Watcher

Quick Start

#include "wtr/watcher.hpp"  // Or wherever yours is
#include <iostream>
#include <string>

using namespace std;
using namespace wtr;

// The event type, and every field within it, has
// string conversions and stream operators. All
// kinds of strings -- Narrow, wide and weird ones.
// If we don't want particular formatting, we can
// json-serialize and show the event like this:
//   some_stream << event
// Here, we'll apply our own formatting.
auto show(event e) {
  auto s = [](auto a) { return to<string>(a); };
  cout << s(e.effect_type) + ' '
        + s(e.path_type)   + ' '
        + s(e.path_name)
        + (e.associated ? " -> " + s(e.associated->path_name) : "")
       << endl;
}

auto main() -> int {
  // Watch the current directory asynchronously,
  // calling the provided function for each event.
  auto watcher = watch(".", show);

  // Do some work. (We'll just wait for a newline.)
  getchar();

  // The watcher would close itself around here,
  // though we can check and close it ourselves:
  return watcher.close() ? 0 : 1;
}

# Sigh
PLATFORM_EXTRAS=$(test "$(uname)" = Darwin && echo '-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -framework CoreFoundation -framework CoreServices')
# Build
eval c++ -std=c++17 -Iinclude src/wtr/tiny_watcher/main.cpp -o watcher $PLATFORM_EXTRAS
# Run
./watcher

modify file /home/e-dant/dev/watcher/.git/refs/heads/next.lock
rename file /home/e-dant/dev/watcher/.git/refs/heads/next.lock -> /home/e-dant/dev/watcher/.git/refs/heads/next
create file /home/e-dant/dev/watcher/.git/HEAD.lock

Enjoy!

---

Tell Me More

A filesystem event watcher which is

1. Simple

The 1613 lines that make up Watcher are kept reasonably simple and the API attempts to be practical:

auto w = watch(path, [](event ev) { cout << ev; });

wtr.watcher ~

2. Modular

Watcher may be used as a library, a program, or both. If you aren't looking to create something with the library, no worries. Just use ours and you've got yourself a filesystem watcher which prints filesystem events as JSON. Neat. Here's how:

# The main branch is the (latest) release branch.
git clone https://github.com/e-dant/watcher.git && cd watcher
# Via Nix
nix run | grep -oE 'cmake-is-tough'
# With the build script
tool/build --no-build-test --no-run && cd out/this/Release # Build the release version for the host platform.
./wtr.watcher | grep -oE 'needle-in-a-haystack/.+"' # Use it, pipe it, whatever. (This is an .exe on Windows.)

3. Efficient

You can watch an entire filesystem with this project. In almost all cases, we use a near-zero amount of resources and make efficient use of the cache. We regularly test that the overhead of detecting and sending an event to the user is an order of magnitude less than the filesystem operations being measured.

4. Safe

We run this project through unit tests against all available sanitiziers. The code is safe (with reasonable certainty) and simple. (This includes thread, memory, bounds, type and resource safety.)

5. Dependency free

Watcher depends on the C++ Standard Library. For efficiency, we use System APIs when possible on Linux, Darwin and Windows. For testing and debugging, we use Snitch and Sanitizers.

6. Runnable anywhere

Watcher is runnable almost anywhere. The only requirement is a filesystem.

---

Usage

Project Content

The important pieces are the (header-only) library and the (optional) CLI program.

• Library: include/wtr/watcher.hpp. Include this to use Watcher in your project.
• Program: src/wtr/watcher/main.cpp. Build this to use Watcher from the command line.

A directory tree is in the notes below.

The Library

Copy the include directory into your project. Include watcher like this:

#include "wtr/watcher.hpp"

The event and watch headers are short and approachable. (You only ever need to include wtr/watcher.hpp.)

There are two things the user needs:

• The watch function
• The event object

watch takes a path, which is a string-like thing, and a callback, with is a function-like thing. Passing watch a character array and a lambda would work well.

