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diff --git a/chdir-notify.h b/chdir-notify.h
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+#ifndef CHDIR_NOTIFY_H
+#define CHDIR_NOTIFY_H
+
+/*
+ * An API to let code "subscribe" to changes to the current working directory.
+ * The general idea is that some code asks to be notified when the working
+ * directory changes, and other code that calls chdir uses a special wrapper
+ * that notifies everyone.
+ */
+
+/*
+ * Callers who need to know about changes can do:
+ *
+ *   void foo(const char *old_path, const char *new_path, void *data)
+ *   {
+ *	warning("switched from %s to %s!", old_path, new_path);
+ *   }
+ *   ...
+ *   chdir_notify_register("description", foo, data);
+ *
+ * In practice most callers will want to move a relative path to the new root;
+ * they can use the reparent_relative_path() helper for that. If that's all
+ * you're doing, you can also use the convenience function:
+ *
+ *   chdir_notify_reparent("description", &my_path);
+ *
+ * Whenever a chdir event occurs, that will update my_path (if it's relative)
+ * to adjust for the new cwd by freeing any existing string and allocating a
+ * new one.
+ *
+ * Registered functions are called in the order in which they were added. Note
+ * that there's currently no way to remove a function, so make sure that the
+ * data parameter remains valid for the rest of the program.
+ *
+ * The "name" argument is used only for printing trace output from
+ * $GIT_TRACE_SETUP. It may be NULL, but if non-NULL should point to
+ * storage which lasts as long as the registration is active.
+ */
+typedef void (*chdir_notify_callback)(const char *name,
+				      const char *old_cwd,
+				      const char *new_cwd,
+				      void *data);
+void chdir_notify_register(const char *name, chdir_notify_callback cb, void *data);
+void chdir_notify_reparent(const char *name, char **path);
+
+/*
+ *
+ * Callers that want to chdir:
+ *
+ *   chdir_notify(new_path);
+ *
+ * to switch to the new path and notify any callbacks.
+ *
+ * Note that you don't need to chdir_notify() if you're just temporarily moving
+ * to a directory and back, as long as you don't call any subscribed code in
+ * between (but it should be safe to do so if you're unsure).
+ */
+int chdir_notify(const char *new_cwd);
+
+/*
+ * Reparent a relative path from old_root to new_root. For example:
+ *
+ *   reparent_relative_path("/a", "/a/b", "b/rel");
+ *
+ * would return the (newly allocated) string "rel". Note that we may return an
+ * absolute path in some cases (e.g., if the resulting path is not inside
+ * new_cwd).
+ */
+char *reparent_relative_path(const char *old_cwd,
+			     const char *new_cwd,
+			     const char *path);
+
+#endif /* CHDIR_NOTIFY_H */