[v15,4/6] fsmonitor: deal with synthetic firmlinks on macOS

Commit Message

Eric DeCosta Oct. 4, 2022, 5:32 p.m. UTC

From: Eric DeCosta <[e-mail]>

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 involves
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 <[e-mail]>
---
 builtin/fsmonitor--daemon.c              |  8 +++
 compat/fsmonitor/fsm-listen-darwin.c     | 14 +++-
 compat/fsmonitor/fsm-path-utils-darwin.c | 92 ++++++++++++++++++++++++
 compat/fsmonitor/fsm-path-utils-win32.c  | 17 +++++
 fsmonitor--daemon.h                      |  3 +
 fsmonitor-path-utils.h                   | 36 +++++++++-
 6 files changed, 167 insertions(+), 3 deletions(-)

Comments

Ævar Arnfjörð Bjarmason Jan. 30, 2023, 10:08 a.m. UTC | #1

On Tue, Oct 04 2022, Eric DeCosta via GitGitGadget wrote:

> From: Eric DeCosta <[e-mail]>

> +	struct strbuf alias;

Rather than init-ing here...
> +	struct strbuf points_to = STRBUF_INIT;
> +
> +	dir = opendir(root);
> +	if (!dir)
> +		return error_errno(_("opendir('%s') failed"), root);
> +
> +	strbuf_init(&alias, 256);

...we pre-grow here...

> +	while ((de = readdir(dir)) != NULL) {

...but as shown here, we may not even use this at all, but even then is
this micro-optimization worth it? If it is a reader would be helped with
an explanation of what the 256 is, is this meant to be some OSX-specific
PATH_MAX, but hardcoded?

> +		strbuf_reset(&alias);
> +		strbuf_addf(&alias, "%s%s", root, de->d_name);
> +
> +		if (lstat(alias.buf, &st) < 0) {
> +			error_errno(_("lstat('%s') failed"), alias.buf);
> +			goto done;
> +		}
> +
> +		if (!S_ISLNK(st.st_mode))
> +			continue;
> +
> +		if (strbuf_readlink(&points_to, alias.buf, st.st_size) < 0) {
> +			error_errno(_("strbuf_readlink('%s') failed"), alias.buf);
> +			goto done;
> +		}
> +

Maybe this code would be simpler if you split it into a trivial static
function, so you could pass in the "alias" and "points_to", and just do
"return error...(...)" here and in the other places.

> +		if (!strncmp(points_to.buf, path, points_to.len) &&
> +			(path[points_to.len] == '/')) {
> +			strbuf_addbuf(&info->alias, &alias);
> +			strbuf_addbuf(&info->points_to, &points_to);

Earlier you use strbuf_addf() for a "append two strings", maybe we could
save ourselves a line and do the same here...

> +			trace_printf_key(&trace_fsmonitor,
> +				"Found alias for '%s' : '%s' -> '%s'",
> +				path, info->alias.buf, info->points_to.buf);

...except we're only doing this to emit this, and then we'll free() it?
Can't we just use %s%s here instead of %s, and e.g. pass
"info->alias.buf, alias" instead of the now-appende-to
"info->alias.buf"?

> +char *fsmonitor__resolve_alias(const char *path,
> +	const struct alias_info *info)

I commented on this in a few other places, and I'll stop noting these
now, but you're mis-indenting function decls consistently, also in a *.h
change later in this commit.

Please look through those for this series.

> +{
> +	if (!info->alias.len)
> +		return NULL;

Maybe "check if we have a zero-length string" should belong in the
caller, as "resolve it as an alias" for "\0" is nonsense?

> +
> +	if ((!strncmp(info->alias.buf, path, info->alias.len))
> +		&& path[info->alias.len] == '/') {
> +		struct strbuf tmp = STRBUF_INIT;
> +		const char *remainder = path + info->alias.len;
> +
> +		strbuf_addbuf(&tmp, &info->points_to);
> +		strbuf_add(&tmp, remainder, strlen(remainder));

There's no point in strbuf_add() if you have to dynamically use strlen()
over just using strbuf_addstr() (which inline resolves to the same),
let's just use that.

Or just a single strbuf_addf()...