Typical use looks like this:

auto watcher = watch(path, [](event ev) { cout << ev; });

watch will happily continue watching until you stop it or it hits an unrecoverable error.

The event object is used to pass information about filesystem events to the (user-supplied) callback given to watch.

The event object will contain:

• associated, another event, associated with this one, such as a renamed-to path. (This is a recursive structure.)
• path_name, which is an absolute path to the event.
• path_type, the type of path. One of:
  • dir
  • file
  • hard_link
  • sym_link
  • watcher
  • other
• effect_type, "what happened". One of:
  • rename
  • modify
  • create
  • destroy
  • owner
  • other
• effect_time, the time of the event in nanoseconds since epoch.

The watcher type is special.

Events with this type will include messages from the watcher. You may recieve error messages or important status updates, such as when it first becomes alive and when it dies.

The last event will always be a destroy event from the watcher. You can parse it like this:

  auto is_last = ev.path_type == path_type::watcher
              && ev.effect_type == effect_type::destroy;

Happy hacking.

Your Project

It is trivial to build programs that yield something useful.

Here is a snapshot of the output taken while preparing this commit, right before writing this paragraph.

{
  "1666393024210001000": {
    "path_name": "./watcher/.git/logs/HEAD",
    "effect_type": "modify",
    "path_type": "file"
  },
  "1666393024210026000": {
    "path_name": "./watcher/.git/logs/refs/heads/next",
    "effect_type": "modify",
    "path_type": "file"
  },
  "1666393024210032000": {
    "path_name": "./watcher/.git/refs/heads/next.lock",
    "effect_type": "create",
    "path_type": "other"
  }
}

Which is pretty cool.

A capable program is here.

Consume

This project is accessible through:

• Conan: Includes the header
• Nix: Provides isolation, determinism, includes header, cli, test and benchmark targets
• Bazel: Provides isolation, include header and cli targets
• tool/build: Includes header, cli, test and benchmark targets
• CMake: Includes header, cli, test and benchmark targets
• Just copying the header file

Conan

See the package here.

Nix

nix build # To just build
nix run # Build and run without arguments
nix run . -- / | jq # Build and run the default target, watch the root directory, pipe it to jq
nix develop # Enter an isolated development shell with everything needed to explore this project

Bazel

bazel build cli # Build, but don't run, the cli
bazel build hdr # Ditto for the single-header
bazel run cli # Run the cli program without arguments

tool/build

tool/build
cd out/this/Release

# watches the current directory forever
./wtr.watcher
# watches some path for 10 seconds
./wtr.watcher 'your/favorite/path' -s 10

This will take care of some platform-specifics, building the release, debug, and sanitizer variants, and running some tests.

CMake

cmake -S . -B out
cmake --build out --config Release
cd out

# watches the current directory forever
./wtr.watcher
# watches some path for 10 seconds
./wtr.watcher 'your/favorite/path' -s 10

Notes

Minimum C++ Version

For the header-only library and the tiny-watcher, C++17 and up should be fine.

We might use C++20 coroutines someday.

Safety and C++

I was comfortable with C++ when I first wrote this. I later rewrote this project in Rust as an experiment. There are benefits and drawbacks to Rust. Some things were a bit safer to express, other things were definitely not. The necessity of doing pointer math on some variably-sized opaque types from the kernel, for example, is not safer to express in Rust. Other things are safer, but this project doesn't benefit much from them.

Rust really shines in usability and expression.

I'm not sure if there is a language that can "just" make the majority of the code in this project safe by definition.

The guts of this project (the adapters) talk to the kernel. They are bound to use unsafe, ill-typed, caveat-rich system-level interfaces.

The public API is just around 100 lines, is well-typed, well-tested, and human-verifiable. Not much happens there.

Creating an FFI by exposing the adapters with a C ABI might be worthwhile. Most languages should be able to hook into that.

