr_appendn, and use them.

diff --git a/bitcoin/psbt.c b/bitcoin/psbt.c
index 68564595..10dada97 100644
--- a/bitcoin/psbt.c
+++ b/bitcoin/psbt.c
@@ -673,9 +673,7 @@ size_t psbt_output_get_weight(const struct wally_psbt *psbt,
 
 static void add(u8 **key, const void *mem, size_t len)
 {
-	size_t oldlen = tal_count(*key);
-	tal_resize(key, oldlen + len);
-	memcpy(*key + oldlen, memcheck(mem, len), len);
+	tal_arr_appendn(key, mem, len);
 }
 
 static void add_type(u8 **key, const u8 num)
diff --git a/bitcoin/script.c b/bitcoin/script.c
index 3b38eae9..ede201e9 100644
--- a/bitcoin/script.c
+++ b/bitcoin/script.c
@@ -37,9 +37,7 @@ static void hash160(struct ripemd160 *redeemhash, const void *mem, size_t len)
 
 static void add(u8 **scriptp, const void *mem, size_t len)
 {
-	size_t oldlen = tal_count(*scriptp);
-	tal_resize(scriptp, oldlen + len);
-	memcpy(*scriptp + oldlen, mem, len);
+	tal_arr_appendn(scriptp, mem, len);
 }
 
 static void add_op(u8 **scriptp, u8 op)
diff --git a/common/bolt11.c b/common/bolt11.c
index 27e5bd24..bc851ed4 100644
--- a/common/bolt11.c
+++ b/common/bolt11.c
@@ -1251,8 +1251,6 @@ static void maybe_encode_9(u5 **data, const u8 *features,
 
 static bool encode_extra(u5 **data, const struct bolt11_field *extra)
 {
-	size_t len;
-
 	/* Can't encode an invalid tag. */
 	if (bech32_charset_rev[(unsigned char)extra->tag] == -1)
 		return false;
@@ -1261,9 +1259,7 @@ static bool encode_extra(u5 **data, const struct bolt11_field *extra)
 	push_varlen_uint(data, tal_count(extra->data), 10);
 
 	/* extra->data is already u5s, so do this raw. */
-	len = tal_count(*data);
-	tal_resize(data, len + tal_count(extra->data));
-	memcpy(*data + len, extra->data, tal_count(extra->data));
+	tal_arr_append(data, extra->data);
 	return true;
 }
 
diff --git a/common/test/run-bolt12_merkle.c b/common/test/run-bolt12_merkle.c
index 902268de..59ab359f 100644
--- a/common/test/run-bolt12_merkle.c
+++ b/common/test/run-bolt12_merkle.c
@@ -48,8 +48,7 @@ static LAST_ARG_NULL void *concat_(const void *p, ...)
 