> +		return strbuf_detach(&tmp, NULL);

...Or actually, these last 3 lines can be replaced by a mere:

	return xstrfmt("%s%s", info->points_to.buf, remainder);

Please just use that. It's not exactly the same due to the pre-sizing
with "addbuf", but again (I commented on a similar case in an earlier
commit), is such micro-optimization worth it here over brevity?

> +/*
> + * No-op for now.
> + */

Please just drop this comment, it doesn't add any information. It would
be useful to say why we're seemingly faking "no aliase on win32", or to
say that it does have them, but implementing them is a "TODO".

But we can see it's a "no-op for now" from the code....

> +int fsmonitor__get_alias(const char *path, struct alias_info *info)
> +{
> +	return 0;
> +}
> +
> +/*
> + * No-op for now.
> + */

...ditto.

> +/*
> + * Get the alias in given path, if any.
> + *
> + * Sets alias to the first alias that matches any part of the path.
> + *
> + * If an alias is found, info.alias and info.points_to are set to the
> + * found mapping.
> + *
> + * Returns -1 on error, 0 otherwise.
> + *
> + * The caller owns the storage that is occupied by info.alias and
> + * info.points_to and is responsible for releasing it.
> + */
> +int fsmonitor__get_alias(const char *path, struct alias_info *info);

I have not looked carefully at this interface, but instead of all of
this explanation & the caller needing to carefully reason about what
parts of this struct it can and can't peek into couldn't we just make it
take two "char **" arguments, one for "alias" and another for
"points_to".

It would then be obvious what the semantics are, and who owns the
memory.

But maybe retaining the strbuf-ness is important (but even then, we
could pass a "struct strbuf *" to populate.
> +
> +/*
> + * Resolve the path against the given alias.
> + *
> + * Returns the resolved path if there is one, NULL otherwise.
> + *
> + * The caller owns the storage that the returned string occupies and
> + * is responsible for releasing it.
> + */
> +char *fsmonitor__resolve_alias(const char *path,
> +	const struct alias_info *info);
> +

Here we don't say anything about the ownership & freeing of "info", does
the same apply? But the API design comment above also applies.

Patch

diff --git a/builtin/fsmonitor--daemon.c b/builtin/fsmonitor--daemon.c
index 0123fc33ed2..7a4cb78c7dd 100644
--- a/builtin/fsmonitor--daemon.c
+++ b/builtin/fsmonitor--daemon.c
@@ -3,6 +3,7 @@ 
 #include "parse-options.h"
 #include "fsmonitor.h"
 #include "fsmonitor-ipc.h"
+#include "fsmonitor-path-utils.h"
 #include "compat/fsmonitor/fsm-health.h"
 #include "compat/fsmonitor/fsm-listen.h"
 #include "fsmonitor--daemon.h"
@@ -1282,6 +1283,11 @@  static int fsmonitor_run_daemon(void)
 	strbuf_addstr(&state.path_worktree_watch, absolute_path(get_git_work_tree()));
 	state.nr_paths_watching = 1;
 
+	strbuf_init(&state.alias.alias, 0);
+	strbuf_init(&state.alias.points_to, 0);
+	if ((err = fsmonitor__get_alias(state.path_worktree_watch.buf, &state.alias)))
+		goto done;
+
 	/*
 	 * We create and delete cookie files somewhere inside the .git
 	 * directory to help us keep sync with the file system.  If
@@ -1391,6 +1397,8 @@  done:
 	strbuf_release(&state.path_gitdir_watch);
 	strbuf_release(&state.path_cookie_prefix);
 	strbuf_release(&state.path_ipc);
+	strbuf_release(&state.alias.alias);
+	strbuf_release(&state.alias.points_to);
 
 	return err;
 }
diff --git a/compat/fsmonitor/fsm-listen-darwin.c b/compat/fsmonitor/fsm-listen-darwin.c
index 8e208e8289e..daeee4e465c 100644
--- a/compat/fsmonitor/fsm-listen-darwin.c
+++ b/compat/fsmonitor/fsm-listen-darwin.c
@@ -26,6 +26,7 @@ 
 #include "fsmonitor.h"
 #include "fsm-listen.h"
 #include "fsmonitor--daemon.h"
+#include "fsmonitor-path-utils.h"
 
 struct fsm_listen_data
 {
@@ -198,8 +199,9 @@  static void fsevent_callback(ConstFSEventStreamRef streamRef,
 	struct string_list cookie_list = STRING_LIST_INIT_DUP;
 	const char *path_k;
 	const char *slash;
-	int k;
+	char *resolved = NULL;
 	struct strbuf tmp = STRBUF_INIT;
+	int k;
 
