core: Add RootDirectory= path to error message if directory does not exist

A colleague reported when RootDirectory= does not exist, systemd reports an error like:
```
Failed to set up mount namespacing: No such file or directory
```

Unfortunately, with large spec files, it can be hard to diagnose which path systemd is talking
about. Thus, to make the error message more helpful and similar to mount error messages, we add
the root directory/image path into the error message like:
```
Failed to set up mount namespacing: /tmp/thisdoesnotexist: No such file or directory
```

1 files changed, 26 insertions, 10 deletions

diff --git a/src/core/exec-invoke.c b/src/core/exec-invoke.c
index 5dfbafb282..c71d6b4c2a 100644
--- a/src/core/exec-invoke.c
+++ b/src/core/exec-invoke.c
@@ -2845,7 +2845,8 @@ static int setup_ephemeral(
                 const ExecContext *context,
                 ExecRuntime *runtime,
                 char **root_image,            /* both input and output! modified if ephemeral logic enabled */
-                char **root_directory) {      /* ditto */
+                char **root_directory,        /* ditto */
+                char **reterr_path) {
 
         _cleanup_close_ int fd = -EBADF;
         _cleanup_free_ char *new_root = NULL;
@@ -2886,9 +2887,11 @@ static int setup_ephemeral(
 
                 fd = copy_file(*root_image, new_root, O_EXCL, 0600,
                                COPY_LOCK_BSD|COPY_REFLINK|COPY_CRTIME|COPY_NOCOW_AFTER);
-                if (fd < 0)
+                if (fd < 0) {
+                        *reterr_path = strdup(*root_image);
                         return log_debug_errno(fd, "Failed to copy image %s to %s: %m",
                                                *root_image, new_root);
+                }
         } else {
                 assert(*root_directory);
 
@@ -2901,9 +2904,11 @@ static int setup_ephemeral(
                                 BTRFS_SNAPSHOT_FALLBACK_DIRECTORY |
                                 BTRFS_SNAPSHOT_RECURSIVE |
                                 BTRFS_SNAPSHOT_LOCK_BSD);
-                if (fd < 0)
+                if (fd < 0) {
+                        *reterr_path = strdup(*root_directory);
                         return log_debug_errno(fd, "Failed to snapshot directory %s to %s: %m",
                                                *root_directory, new_root);
+                }
         }
 
         r = send_one_fd(runtime->ephemeral_storage_socket[1], fd, MSG_DONTWAIT);
@@ -2980,7 +2985,8 @@ static int pick_versions(
                 const ExecContext *context,
                 const ExecParameters *params,
                 char **ret_root_image,
-                char **ret_root_directory) {
+                char **ret_root_directory,
+                char **reterr_path) {
 
         int r;
 
@@ -2998,11 +3004,15 @@ static int pick_versions(
                               &pick_filter_image_raw,
                               PICK_ARCHITECTURE|PICK_TRIES|PICK_RESOLVE,
                               &result);
-                if (r < 0)
+                if (r < 0) {
+                        *reterr_path = strdup(context->root_image);
                         return r;
+                }
 
-                if (!result.path)
+                if (!result.path) {
+                        *reterr_path = strdup(context->root_image);
                         return log_exec_debug_errno(context, params, SYNTHETIC_ERRNO(ENOENT), "No matching entry in .v/ directory %s found.", context->root_image);
+                }
 
                 *ret_root_image = TAKE_PTR(result.path);
                 *ret_root_directory = NULL;
@@ -3018,11 +3028,15 @@ static int pick_versions(
                               &pick_filter_image_dir,
                               PICK_ARCHITECTURE|PICK_TRIES|PICK_RESOLVE,
                               &result);
-                if (r < 0)
+                if (r < 0) {
+                        *reterr_path = strdup(context->root_directory);
                         return r;
+                }
 
-                if (!result.path)
+                if (!result.path) {
+                        *reterr_path = strdup(context->root_directory);
                         return log_exec_debug_errno(context, params, SYNTHETIC_ERRNO(ENOENT), "No matching entry in .v/ directory %s found.", context->root_directory);
+                }
 
                 *ret_root_image = NULL;
                 *ret_root_directory = TAKE_PTR(result.path);
@@ -3063,7 +3077,8 @@ static int apply_mount_namespace(
                                 context,
                                 params,
                                 &root_image,
-                                &root_dir);
+                                &root_dir,
+                                reterr_path);
                 if (r < 0)
                         return r;
 
@@ -3071,7 +3086,8 @@ static int apply_mount_namespace(
                                 context,
                                 runtime,
                                 &root_image,
-                                &root_dir);
+                                &root_dir,
+                                reterr_path);
                 if (r < 0)
                         return r;
         }