 	va_start(ap, p);
 	do {
-		tal_resize(&ret, len + tal_bytelen(p));
-		memcpy(ret + len, p, tal_bytelen(p));
+		tal_arr_append(&ret, p);
 		len += tal_bytelen(p);
 	} while ((p = va_arg(ap, const void *)) != NULL);
 	va_end(ap);
diff --git a/common/test/run-utils-tal_arr_append.c b/common/test/run-utils-tal_arr_append.c
new file mode 100644
index 00000000..483d8b28
--- /dev/null
+++ b/common/test/run-utils-tal_arr_append.c
@@ -0,0 +1,238 @@
+/* ChatGPT-derived */
+#include "config.h"
+#include <assert.h>
+#include <ccan/array_size/array_size.h>
+#include <ccan/tal/tal.h>
+#include <common/amount.h>
+#include <common/pseudorand.h>
+#include <common/setup.h>
+#include <common/utils.h>
+#include <wire/wire.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* AUTOGENERATED MOCKS START */
+/* Generated stub for amount_asset_is_main */
+bool amount_asset_is_main(struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "amount_asset_is_main called!\n"); abort(); }
+/* Generated stub for amount_asset_to_sat */
+struct amount_sat amount_asset_to_sat(struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "amount_asset_to_sat called!\n"); abort(); }
+/* Generated stub for amount_feerate */
+ bool amount_feerate(u32 *feerate UNNEEDED, struct amount_sat fee UNNEEDED, size_t weight UNNEEDED)
+{ fprintf(stderr, "amount_feerate called!\n"); abort(); }
+/* Generated stub for amount_sat */
+struct amount_sat amount_sat(u64 satoshis UNNEEDED)
+{ fprintf(stderr, "amount_sat called!\n"); abort(); }
+/* Generated stub for amount_sat_add */
+ bool amount_sat_add(struct amount_sat *val UNNEEDED,
+				       struct amount_sat a UNNEEDED,
+				       struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_add called!\n"); abort(); }
+/* Generated stub for amount_sat_eq */
+bool amount_sat_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_eq called!\n"); abort(); }
+/* Generated stub for amount_sat_greater_eq */
+bool amount_sat_greater_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_greater_eq called!\n"); abort(); }
+/* Generated stub for amount_sat_sub */
+ bool amount_sat_sub(struct amount_sat *val UNNEEDED,
+				       struct amount_sat a UNNEEDED,
+				       struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_sub called!\n"); abort(); }
+/* Generated stub for amount_sat_to_asset */
+struct amount_asset amount_sat_to_asset(struct amount_sat *sat UNNEEDED, const u8 *asset UNNEEDED)
+{ fprintf(stderr, "amount_sat_to_asset called!\n"); abort(); }
+/* Generated stub for amount_tx_fee */
+struct amount_sat amount_tx_fee(u32 fee_per_kw UNNEEDED, size_t weight UNNEEDED)
+{ fprintf(stderr, "amount_tx_fee called!\n"); abort(); }
+/* Generated stub for fromwire */
+const u8 *fromwire(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, void *copy UNNEEDED, size_t n UNNEEDED)
+{ fprintf(stderr, "fromwire called!\n"); abort(); }
+/* Generated stub for fromwire_bool */
+bool fromwire_bool(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_bool called!\n"); abort(); }
+/* Generated stub for fromwire_fail */
+void *fromwire_fail(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_fail called!\n"); abort(); }
+/* Generated stub for fromwire_secp256k1_ecdsa_signature */
+void fromwire_secp256k1_ecdsa_signature(const u8 **cursor UNNEEDED, size_t *max UNNEEDED,
+					secp256k1_ecdsa_signature *signature UNNEEDED)
+{ fprintf(stderr, "fromwire_secp256k1_ecdsa_signature called!\n"); abort(); }
+/* Generated stub for fromwire_sha256 */
+void fromwire_sha256(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, struct sha256 *sha256 UNNEEDED)
+{ fprintf(stderr, "fromwire_sha256 called!\n"); abort(); }
+/* Generated stub for fromwire_tal_arrn */
+u8 *fromwire_tal_arrn(const tal_t *ctx UNNEEDED,
+		       const u8 **cursor UNNEEDED, size_t *max UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "fromwire_tal_arrn called!\n"); abort(); }
+/* Generated stub for fromwire_u32 */
+u32 fromwire_u32(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_u32 called!\n"); abort(); }
+/* Generated stub for fromwire_u64 */
+u64 fromwire_u64(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_u64 called!\n"); abort(); }
+/* Generated stub for fromwire_u8 */
+u8 fromwire_u8(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_u8 called!\n"); abort(); }
+/* Generated stub for fromwire_u8_array */
