analyze: add 'cat-config' verb

This is used as 'systemd-analyze show-config systemd/logind.conf', which
will dump
   /etc/systemd/system/user@.service
   /etc/systemd/system/user@.service.d/*.conf
   /run/systemd/system/user@.service.d/*.conf
   /usr/local/lib/systemd/system/user@.service.d/*.conf
   /usr/lib/systemd/system/user@.service.d/*.conf

The idea is to make it easy to dump the configuration using the same locations
and order that systemd programs use themselves (including masking, in the right
order, etc.). This is the generic variant that works with any configuration
scheme that follows the same general rules:

$ systemd-analyze cat-config systemd/system.conf
$ systemd-analyze cat-config systemd/user.conf
$ systemd-analyze cat-config systemd/logind.conf
$ systemd-analyze cat-config systemd/sleep.conf
$ systemd-analyze cat-config systemd/journald.conf
$ systemd-analyze cat-config systemd/journal-remote.conf
$ systemd-analyze cat-config systemd/journal-upload.conf
$ systemd-analyze cat-config systemd/coredump.conf
$ systemd-analyze cat-config systemd/resolved.conf
$ systemd-analyze cat-config systemd/timesyncd.conf
$ systemd-analyze cat-config udev/udev.conf

1 files changed, 7 insertions, 2 deletions

diff --git a/src/basic/terminal-util.c b/src/basic/terminal-util.c
index 5624c03bce..c2b7cd799e 100644
--- a/src/basic/terminal-util.c
+++ b/src/basic/terminal-util.c
@@ -1381,13 +1381,18 @@ static int cat_file(const char *filename, bool newline) {
         return copy_bytes(fd, STDOUT_FILENO, (uint64_t) -1, 0);
 }
 
-int cat_files(const char *file, char **dropins) {
+int cat_files(const char *file, char **dropins, CatFlags flags) {
         char **path;
         int r;
 
         if (file) {
                 r = cat_file(file, false);
-                if (r < 0)
+                if (r == -ENOENT && (flags & CAT_FLAGS_MAIN_FILE_OPTIONAL))
+                        printf("%s# config file %s not found%s\n",
+                               ansi_highlight_magenta(),
+                               file,
+                               ansi_normal());
+                else if (r < 0)
                         return log_warning_errno(r, "Failed to cat %s: %m", file);
         }