path-util: unify code for detecting OS trees

This also makes sure we always detect an OS tree the same way, by
checking for /etc/os-release.

src/shared/path-util.c

@@ -413,3 +413,15 @@ int path_is_read_only_fs(const char *path) {
 
         return !!(st.f_flag & ST_RDONLY);
 }
+
+int path_is_os_tree(const char *path) {
+        char *p;
+        int r;
+
+        /* We use /etc/os-release as flag file if something is an OS */
+
+        p = strappenda(path, "/etc/os-release");
+        r = access(p, F_OK);
+
+        return r < 0 ? 0 : 1;
+}

src/shared/path-util.h

@@ -40,3 +40,4 @@ char **path_strv_canonicalize_uniq(char **l);
 
 int path_is_mount_point(const char *path, bool allow_symlink);
 int path_is_read_only_fs(const char *path);
+int path_is_os_tree(const char *path);

src/shared/util.h

@@ -684,3 +684,15 @@ int unlink_noerrno(const char *path);
                 _new_ = alloca(_len_);                  \
                 (void *) memset(_new_, 0, _len_);       \
         })
+
+#define strappenda(a, b)                                \
+        ({                                              \
+                const char *_a_ = (a), *_b_ = (b);      \
+                char *_c_;                              \
+                size_t _x_, _y_;                        \
+                _x_ = strlen(_a_);                      \
+                _y_ = strlen(_b_);                      \
+                _c_ = alloca(_x_ + _y_ + 1);            \
+                strcpy(stpcpy(_c_, _a_), _b_);          \
+                _c_;                                    \
+        })