nspawn: Add try-{host,guest} journal link modes

--link-journal={host,guest} fail if the host does not have persistent
journalling enabled and /var/log/journal/ does not exist. Even worse, as there
is no stdout/err any more, there is no error message to point that out.

Introduce two new modes "try-host" and "try-guest" which don't fail in this
case, and instead just silently skip the guest journal setup.

Change -j to mean "try-guest" instead of "guest", and fix the wrong --help
output for it (it said "host" before).

Change systemd-nspawn@.service.in to use "try-guest" so that this unit works
with both persistent and non-persistent journals on the host without failing.

https://bugs.debian.org/770275

3 files changed, 38 insertions, 12 deletions

diff --git a/man/systemd-nspawn.xml b/man/systemd-nspawn.xml
index b3a2d32855..75db65eac0 100644
--- a/man/systemd-nspawn.xml
+++ b/man/systemd-nspawn.xml
@@ -439,7 +439,9 @@
                                 versa). Takes one of
                                 <literal>no</literal>,
                                 <literal>host</literal>,
+                                <literal>try-host</literal>,
                                 <literal>guest</literal>,
+                                <literal>try-guest</literal>,
                                 <literal>auto</literal>. If
                                 <literal>no</literal>, the journal is
                                 not linked. If <literal>host</literal>,
@@ -453,8 +455,11 @@
                                 guest file system (beneath
                                 <filename>/var/log/journal/<replaceable>machine-id</replaceable></filename>)
                                 and the subdirectory is symlinked into the host
-                                at the same location. If
-                                <literal>auto</literal> (the default),
+                                at the same location. <literal>try-host</literal>
+                                and <literal>try-guest</literal> do the same
+                                but do not fail if the host does not have
+                                persistant journalling enabled.
+                                If <literal>auto</literal> (the default),
                                 and the right subdirectory of
                                 <filename>/var/log/journal</filename>
                                 exists, it will be bind mounted
@@ -473,7 +478,7 @@
                                 <term><option>-j</option></term>
 
                                 <listitem><para>Equivalent to
-                                <option>--link-journal=guest</option>.</para></listitem>
+                                <option>--link-journal=try-guest</option>.</para></listitem>
                         </varlistentry>
 
                         <varlistentry>
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index c2311b36d5..b4dcf39e83 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -124,6 +124,7 @@ static bool arg_private_network = false;
 static bool arg_read_only = false;
 static bool arg_boot = false;
 static LinkJournal arg_link_journal = LINK_AUTO;
+static bool arg_link_journal_try = false;
 static uint64_t arg_retain =
         (1ULL << CAP_CHOWN) |
         (1ULL << CAP_DAC_OVERRIDE) |
@@ -202,8 +203,9 @@ static void help(void) {
                "     --capability=CAP       In addition to the default, retain specified\n"
                "                            capability\n"
                "     --drop-capability=CAP  Drop the specified capability from the default set\n"
-               "     --link-journal=MODE    Link up guest journal, one of no, auto, guest, host\n"
-               "  -j                        Equivalent to --link-journal=host\n"
+               "     --link-journal=MODE    Link up guest journal, one of no, auto, guest, host,\n"
+               "                            try-guest, try-host\n"
+               "  -j                        Equivalent to --link-journal=try-guest\n"
                "     --read-only            Mount the root directory read-only\n"
                "     --bind=PATH[:PATH]     Bind mount a file or directory from the host into\n"
                "                            the container\n"
@@ -428,6 +430,7 @@ static int parse_argv(int argc, char *argv[]) {
 
                 case 'j':
                         arg_link_journal = LINK_GUEST;
+                        arg_link_journal_try = true;
                         break;
 
                 case ARG_LINK_JOURNAL:
@@ -439,7 +442,13 @@ static int parse_argv(int argc, char *argv[]) {
                                 arg_link_journal = LINK_GUEST;
                         else if (streq(optarg, "host"))
                                 arg_link_journal = LINK_HOST;
-                        else {
+                        else if (streq(optarg, "try-guest")) {
+                                arg_link_journal = LINK_GUEST;
+                                arg_link_journal_try = true;
+                        } else if (streq(optarg, "try-host")) {
+                                arg_link_journal = LINK_HOST;
+                                arg_link_journal_try = true;
+                        } else {
                                 log_error("Failed to parse link journal mode %s", optarg);
                                 return -EINVAL;
                         }
@@ -1404,8 +1413,13 @@ static int setup_journal(const char *directory) {
         if (arg_link_journal == LINK_GUEST) {
 
                 if (symlink(q, p) < 0) {
-                        log_error("Failed to symlink %s to %s: %m", q, p);
-                        return -errno;
+                        if (arg_link_journal_try) {
+                                log_debug("Failed to symlink %s to %s, skipping journal setup: %m", q, p);
+                                return 0;
+                        } else {
+                                log_error("Failed to symlink %s to %s: %m", q, p);
+                                return -errno;
+                        }
                 }
 
                 r = mkdir_p(q, 0755);
@@ -1415,10 +1429,17 @@ static int setup_journal(const char *directory) {
         }
 
         if (arg_link_journal == LINK_HOST) {
-                r = mkdir_p(p, 0755);
+                /* don't create parents here -- if the host doesn't have
+                 * permanent journal set up, don't force it here */
+                r = mkdir(p, 0755);
                 if (r < 0) {
-                        log_error("Failed to create %s: %m", p);
-                        return r;
+                        if (arg_link_journal_try) {
+                                log_debug("Failed to create %s, skipping journal setup: %m", p);
+                                return 0;
+                        } else {
+                                log_error("Failed to create %s: %m", p);
+                                return r;
+                        }
                 }
 
         } else if (access(p, F_OK) < 0)
diff --git a/units/systemd-nspawn@.service.in b/units/systemd-nspawn@.service.in
index dec2ce7df7..e3eaa53239 100644
--- a/units/systemd-nspawn@.service.in
+++ b/units/systemd-nspawn@.service.in
@@ -10,7 +10,7 @@ Description=Container %i
 Documentation=man:systemd-nspawn(1)
 
 [Service]
-ExecStart=@bindir@/systemd-nspawn --quiet --keep-unit --boot --link-journal=guest --directory=/var/lib/container/%i
+ExecStart=@bindir@/systemd-nspawn --quiet --keep-unit --boot --link-journal=try-guest --directory=/var/lib/container/%i
 KillMode=mixed
 Type=notify
 RestartForceExitStatus=133