core/path: install inotify watches top-down instead of bottom-up

When watches are installed from the bottom, it is always possible
to race, and miss a file creation event. The race can be avoided
if a watch is first established for a parent directory, and then for
the file in the directory. If the file is created in the time between,
the watch on the parent directory will fire.

Some messages (mostly at debug level) are added to help diagnose
pidfile issues.

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=917075.

1 files changed, 48 insertions, 30 deletions

diff --git a/src/core/path.c b/src/core/path.c
index fc101280a1..01a2b0810e 100644
--- a/src/core/path.c
+++ b/src/core/path.c
@@ -53,8 +53,8 @@ int path_spec_watch(PathSpec *s, Unit *u) {
         };
 
         bool exists = false;
-        char _cleanup_free_ *k = NULL;
-        char *slash;
+        char _cleanup_free_ *path = NULL;
+        char *slash, *oldslash = NULL;
         int r;
 
         assert(u);
@@ -62,8 +62,8 @@ int path_spec_watch(PathSpec *s, Unit *u) {
 
         path_spec_unwatch(s, u);
 
-        k = strdup(s->path);
-        if (!k)
+        path = strdup(s->path);
+        if (!path)
                 return -ENOMEM;
 
         s->inotify_fd = inotify_init1(IN_NONBLOCK|IN_CLOEXEC);
@@ -76,43 +76,61 @@ int path_spec_watch(PathSpec *s, Unit *u) {
         if (r < 0)
                 goto fail;
 
-        s->primary_wd = inotify_add_watch(s->inotify_fd, k, flags_table[s->type]);
-        if (s->primary_wd >= 0)
-                exists = true;
-        else if (errno != EACCES && errno != ENOENT) {
-                log_error("Failed to add watch on %s: %m", k);
-                r = -errno;
-                goto fail;
-        }
+        /* This assumes the path was passed through path_kill_slashes()! */
 
-        do {
+        for(slash = strchr(path, '/'); ; slash = strchr(slash+1, '/')) {
                 int flags;
+                char tmp;
+
+                if (slash) {
+                        tmp = slash[slash == path];
+                        slash[slash == path] = '\0';
+                        flags = IN_MOVE_SELF | IN_DELETE_SELF | IN_ATTRIB | IN_CREATE | IN_MOVED_TO;
+                } else {
+                        flags = flags_table[s->type];
+                }
 
-                /* This assumes the path was passed through path_kill_slashes()! */
-                slash = strrchr(k, '/');
-                if (!slash)
-                        break;
-
-                /* Trim the path at the last slash. Keep the slash if it's the root dir. */
-                slash[slash == k] = 0;
-
-                flags = IN_MOVE_SELF;
-                if (!exists)
-                        flags |= IN_DELETE_SELF | IN_ATTRIB | IN_CREATE | IN_MOVED_TO;
+                r = inotify_add_watch(s->inotify_fd, path, flags);
+                if (r < 0) {
+                        if (errno == EACCES || errno == ENOENT)
+                                break;
 
-                if (inotify_add_watch(s->inotify_fd, k, flags) >= 0)
-                        exists = true;
-                else if (errno != EACCES && errno != ENOENT) {
-                        log_error("Failed to add watch on %s: %m", k);
+                        log_warning("Failed to add watch on %s: %m", path);
                         r = -errno;
                         goto fail;
+                } else {
+                        exists = true;
+
+                        /* Path exists, we don't need to watch parent
+                           too closely. */
+                        if (oldslash) {
+                                char tmp2 = oldslash[oldslash == path];
+                                oldslash[oldslash == path] = '\0';
+
+                                inotify_add_watch(s->inotify_fd, path, IN_MOVE_SELF);
+                                /* Error is ignored, the worst can happen is
+                                   we get spurious events. */
+
+                                oldslash[oldslash == path] = tmp2;
+                        }
                 }
-        } while (slash != k);
+
+                if (slash) {
+                        slash[slash == path] = tmp;
+                        oldslash = slash;
+                } else {
+                        /* whole path has been iterated over */
+                        s->primary_wd = r;
+                        break;
+                }
+        }
+
+        assert(errno == EACCES || errno == ENOENT || streq(path, s->path));
 
         if (!exists) {
                 log_error("Failed to add watch on any of the components of %s: %m",
                           s->path);
-                r = -errno;
+                r = -errno; /* either EACCESS or ENOENT */
                 goto fail;
         }