The safety of the platform adapters necessarily depends on each platform's documentation for their interfaces. Like with all system-level interfaces, as long as we ensure the correct pre-and-post-conditions, and those conditions are well-defined, we should be fine.

Limitations

• Ready State

There is no reliable way to communicate when a watcher is ready to send events to the callback. For a few thousand paths, this may take a few milliseconds. For a few million, consider waiting a second or so.

Exception to Efficient Scanning

Efficiency takes a hit when we bring out the warthog, our platform-independent adapter. This adapter is used on platforms that lack better alternatives, such as (not Darwin) BSD and Solaris (because warthog beats kqueue).

Watcher is still relatively efficient when it has no alternative better than warthog. As a thumb-rule, scanning more than one-hundred-thousand paths with warthog might stutter.

I'll keep my eyes open for better kernel APIs on BSD.

OS APIs Used

Linux

• inotify
• fanotify
• epoll
• eventfd

Darwin

• FSEvents
• dispatch

Windows

• ReadDirectoryChangesW
• IoCompletionPort

Cache Efficiency

$ tool/test/dir &
$ tool/test/file &
$ valgrind --tool=cachegrind wtr.watcher ~ -s 30

I   refs:      797,368,564
I1  misses:          6,807
LLi misses:          2,799
I1  miss rate:        0.00%
LLi miss rate:        0.00%

D   refs:      338,544,669  (224,680,988 rd   + 113,863,681 wr)
D1  misses:         35,331  (     24,823 rd   +      10,508 wr)
LLd misses:         11,884  (      8,121 rd   +       3,763 wr)
D1  miss rate:         0.0% (        0.0%     +         0.0%  )

LLd miss rate:         0.0% (        0.0%     +         0.0%  )
LL refs:            42,138  (     31,630 rd   +      10,508 wr)
LL misses:          14,683  (     10,920 rd   +       3,763 wr)
LL miss rate:          0.0% (        0.0%     +         0.0%  )

Namespaces and the Directory Tree

Namespaces and symbols closely follow the directories in the devel/include folder. Inline namespaces are in directories with the - affix.

For example, wtr::watch is inside the file devel/include/wtr/watcher-/watch.hpp. The namespace watcher in wtr::watcher::watch is anonymous by this convention.

More in depth: the function ::detail::wtr::watcher::adapter::watch() is defined inside one (and only one!) of the files devel/include/detail/wtr/watcher/adapter/*/watch.hpp, where * is decided at compile-time (depending on the host's operating system).

All of the headers in devel/include are amalgamated into include/wtr/watcher.hpp and an include guard is added to the top. The include guard doesn't change with the release version. In the future, it might.

watcher
├── flake.nix
├── flake.lock
├── src
│  └── wtr
│     ├── watcher
│     │  └── main.cpp
│     └── tiny_watcher
│        └── main.cpp
├── include
│  └── wtr
│     └── watcher.hpp
└── devel
   └── include
      ├── wtr
      │  ├── watcher.hpp
      │  └── watcher-
      │     ├── watch.hpp
      │     └── event.hpp
      └── detail
         └── wtr
            └── watcher
               └── adapter
                  ├── windows
                  │  └── watch.hpp
                  ├── warthog
                  │  └── watch.hpp
                  ├── linux
                  │  ├── watch.hpp
                  │  ├── inotify
                  │  │  └── watch.hpp
                  │  └── fanotify
                  │     └── watch.hpp
                  └── darwin
                     └── watch.hpp

You can run tool/tree to view this tree locally.

