iovec-util: add exported constant empty but valid (i.e. non-NULL) iovec

Also, make sure the NUL byte iovec becomes an exported constant too.

This is better than the previous situation where this was a macro
resolving to a compount expression, since the lifetime of the expression
is limited to its invoking scope. By turning this into a proper variable
the lifetime becomes unbounded, which makes it easier to use in various
scenarios, such as "if" blocks.

src/basic/iovec-util.c

@@ -3,6 +3,18 @@
 #include "iovec-util.h"
 #include "string-util.h"
 
+static const uint8_t nul_byte = 0;
+
+const struct iovec iovec_nul_byte = {
+        .iov_base = (void*) &nul_byte,
+        .iov_len = 1,
+};
+
+const struct iovec iovec_empty = {
+        .iov_base = (void*) &nul_byte,
+        .iov_len = 0,
+};
+
 size_t iovec_total_size(const struct iovec *iovec, size_t n) {
         size_t sum = 0;
 

src/basic/iovec-util.h

@@ -8,11 +8,8 @@
 #include "alloc-util.h"
 #include "macro.h"
 
-/* An iovec pointing to a single NUL byte */
-#define IOVEC_NUL_BYTE (const struct iovec) {                   \
-                .iov_base = (void*) (const uint8_t[1]) { 0 },   \
-                .iov_len = 1,                                   \
-        }
+extern const struct iovec iovec_nul_byte; /* Points to a single NUL byte */
+extern const struct iovec iovec_empty;    /* Points to an empty, but valid (i.e. non-NULL) pointer */
 
 size_t iovec_total_size(const struct iovec *iovec, size_t n);
 

src/ssh-generator/ssh-proxy.c

@@ -41,7 +41,7 @@ static int process_vsock(const char *host, const char *port) {
                 return log_error_errno(errno, "Failed to connect to vsock:%u:%u: %m", sa.vm.svm_cid, sa.vm.svm_port);
 
         /* OpenSSH wants us to send a single byte along with the file descriptor, hence do so */
-        r = send_one_fd_iov(STDOUT_FILENO, fd, &IOVEC_NUL_BYTE, /* n_iovec= */ 1, /* flags= */ 0);
+        r = send_one_fd_iov(STDOUT_FILENO, fd, &iovec_nul_byte, /* n_iovec= */ 1, /* flags= */ 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to send socket via STDOUT: %m");
 
@@ -72,7 +72,7 @@ static int process_unix(const char *path) {
         if (r < 0)
                 return log_error_errno(r, "Failed to connect to AF_UNIX socket %s: %m", path);
 
-        r = send_one_fd_iov(STDOUT_FILENO, fd, &IOVEC_NUL_BYTE, /* n_iovec= */ 1, /* flags= */ 0);
+        r = send_one_fd_iov(STDOUT_FILENO, fd, &iovec_nul_byte, /* n_iovec= */ 1, /* flags= */ 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to send socket via STDOUT: %m");
 
@@ -116,7 +116,7 @@ static int process_vsock_mux(const char *path, const char *port) {
         if (r < 0)
                 return log_error_errno(r, "Failed to send CONNECT to %s:%s: %m", path, port);
 
-        r = send_one_fd_iov(STDOUT_FILENO, fd, &IOVEC_NUL_BYTE, /* n_iovec= */ 1, /* flags= */ 0);
+        r = send_one_fd_iov(STDOUT_FILENO, fd, &iovec_nul_byte, /* n_iovec= */ 1, /* flags= */ 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to send socket via STDOUT: %m");
 

src/test/test-socket-util.c

@@ -453,13 +453,10 @@ TEST(send_emptydata) {
 
         if (r == 0) {
                 /* Child */
-                struct iovec iov = IOVEC_MAKE_STRING("");  /* zero-length iov */
-                assert_se(iov.iov_len == 0);
-
                 pair[0] = safe_close(pair[0]);
 
                 /* This will succeed, since iov is set. */
-                assert_se(send_one_fd_iov(pair[1], -1, &iov, 1, MSG_DONTWAIT) == 0);
+                assert_se(send_one_fd_iov(pair[1], -1, &iovec_empty, 1, MSG_DONTWAIT) == 0);
                 _exit(EXIT_SUCCESS);
         }