hexdump: add hexdump_iovw_compare()

Add a helper dumping the differing elements of two iovec wrappers,
element by element, reusing hexdump_memcmp(). Meant for assertion
failure messages.

src/shared/hexdump.c

@@ -7,6 +7,8 @@
 
 #include "alloc-util.h"
 #include "hexdump.h"
+#include "iovec-util.h"
+#include "iovec-wrapper.h"
 #include "memstream-util.h"
 
 void hexdump(FILE *f, const void *p, size_t s) {
@@ -148,3 +150,34 @@ char* hexdump_memcmp_byte(uint8_t byte, const void *data, size_t length) {
 
         return hexdump_memcmp(data, length, expected, length);
 }
+
+char* hexdump_iovw_compare(const struct iovec_wrapper *a, const struct iovec_wrapper *b) {
+        _cleanup_(memstream_done) MemStream m = {};
+        size_t na = a ? a->count : 0, nb = b ? b->count : 0, mx = MAX(na, nb);
+        FILE *f;
+
+        /* Dumps the elements of two iovec wrappers that differ, element by element, reusing
+         * hexdump_memcmp(). Used by ASSERT_IOVW_EQ(). */
+
+        f = memstream_init(&m);
+        if (!f)
+                return NULL;
+
+        for (size_t i = 0; i < mx; i++) {
+                const struct iovec *ia = i < na ? a->iovec + i : NULL;
+                const struct iovec *ib = i < nb ? b->iovec + i : NULL;
+
+                if (ia && ib && iovec_memcmp(ia, ib) == 0)
+                        continue;
+
+                _cleanup_free_ char *d = hexdump_memcmp(ia ? ia->iov_base : NULL, ia ? ia->iov_len : 0,
+                                                        ib ? ib->iov_base : NULL, ib ? ib->iov_len : 0);
+                fprintf(f, "element %zu differs:\n%s", i, d ?: "");
+        }
+
+        char *ret;
+        if (memstream_finalize(&m, &ret, NULL) < 0)
+                return NULL;
+
+        return ret;
+}

src/shared/hexdump.h

@@ -11,3 +11,6 @@ char* hexdump_memcmp(const void *m1, size_t l1, const void *m2, size_t l2);
 
 /* Like hexdump_memcmp(), but compares against a region of the same length filled with 'byte'. */
 char* hexdump_memcmp_byte(uint8_t byte, const void *data, size_t length);
+
+/* Dumps the differing elements of two iovec wrappers, element by element. Used by ASSERT_IOVW_EQ(). */
+char* hexdump_iovw_compare(const struct iovec_wrapper *a, const struct iovec_wrapper *b);

src/test/test-hexdump.c

@@ -2,6 +2,8 @@
 
 #include "alloc-util.h"
 #include "hexdump.h"
+#include "iovec-util.h"
+#include "iovec-wrapper.h"
 #include "tests.h"
 
 TEST(hexdump) {
@@ -68,4 +70,26 @@ TEST(hexdump_memcmp_byte) {
         ASSERT_STREQ(s, "");
 }
 
+TEST(hexdump_iovw_compare) {
+        struct iovec iov_a[] = { IOVEC_MAKE_STRING("foo"), IOVEC_MAKE_STRING("bar") };
+        struct iovec iov_b[] = { IOVEC_MAKE_STRING("foo"), IOVEC_MAKE_STRING("baz") };
+        struct iovec_wrapper wa = { .iovec = iov_a, .count = ELEMENTSOF(iov_a) };
+        struct iovec_wrapper wb = { .iovec = iov_b, .count = ELEMENTSOF(iov_b) };
+
+        /* Equal wrappers: empty dump. */
+        _cleanup_free_ char *s = ASSERT_NOT_NULL(hexdump_iovw_compare(&wa, &wa));
+        ASSERT_STREQ(s, "");
+
+        /* One element differs. */
+        free(s);
+        s = ASSERT_NOT_NULL(hexdump_iovw_compare(&wa, &wb));
+        log_info("differ:\n%s", s);
+
+        /* Different element count. */
+        struct iovec_wrapper wc = { .iovec = iov_a, .count = 1 };
+        free(s);
+        s = ASSERT_NOT_NULL(hexdump_iovw_compare(&wa, &wc));
+        log_info("count differ:\n%s", s);
+}
+
 DEFINE_TEST_MAIN(LOG_INFO);