Comparison with Similar Projects

https://github.com/notify-rs/notify:
  lines of code: 2799
  lines of tests: 475
  lines of docs: 1071
  implementation languages: rust
  interface languages: rust
  supported platforms: linux, windows, darwin, bsd
  kernel apis: inotify, readdirectorychanges, fsevents, kqueue
  non-blocking: yes (with api caveats)
  dependencies: none
  tests: yes (many)
  static analysis: yes (borrow checked)

https://github.com/e-dant/watcher:
  lines of code: 1613
  lines of tests: 865
  lines of docs: 1908
  implementation languages: cpp
  interface languages: cpp, shells
  supported platforms: linux, darwin, windows, bsd
  kernel apis: inotify, fanotify, fsevents, readdirectorychanges
  non-blocking: yes
  dependencies: none
  tests: yes (many)
  static analysis: yes (all available)

https://github.com/facebook/watchman.git:
  lines of code: 37435
  lines of tests: unknown
  lines of docs: unknown
  implementation languages: cpp, c
  interface languages: cpp, js, java, python, ruby, rust, shells
  supported platforms: linux, darwin, windows, maybe bsd
  kernel apis: inotify, fsevents, readdirectorychanges
  non-blocking: yes
  dependencies: none
  tests: yes (many)
  static analysis: yes (all available)

https://github.com/p-ranav/fswatch:
  lines of code: 245
  lines of tests: 19
  lines of docs: 114
  implementation languages: cpp
  interface languages: cpp, shells
  supported platforms: linux, darwin, windows, bsd
  kernel apis: inotify
  non-blocking: maybe
  dependencies: none
  tests: some
  static analysis: none

https://github.com/tywkeene/go-fsevents:
  lines of code: 413
  lines of tests: 401
  lines of docs: 384
  implementation languages: go
  interface languages: go
  supported platforms: linux
  kernel apis: inotify
  non-blocking: yes
  dependencies: yes
  tests: yes
  static analysis: none (gc language)

https://github.com/radovskyb/watcher:
  lines of code: 552
  lines of tests: 767
  lines of docs: 399
  implementation languages: go
  interface languages: go
  supported platforms: linux, darwin, windows
  kernel apis: none
  non-blocking: no
  dependencies: none
  tests: yes
  static analysis: none

https://github.com/parcel-bundler/watcher:
  lines of code: 2862
  lines of tests: 474
  lines of docs: 791
  implementation languages: cpp
  interface languages: js
  supported platforms: linux, darwin, windows, maybe bsd
  kernel apis: fsevents, inotify, readdirectorychanges
  non-blocking: yes
  dependencies: none
  tests: some (js bindings)
  static analysis: none (interpreted language)

https://github.com/atom/watcher:
  lines of code: 7789
  lines of tests: 1864
  lines of docs: 1334
  implementation languages: cpp
  interface languages: js
  supported platforms: linux, darwin, windows, maybe bsd
  kernel apis: inotify, fsevents, readdirectorychanges
  non-blocking: yes
  dependencies: none
  tests: some (js bindings)
  static analysis: none

https://github.com/paulmillr/chokidar:
  lines of code: 1544
  lines of tests: 1823
  lines of docs: 1377
  implementation languages: js
  interface languages: js
  supported platforms: linux, darwin, windows, bsd
  kernel apis: fsevents
  non-blocking: maybe
  dependencies: yes
  tests: yes (many)
  static analysis: none (interpreted language)

https://github.com/Axosoft/nsfw:
  lines of code: 2536
  lines of tests: 1085
  lines of docs: 148
  implementation languages: cpp
  interface languages: js
  supported platforms: linux, darwin, windows, maybe bsd
  kernel apis: fsevents
  non-blocking: maybe
  dependencies: yes (many)
  tests: yes (js bindings)
  static analysis: none

https://github.com/canton7/SyncTrayzor:
  lines of code: 17102
  lines of tests: 0
  lines of docs: 2303
  implementation languages: c#
  interface languages: c#
  supported platforms: windows
  kernel apis: unknown
  non-blocking: yes
  dependencies: unknown
  tests: none
  static analysis: none (managed language)

https://github.com/g0t4/Rx-FileSystemWatcher:
  lines of code: 360
  lines of tests: 0
  lines of docs: 46
  implementation languages: c#
  interface languages: c#
  supported platforms: windows
  kernel apis: unknown
  non-blocking: yes
  dependencies: unknown
  tests: yes
  static analysis: none (managed language)