+void fromwire_u8_array(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, u8 *arr UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "fromwire_u8_array called!\n"); abort(); }
+/* Generated stub for siphash_seed */
+const struct siphash_seed *siphash_seed(void)
+{ fprintf(stderr, "siphash_seed called!\n"); abort(); }
+/* Generated stub for towire */
+void towire(u8 **pptr UNNEEDED, const void *data UNNEEDED, size_t len UNNEEDED)
+{ fprintf(stderr, "towire called!\n"); abort(); }
+/* Generated stub for towire_bool */
+void towire_bool(u8 **pptr UNNEEDED, bool v UNNEEDED)
+{ fprintf(stderr, "towire_bool called!\n"); abort(); }
+/* Generated stub for towire_secp256k1_ecdsa_signature */
+void towire_secp256k1_ecdsa_signature(u8 **pptr UNNEEDED,
+			      const secp256k1_ecdsa_signature *signature UNNEEDED)
+{ fprintf(stderr, "towire_secp256k1_ecdsa_signature called!\n"); abort(); }
+/* Generated stub for towire_sha256 */
+void towire_sha256(u8 **pptr UNNEEDED, const struct sha256 *sha256 UNNEEDED)
+{ fprintf(stderr, "towire_sha256 called!\n"); abort(); }
+/* Generated stub for towire_u32 */
+void towire_u32(u8 **pptr UNNEEDED, u32 v UNNEEDED)
+{ fprintf(stderr, "towire_u32 called!\n"); abort(); }
+/* Generated stub for towire_u64 */
+void towire_u64(u8 **pptr UNNEEDED, u64 v UNNEEDED)
+{ fprintf(stderr, "towire_u64 called!\n"); abort(); }
+/* Generated stub for towire_u8 */
+void towire_u8(u8 **pptr UNNEEDED, u8 v UNNEEDED)
+{ fprintf(stderr, "towire_u8 called!\n"); abort(); }
+/* Generated stub for towire_u8_array */
+void towire_u8_array(u8 **pptr UNNEEDED, const u8 *arr UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "towire_u8_array called!\n"); abort(); }
+/* AUTOGENERATED MOCKS END */
+
+static void test_append_empty(void)
+{
+	int *arr = tal_arr(NULL, int, 0);
+	int one[] = { 1, 2, 3 };
+
+	tal_arr_append(&arr, tal_dup_arr(arr, int, one, ARRAY_SIZE(one), 0));
+
+	assert(tal_count(arr) == 3);
+	assert(arr[0] == 1);
+	assert(arr[1] == 2);
+	assert(arr[2] == 3);
+
+	tal_free(arr);
+}
+
+static void test_append_nonempty(void)
+{
+	int *arr = tal_arr(NULL, int, 2);
+	arr[0] = 10;
+	arr[1] = 20;
+
+	int more[] = { 30, 40 };
+
+	tal_arr_append(&arr, tal_dup_arr(arr, int, more, ARRAY_SIZE(more), 0));
+
+	assert(tal_count(arr) == 4);
+	assert(arr[0] == 10);
+	assert(arr[1] == 20);
+	assert(arr[2] == 30);
+	assert(arr[3] == 40);
+
+	tal_free(arr);
+}
+
+static void test_appendn_partial(void)
+{
+	int *arr = tal_arr(NULL, int, 1);
+	arr[0] = 5;
+
+	int more[] = { 6, 7, 8, 9 };
+
+	tal_arr_appendn(&arr, more, 2);
+
+	assert(tal_count(arr) == 3);
+	assert(arr[0] == 5);
+	assert(arr[1] == 6);
+	assert(arr[2] == 7);
+
+	tal_free(arr);
+}
+
+static void test_appendn_zero(void)
+{
+	int *arr = tal_arr(NULL, int, 3);
+	arr[0] = 1;
+	arr[1] = 2;
+	arr[2] = 3;
+
+	int more[] = { 4, 5 };
+
+	tal_arr_appendn(&arr, more, 0);
+
+	assert(tal_count(arr) == 3);
+	assert(arr[0] == 1);
+	assert(arr[1] == 2);
+	assert(arr[2] == 3);
+
+	tal_free(arr);
+}
+
+static void test_append_structs(void)
+{
+	struct foo {
+		int a;
+		int b;
+	};
+
+	struct foo *arr = tal_arr(NULL, struct foo, 1);
+	arr[0].a = 1;
+	arr[0].b = 2;
+
+	struct foo more[] = {
+		{ 3, 4 },
+		{ 5, 6 },
+	};
+
+	tal_arr_append(&arr, tal_dup_arr(arr, struct foo, more, ARRAY_SIZE(more), 0));
+
+	assert(tal_count(arr) == 3);
+	assert(arr[0].a == 1 && arr[0].b == 2);
+	assert(arr[1].a == 3 && arr[1].b == 4);
+	assert(arr[2].a == 5 && arr[2].b == 6);
+
+	tal_free(arr);
+}
+
+static void test_append_take(void)
+{
+	int *arr = tal_arr(NULL, int, 0);
+	int more[] = { 4, 5 };
+	const int *moreptr = tal_dup_arr(NULL, int, more, ARRAY_SIZE(more), 0);
+
+	tal_arr_appendn(&arr, take(moreptr), 1);
+	moreptr = tal_dup_arr(NULL, int, more, ARRAY_SIZE(more), 0);
+	tal_arr_append(&arr, take(moreptr));
+
+	assert(tal_count(arr) == 3);
+	assert(arr[0] == 4);
+	assert(arr[1] == 4);
+	assert(arr[2] == 5);
+
+	tal_free(arr);
+}
+
+int main(int argc, char *argv[])
+{
+	common_setup(argv[0]);
+
+	test_append_empty();
+	test_append_nonempty();
+	test_appendn_partial();
+	test_appendn_zero();
+	test_append_structs();
+	test_append_take();
+
+	common_shutdown();
+}
diff --git a/common/utils.c b/common/utils.c
index 102eed7a..860cd446 100644
--- a/common/utils.c
+++ b/common/utils.c
@@ -147,6 +147,28 @@ void tal_arr_remove_(void *p, size_t elemsize, size_t n)
     tal_resize((char **)p, len - elemsize);
 }
 
