xattr-util: merge various getxattr()/listxattr() helpers into getxattr_at_malloc() + listxattr_at_malloc()

Unfortunately fgetxattr() and flistxattr() don't work via O_PATH fds.
Let's thus add fallbacks to go via /proc/self/fd/ in these cases.

Also, let's merge all the various flavours we have here into singular
implementations that can do everything we need:

1. malloc() loop handling
2. by fd, by path, or combination (i.e. a proper openat() like API)
3. work on O_PATH

src/test/test-copy.c

@@ -149,7 +149,7 @@ static void test_copy_tree(void) {
                 assert_se(read_full_file(f, &buf, &sz) == 0);
                 assert_se(streq(buf, "file\n"));
 
-                k = getxattr_malloc(f, "user.testxattr", &c, false);
+                k = lgetxattr_malloc(f, "user.testxattr", &c);
                 assert_se(xattr_worked < 0 || ((k >= 0) == !!xattr_worked));
 
                 if (k >= 0) {

src/test/test-xattr-util.c

@@ -15,14 +15,12 @@
 #include "tmpfile-util.h"
 #include "xattr-util.h"
 
-static void test_fgetxattrat_fake(void) {
+static void test_getxattr_at_malloc(void) {
         char t[] = "/var/tmp/xattrtestXXXXXX";
         _cleanup_free_ char *value = NULL;
         _cleanup_close_ int fd = -1;
         const char *x;
-        char v[3];
         int r;
-        size_t size;
 
         log_info("/* %s */", __func__);
 
@@ -38,21 +36,24 @@ static void test_fgetxattrat_fake(void) {
         fd = open(t, O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY);
         assert_se(fd >= 0);
 
-        assert_se(fgetxattrat_fake(fd, "test", "user.foo", v, 3, 0, &size) >= 0);
-        assert_se(size == 3);
-        assert_se(memcmp(v, "bar", 3) == 0);
+        assert_se(getxattr_at_malloc(fd, "test", "user.foo", 0, &value) == 3);
+        assert_se(memcmp(value, "bar", 3) == 0);
+        value = mfree(value);
+
+        assert_se(getxattr_at_malloc(AT_FDCWD, x, "user.foo", 0, &value) == 3);
+        assert_se(memcmp(value, "bar", 3) == 0);
+        value = mfree(value);
 
         safe_close(fd);
         fd = open("/", O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY);
         assert_se(fd >= 0);
-        r = fgetxattrat_fake(fd, "usr", "user.idontexist", v, 3, 0, &size);
+        r = getxattr_at_malloc(fd, "usr", "user.idontexist", 0, &value);
         assert_se(r == -ENODATA || ERRNO_IS_NOT_SUPPORTED(r));
 
         safe_close(fd);
         fd = open(x, O_PATH|O_CLOEXEC);
         assert_se(fd >= 0);
-        r = fgetxattrat_fake_malloc(fd, NULL, "user.foo", AT_EMPTY_PATH, &value);
-        assert_se(r == 3);
+        assert_se(getxattr_at_malloc(fd, NULL, "user.foo", 0, &value) == 3);
         assert_se(streq(value, "bar"));
 
 cleanup:
@@ -68,7 +69,7 @@ static void test_getcrtime(void) {
 
         log_info("/* %s */", __func__);
 
-        assert_se(tmp_dir(&vt) >= 0);
+        assert_se(var_tmp_dir(&vt) >= 0);
 
         fd = open_tmpfile_unlinkable(vt, O_RDWR);
         assert_se(fd >= 0);
@@ -92,7 +93,7 @@ static void test_getcrtime(void) {
 int main(void) {
         test_setup_logging(LOG_DEBUG);
 
-        test_fgetxattrat_fake();
+        test_getxattr_at_malloc();
         test_getcrtime();
 
         return 0;