Re: [PATCH v5 4/4] fsmonitor: deal with synthetic firmlinks on macOS

> Starting with macOS 10.15 (Catalina), Apple introduced a new feature
> called 'firmlinks' in order to separate the boot volume into two
> volumes, one read-only and one writable but still present them to the
> user as a single volume. Along with this change, Apple removed the
> ability to create symlinks in the root directory and replaced them with
> 'synthetic firmlinks'. See 'man synthetic.conf'
>
> When FSEevents reports the path of changed files, if the path invloves
> a synthetic firmlink, the path is reported from the point of the
> synthetic firmlink and not the real path. For example:

> Real path:
> /System/Volumes/Data/network/working/directory/foo.txt
>
> Synthetic firmlink:
> /network -> /System/Volumes/Data/network
>
> FSEvents path:
> /network/working/directory/foo.txt
>
> This causes the FSEvents path to not match against the worktree
> directory.
>
> There are several ways in which synthetic firmlinks can be created:
> they can be defined in /etc/synthetic.conf, the automounter can create
> them, and there may be other means. Simply reading /etc/synthetic.conf
> is insufficient. No matter what process creates synthetic firmlinks,
> they all get created in the root directory.
>
> Therefore, in order to deal with synthetic firmlinks, the root directory
> is scanned and the first possible synthetic firmink that, when resolved,
> is a prefix of the worktree is used to map FSEvents paths to worktree
> paths.
>
> Signed-off-by: Eric DeCosta <edecosta@mathworks.com>
> ---
> diff --git a/compat/fsmonitor/fsm-path-utils-darwin.c b/compat/fsmonitor/fsm-path-utils-darwin.c
> @@ -38,3 +41,91 @@ int fsmonitor__is_fs_remote(const char *path)
> +/*
> + * Scan the root directory for synthetic firmlinks that when resolved
> + * are a prefix of the path, stopping at the first one found.
> + *
> + * Some information about firmlinks and synthetic firmlinks:
> + * https://eclecticlight.co/2020/01/23/catalina-boot-volumes/
> + *
> + * macOS no longer allows symlinks in the root directory; any link found
> + * there is therefore a synthetic firmlink.
> + *
> + * If this function gets called often, will want to cache all the firmlink
> + * information, but for now there is only one caller of this function.
> + *
> + * If there is more than one alias for the path, that is another
> + * matter altogteher.
> + */

> diff --git a/fsmonitor-path-utils.h b/fsmonitor-path-utils.h
> @@ -1,6 +1,14 @@
> +struct alias_info
> +{
> +       struct strbuf alias;
> +       struct strbuf points_to;
> +};
> +/*
> + * Get the alias in given path, if any.
> + *
> + * Sets alias to the first alias that matches any part of the path.
> + *
> + * Returns -1 on error, 0 otherwise.
> + */
> +int fsmonitor__get_alias(const char *path, struct alias_info *info);

> +/*
> + * Resolve the path against the given alias.
> + *
> + * Returns the resolved path if there is one, NULL otherwise.
> + */
> +char *fsmonitor__resolve_alias(const char *path,
> +       const struct alias_info *info);