+static void tal_arr_append_bytes(void *p, const void *append, size_t bytes)
+{
+	void **pptr = p;
+	size_t oldsize = tal_bytelen(*pptr);
+	tal_resize(pptr, oldsize + bytes);
+	/* Blah blah blah memcpy NULL blah blah */
+	if (append || bytes)
+		memcpy(*pptr + oldsize, memcheck(append, bytes), bytes);
+	if (taken(append))
+		tal_free(append);
+}
+
+void tal_arr_append_(void *p, const void *append TAKES)
+{
+	tal_arr_append_bytes(p, append, tal_bytelen(append));
+}
+
+void tal_arr_appendn_(void *p, const void *append TAKES, size_t bytes)
+{
+	tal_arr_append_bytes(p, append, bytes);
+}
+
 /* Check for valid UTF-8 */
 bool utf8_check(const void *vbuf, size_t buflen)
 {
diff --git a/common/utils.h b/common/utils.h
index cc378e62..7a5c98aa 100644
--- a/common/utils.h
+++ b/common/utils.h
@@ -104,6 +104,21 @@ static inline void tal_free_if_taken(const tal_t *p)
 		tal_free(p);
 }
 
+/**
+ * Append one tal array to another (TAKES)
+ */
+#define tal_arr_append(pptr, append)				\
+	tal_arr_append_((pptr) + 0*sizeof(*(pptr) == (append)), (append))
+
+/**
+ * Append an array of appendnum elements to a tal arr (TAKES)
+ */
+#define tal_arr_appendn(pptr, append, appendnum)					\
+	tal_arr_appendn_((pptr), (append), (appendnum) * sizeof(*append) + 0*sizeof(*(pptr) == (append)))
+
+void tal_arr_append_(void *p, const void *append TAKES);
+void tal_arr_appendn_(void *p, const void *append TAKES, size_t bytes);
+
 /* Check for valid UTF-8 */
 bool utf8_check(const void *buf, size_t buflen);
 
diff --git a/connectd/multiplex.c b/connectd/multiplex.c
index 46e3aeb0..59e483d9 100644
--- a/connectd/multiplex.c
+++ b/connectd/multiplex.c
@@ -376,7 +376,6 @@ static bool is_urgent(enum peer_wire type)
 static u8 *process_batch_elements(const tal_t *ctx, struct peer *peer, const u8 *msg TAKES)
 {
 	u8 *ret = tal_arr(ctx, u8, 0);
-	size_t ret_size = 0;
 	const u8 *cursor = msg;
 	size_t plen = tal_count(msg);
 
