fstab-generator: add x-systemd.requires and x-systemd.requires-mounts-for

Currently we have no way how to specify dependencies between fstab
entries (or another units) in the /etc/fstab. It means that users are
forced to bypass fstab and write .mount units manually.

The patch introduces new systemd fstab options:

x-systemd.requires=<PATH>

 - to specify dependence an another mount (PATH is translated to unit name)

x-systemd.requires=<UNIT>

 - to specify dependence on arbitrary UNIT

x-systemd.requires-mounts-for=<PATH ...>

 - to specify dependence on another paths, implemented by
   RequiresMountsFor=. The option may be specified more than once.

For example two bind mounts where B depends on A:

 /mnt/test/A    /mnt/test/A     none    bind,defaults
 /mnt/test/A    /mnt/test/B     none    bind,x-systemd.requires=/mnt/test/A

More complex example with overlay FS where one mount point depends on
"low" and "upper" directories:

 /dev/sdc1   /mnt/low    ext4     defaults
 /dev/sdc2   /mnt/high   ext4     defaults
 overlay     /mnt/merged overlay  lowerdir=/mnt/low,upperdir=/mnt/high/data,workdir=/mnt/high/work,x-systemd.requires-mounts-for=/mnt/low,x-systemd.requires-mounts-for=mnt/high

https://bugzilla.redhat.com/show_bug.cgi?id=812826
https://bugzilla.redhat.com/show_bug.cgi?id=1164334

src/fstab-generator/fstab-generator.c

@@ -177,6 +177,65 @@ static int write_idle_timeout(FILE *f, const char *where, const char *opts) {
         return 0;
 }
 
+static int write_requires_after(FILE *f, const char *opts) {
+        _cleanup_strv_free_ char **names = NULL, **units = NULL;
+        _cleanup_free_ char *res = NULL;
+        char **s;
+        int r;
+
+        assert(f);
+        assert(opts);
+
+        r = fstab_extract_values(opts, "x-systemd.requires", &names);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to parse options: %m");
+        if (r == 0)
+                return 0;
+
+        STRV_FOREACH(s, names) {
+                char *x;
+
+                r = unit_name_mangle_with_suffix(*s, UNIT_NAME_NOGLOB, ".mount", &x);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to generate unit name: %m");
+                r = strv_consume(&units, x);
+                if (r < 0)
+                        return log_oom();
+        }
+
+        if (units) {
+                res = strv_join(units, " ");
+                if (!res)
+                        return log_oom();
+                fprintf(f, "After=%1$s\nRequires=%1$s\n", res);
+        }
+
+        return 0;
+}
+
+static int write_requires_mounts_for(FILE *f, const char *opts) {
+        _cleanup_strv_free_ char **paths = NULL;
+        _cleanup_free_ char *res = NULL;
+        int r;
+
+        assert(f);
+        assert(opts);
+
+        r = fstab_extract_values(opts, "x-systemd.requires-mounts-for", &paths);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to parse options: %m");
+        if (r == 0)
+                return 0;
+
+        res = strv_join(paths, " ");
+        if (!res)
+                return log_oom();
+
+        fprintf(f, "RequiresMountsFor=%s\n", res);
+
+        return 0;
+}
+
 static int add_mount(
                 const char *what,
                 const char *where,
@@ -251,6 +310,15 @@ static int add_mount(
         if (post && !noauto && !nofail && !automount)
                 fprintf(f, "Before=%s\n", post);
 
+        if (!automount && opts) {
+                 r = write_requires_after(f, opts);
+                 if (r < 0)
+                         return r;
+                 r = write_requires_mounts_for(f, opts);
+                 if (r < 0)
+                         return r;
+        }
+
         if (passno != 0) {
                 r = generator_write_fsck_deps(f, arg_dest, what, where, fstype);
                 if (r < 0)
@@ -315,6 +383,15 @@ static int add_mount(
                                 "Before=%s\n",
                                 post);
 
+                if (opts) {
+                        r = write_requires_after(f, opts);
+                        if (r < 0)
+                                return r;
+                        r = write_requires_mounts_for(f, opts);
+                        if (r < 0)
+                                return r;
+                }
+
                 fprintf(f,
                         "[Automount]\n"
                         "Where=%s\n",

src/shared/fstab-util.c

@@ -125,6 +125,36 @@ answer:
         return !!n;
 }
 
+int fstab_extract_values(const char *opts, const char *name, char ***values) {
+        _cleanup_strv_free_ char **optsv = NULL, **res = NULL;
+        char **s;
+
+        assert(opts);
+        assert(name);
+        assert(values);
+
+        optsv = strv_split(opts, ",");
+        if (!optsv)
+                return -ENOMEM;
+
+        STRV_FOREACH(s, optsv) {
+                char *arg;
+                int r;
+
+                arg = startswith(*s, name);
+                if (!arg || *arg != '=')
+                        continue;
+                r = strv_extend(&res, arg + 1);
+                if (r < 0)
+                        return r;
+        }
+
+        *values = res;
+        res = NULL;
+
+        return !!*values;
+}
+
 int fstab_find_pri(const char *options, int *ret) {
         _cleanup_free_ char *opt = NULL;
         int r;

src/shared/fstab-util.h

@@ -28,6 +28,8 @@
 int fstab_filter_options(const char *opts, const char *names,
                          const char **namefound, char **value, char **filtered);
 
+int fstab_extract_values(const char *opts, const char *name, char ***values);
+
 static inline bool fstab_test_option(const char *opts, const char *names) {
         return !!fstab_filter_options(opts, names, NULL, NULL, NULL);
 }