fs: implement inotify(2)

inotify was stubbed (inotify_init returned EMFILE, add_watch/rm_watch returned
EINVAL), so programs that watch the filesystem for changes could not run.

Implement it against OSv's VFS.  All path mutations route through a handful of
central functions in fs/vfs/vfs_syscalls.cc; those now call osv_inotify_notify()
with the absolute path affected and the event mask.  libc/inotify.cc keeps a
registry of inotify instances (each a pollable special_file, modeled on
eventfd), matches every event against their watches, and queues a
struct inotify_event for the fd, waking any poller.

- inotify_init/inotify_init1 create the fd (IN_CLOEXEC/IN_NONBLOCK honored).
- inotify_add_watch resolves the path to absolute, adds (or merges, with
  IN_MASK_ADD) a watch, and returns a watch descriptor.
- inotify_rm_watch removes a watch and queues IN_IGNORED.
- read() returns as many packed inotify_event structures as fit in the buffer
  (blocking, or EAGAIN when non-blocking), rejecting a too-small buffer with
  EINVAL; poll() reports POLLIN when events are queued.

A directory watch fires for changes to entries within it (reporting the entry
name); a watch on the object itself fires with no name.  Events wired:
IN_CREATE (mkdir, open O_CREAT), IN_DELETE (rmdir, unlink), IN_MOVED_FROM /
IN_MOVED_TO (rename), each with IN_ISDIR when the object is a directory.

The rename hook rebuilds the full destination path because sys_rename truncates
`dest` to its parent in place before the VOP.

Not yet covered (follow-ups): IN_MODIFY/IN_CLOSE_WRITE on writes (needs an
fd-to-path lookup), IN_ACCESS/IN_OPEN, and recursive watches.

Add tests/tst-inotify.cc covering create/delete/move on files and a
subdirectory (names and IN_ISDIR), IN_IGNORED on watch removal, and the
EAGAIN/EINVAL paths.  Passes on OSv under KVM; tst-remove (unlink/rename/rmdir)
continues to pass, so the VFS mutation paths are unaffected.

Author: gburd

fs/vfs/vfs.h

@@ -113,6 +113,9 @@ int	 sys_link(char *oldpath, char *newpath);
 int	 sys_unlink(char *path);
 int	 sys_symlink(const char *oldpath, const char *newpath);
 int	 sys_access(char *path, int mode);
+
+/* inotify VFS notification hook (libc/inotify.cc). */
+void osv_inotify_notify(const char *path, uint32_t mask, int is_dir);
 int	 sys_stat(char *path, struct stat *st);
 int	 sys_lstat(char *path, struct stat *st);
 int	 sys_statfs(char *path, struct statfs *buf);

fs/vfs/vfs_syscalls.cc

@@ -41,6 +41,7 @@
 
 #include <sys/stat.h>
 #include <dirent.h>
+#include <sys/inotify.h>
 
 #include <limits.h>
 #include <unistd.h>
@@ -141,6 +142,7 @@ sys_open(char *path, int flags, mode_t mode, struct file **fpp)
 
 			if (error)
 				return error;
+			osv_inotify_notify(path, IN_CREATE, 0);
 			if ((error = namei(path, &dp)) != 0)
 				return error;
 
@@ -568,6 +570,9 @@ sys_mkdir(char *path, mode_t mode)
 	mode |= S_IFDIR;
 
 	error = VOP_MKDIR(ddp->d_vnode, name, mode);
+	if (!error) {
+		osv_inotify_notify(path, IN_CREATE, 1);
+	}
  out:
 	vn_unlock(ddp->d_vnode);
 	drele(ddp);
@@ -609,6 +614,9 @@ sys_rmdir(char *path)
 	error = VOP_RMDIR(ddp->d_vnode, vp, name);
 	vn_unlock(ddp->d_vnode);
 
+	if (!error) {
+		osv_inotify_notify(path, IN_DELETE, 1);
+	}
 	vn_unlock(vp);
 	dentry_remove(dp);
 	drele(ddp);
@@ -824,6 +832,19 @@ sys_rename(char *src, char *dest)
 
 	error = VOP_RENAME(dvp1, vp1, sname, dvp2, vp2, dname);
 
+	if (!error) {
+		int is_dir = (vp1->v_type == VDIR) ? 1 : 0;
+		// `src` is still the full source path, but `dest` was truncated to its
+		// parent above (*dname was NUL'd), so rebuild the full dest path.
+		char full_dest[PATH_MAX];
+		if (dest[0] == '/' && dest[1] == '\0') {
+			snprintf(full_dest, sizeof(full_dest), "/%s", dname);
+		} else {
+			snprintf(full_dest, sizeof(full_dest), "%s/%s", dest, dname);
+		}
+		osv_inotify_notify(src, IN_MOVED_FROM, is_dir);
+		osv_inotify_notify(full_dest, IN_MOVED_TO, is_dir);
+	}
 	dentry_move(dp1, ddp2, dname);
 	if (dp2)
 		dentry_remove(dp2);
@@ -1016,6 +1037,9 @@ sys_unlink(char *path)
 	error = VOP_REMOVE(ddp->d_vnode, vp, name);
 	vn_unlock(ddp->d_vnode);
 
+	if (!error) {
+		osv_inotify_notify(path, IN_DELETE, 0);
+	}
 	vn_unlock(vp);
 	dentry_remove(dp);
 	drele(ddp);