@@ -422,9 +421,7 @@ static u8 *process_batch_elements(const tal_t *ctx, struct peer *peer, const u8
 		enc_msg = cryptomsg_encrypt_msg(tmpctx, &peer->cs,
 						take(element_bytes));
 
-		tal_resize(&ret, ret_size + tal_bytelen(enc_msg));
-		memcpy(&ret[ret_size], enc_msg, tal_bytelen(enc_msg));
-		ret_size += tal_bytelen(enc_msg);
+		tal_arr_append(&ret, enc_msg);
 
 	} while(plen);
 
diff --git a/connectd/test/run-websocket.c b/connectd/test/run-websocket.c
index d31e889b..2d133f06 100644
--- a/connectd/test/run-websocket.c
+++ b/connectd/test/run-websocket.c
@@ -2,6 +2,7 @@
 #include <assert.h>
 #include <common/amount.h>
 #include <common/memleak.h>
+#include <common/utils.h>
 #include <ccan/io/io.h>
 #include <ccan/read_write_all/read_write_all.h>
 #include <wire/wire.h>
@@ -31,9 +32,7 @@ static bool my_read_all(int fd, void *buf, size_t count)
 
 static ssize_t my_write(int fd, const void *buf, size_t count)
 {
-	size_t buflen = tal_bytelen(my_wbuf);
-	tal_resize(&my_wbuf, buflen + count);
-	memcpy(my_wbuf + buflen, buf, count);
+	tal_arr_appendn(&my_wbuf, buf, count);
 	return count;
 }
 static bool my_write_all(int fd, const void *buf, size_t count)
diff --git a/lightningd/plugin.c b/lightningd/plugin.c
index a9098ad0..ca4a2f64 100644
--- a/lightningd/plugin.c
+++ b/lightningd/plugin.c
@@ -190,23 +190,15 @@ struct command_result *plugin_register_all_complete(struct lightningd *ld,
 static void tell_connectd_custommsgs(struct plugins *plugins)
 {
 	struct plugin *p;
-	size_t n = 0;
-	u16 *all_msgs = tal_arr(tmpctx, u16, n);
+	u16 *all_msgs = tal_arr(tmpctx, u16, 0);
 
 	/* Not when shutting down */
 	if (!plugins->ld->connectd)
 		return;
 
 	/* Gather from all plugins. */
-	list_for_each(&plugins->plugins, p, list) {
-		size_t num = tal_count(p->custom_msgs);
-		/* Blah blah blah memcpy NULL blah blah */
-		if (num == 0)
-			continue;
-		tal_resize(&all_msgs, n + num);
-		memcpy(all_msgs + n, p->custom_msgs, num * sizeof(*p->custom_msgs));
-		n += num;
-	}
+	list_for_each(&plugins->plugins, p, list)
+		tal_arr_append(&all_msgs, p->custom_msgs);
 
 	/* Don't bother sorting or uniquifying.  If plugins are dumb, they deserve it. */
 	subd_send_msg(plugins->ld->connectd,
diff --git a/plugins/commando.c b/plugins/commando.c
index d0d2e91c..a256f0b0 100644
--- a/plugins/commando.c
+++ b/plugins/commando.c
@@ -102,8 +102,7 @@ static void append_contents(struct commando *commando, const u8 *msg, size_t msg
 		return;
 	}
 
-	tal_resize(&commando->contents, len + msglen);
-	memcpy(commando->contents + len, msg, msglen);
+	tal_arr_appendn(&commando->contents, msg, msglen);
 }
 
 struct reply {
diff --git a/tools/lightning-downgrade.c b/tools/lightning-downgrade.c
index 70a8f220..a1fc9beb 100644
--- a/tools/lightning-downgrade.c
+++ b/tools/lightning-downgrade.c
@@ -41,10 +41,7 @@ struct layer {
 
 static void copy_data(u8 **out, const u8 *in, size_t len)
 {
-	size_t oldlen = tal_bytelen(*out);
-
-	tal_resize(out, oldlen + len);
-	memcpy(*out + oldlen, in, len);
+	tal_arr_appendn(out, in, len);
 }
 
 /* askrene added DSTORE_CHANNEL_BIAS_V2 (convertable) and