 	/*
 	 * Build a list of all filesystem changes into a private/local
@@ -209,7 +211,12 @@  static void fsevent_callback(ConstFSEventStreamRef streamRef,
 		/*
 		 * On Mac, we receive an array of absolute paths.
 		 */
-		path_k = paths[k];
+		free(resolved);
+		resolved = fsmonitor__resolve_alias(paths[k], &state->alias);
+		if (resolved)
+			path_k = resolved;
+		else
+			path_k = paths[k];
 
 		/*
 		 * If you want to debug FSEvents, log them to GIT_TRACE_FSMONITOR.
@@ -238,6 +245,7 @@  static void fsevent_callback(ConstFSEventStreamRef streamRef,
 			fsmonitor_force_resync(state);
 			fsmonitor_batch__free_list(batch);
 			string_list_clear(&cookie_list, 0);
+			batch = NULL;
 
 			/*
 			 * We assume that any events that we received
@@ -360,12 +368,14 @@  static void fsevent_callback(ConstFSEventStreamRef streamRef,
 		}
 	}
 
+	free(resolved);
 	fsmonitor_publish(state, batch, &cookie_list);
 	string_list_clear(&cookie_list, 0);
 	strbuf_release(&tmp);
 	return;
 
 force_shutdown:
+	free(resolved);
 	fsmonitor_batch__free_list(batch);
 	string_list_clear(&cookie_list, 0);
 
diff --git a/compat/fsmonitor/fsm-path-utils-darwin.c b/compat/fsmonitor/fsm-path-utils-darwin.c
index d46d7f13538..ce5a8febe09 100644
--- a/compat/fsmonitor/fsm-path-utils-darwin.c
+++ b/compat/fsmonitor/fsm-path-utils-darwin.c
@@ -1,5 +1,8 @@ 
 #include "fsmonitor.h"
 #include "fsmonitor-path-utils.h"
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <sys/param.h>
 #include <sys/mount.h>
 
@@ -41,3 +44,92 @@  int fsmonitor__is_fs_remote(const char *path)
 
 	return fs.is_remote;
 }
+
+/*
+ * 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 altogether.
+ */
+int fsmonitor__get_alias(const char *path, struct alias_info *info)
+{
+	DIR *dir;
+	int retval = -1;
+	const char *const root = "/";
+	struct stat st;
+	struct dirent *de;
+	struct strbuf alias;
+	struct strbuf points_to = STRBUF_INIT;
+
+	dir = opendir(root);
+	if (!dir)
+		return error_errno(_("opendir('%s') failed"), root);
+
+	strbuf_init(&alias, 256);
+
+	while ((de = readdir(dir)) != NULL) {
+		strbuf_reset(&alias);
+		strbuf_addf(&alias, "%s%s", root, de->d_name);
+
+		if (lstat(alias.buf, &st) < 0) {
+			error_errno(_("lstat('%s') failed"), alias.buf);
+			goto done;
+		}
+
+		if (!S_ISLNK(st.st_mode))
+			continue;
+
+		if (strbuf_readlink(&points_to, alias.buf, st.st_size) < 0) {
+			error_errno(_("strbuf_readlink('%s') failed"), alias.buf);
+			goto done;
+		}
+
+		if (!strncmp(points_to.buf, path, points_to.len) &&
+			(path[points_to.len] == '/')) {
+			strbuf_addbuf(&info->alias, &alias);
+			strbuf_addbuf(&info->points_to, &points_to);
+			trace_printf_key(&trace_fsmonitor,
+				"Found alias for '%s' : '%s' -> '%s'",
+				path, info->alias.buf, info->points_to.buf);
+			retval = 0;
+			goto done;
+		}
+	}
+	retval = 0; /* no alias */
+
+done:
+	strbuf_release(&alias);
+	strbuf_release(&points_to);
+	if (closedir(dir) < 0)
+		return error_errno(_("closedir('%s') failed"), root);
+	return retval;
+}
+
+char *fsmonitor__resolve_alias(const char *path,
+	const struct alias_info *info)
+{
+	if (!info->alias.len)
+		return NULL;
+
+	if ((!strncmp(info->alias.buf, path, info->alias.len))
+		&& path[info->alias.len] == '/') {
+		struct strbuf tmp = STRBUF_INIT;
+		const char *remainder = path + info->alias.len;
+
+		strbuf_addbuf(&tmp, &info->points_to);
+		strbuf_add(&tmp, remainder, strlen(remainder));
+		return strbuf_detach(&tmp, NULL);
+	}
+
+	return NULL;
+}
diff --git a/compat/fsmonitor/fsm-path-utils-win32.c b/compat/fsmonitor/fsm-path-utils-win32.c
index a90b8f7925b..0d95bbb416f 100644
--- a/compat/fsmonitor/fsm-path-utils-win32.c
+++ b/compat/fsmonitor/fsm-path-utils-win32.c
@@ -126,3 +126,20 @@  int fsmonitor__is_fs_remote(const char *path)
 		return -1;
 	return fs.is_remote;
 }
+
+/*
+ * No-op for now.
+ */
+int fsmonitor__get_alias(const char *path, struct alias_info *info)
+{
+	return 0;
+}
+
+/*
+ * No-op for now.
+ */
+char *fsmonitor__resolve_alias(const char *path,
+	const struct alias_info *info)
+{
+	return NULL;
+}
diff --git a/fsmonitor--daemon.h b/fsmonitor--daemon.h
index 2102a5c9ff5..e24838f9a86 100644
--- a/fsmonitor--daemon.h
+++ b/fsmonitor--daemon.h
@@ -8,6 +8,7 @@ 
 #include "run-command.h"
 #include "simple-ipc.h"
 #include "thread-utils.h"
+#include "fsmonitor-path-utils.h"
 
 struct fsmonitor_batch;
 struct fsmonitor_token_data;
@@ -43,6 +44,7 @@  struct fsmonitor_daemon_state {
 
 	struct strbuf path_worktree_watch;
 	struct strbuf path_gitdir_watch;
+	struct alias_info alias;
 	int nr_paths_watching;
 
 	struct fsmonitor_token_data *current_token_data;
@@ -59,6 +61,7 @@  struct fsmonitor_daemon_state {
 
 	struct ipc_server_data *ipc_server_data;
 	struct strbuf path_ipc;
+
 };
 
 /*
diff --git a/fsmonitor-path-utils.h b/fsmonitor-path-utils.h
index 41edf5b934f..5bfdfb81c14 100644
--- a/fsmonitor-path-utils.h
+++ b/fsmonitor-path-utils.h
@@ -1,13 +1,21 @@ 
 #ifndef FSM_PATH_UTILS_H
 #define FSM_PATH_UTILS_H
 
+#include "strbuf.h"
+
+struct alias_info
+{
+	struct strbuf alias;
+	struct strbuf points_to;
+};
+
 struct fs_info {
 	int is_remote;
 	char *typename;
 };
 
 /*
- * Get some basic filesystem informtion for the given path
+ * Get some basic filesystem information for the given path
  *
  * The caller owns the storage that is occupied by fs_info and
  * is responsible for releasing it.
@@ -23,4 +31,30 @@  int fsmonitor__get_fs_info(const char *path, struct fs_info *fs_info);
  */
 int fsmonitor__is_fs_remote(const char *path);
 
+/*
+ * Get the alias in given path, if any.
+ *
+ * Sets alias to the first alias that matches any part of the path.
+ *
+ * If an alias is found, info.alias and info.points_to are set to the
+ * found mapping.
+ *
+ * Returns -1 on error, 0 otherwise.
+ *
+ * The caller owns the storage that is occupied by info.alias and
+ * info.points_to and is responsible for releasing it.
+ */
+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.
+ *
+ * The caller owns the storage that the returned string occupies and
+ * is responsible for releasing it.
+ */
+char *fsmonitor__resolve_alias(const char *path,
+	const struct alias_info *info);
+
 #endif