docs: add changelog

[skip ci]

Author: tomeon

CHANGELOG.md

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+# Changelog for `anything-sync-daemon`
+
+All notable changes to this project will be documented in this file.
+
+## [Unreleased]
+
+### Added
+
+- Support running as an unprivileged user.
+- Document usage in [a Markdown file](/USAGE.md) readable in GitHub's web UI.
+- Document significant environment variables in the manual page.
+- Add [a Nix derivation](/nix/packages.nix) for `anything-sync-daemon`.
+- Add [NixOS modules](/nix/nixos-modules.nix) for managing
+  `anything-sync-daemon`.
+- Support OverlayFS when running as an unprivileged user by managing mounts
+  through the new [`asd-mount-helper`](/common/asd-mount-helper) script.
+  For this to work, the user in question must be able to run `asd-mount-helper`
+  as root via `sudo` without entering a password.
+- Install the example systemd unit files to the `/user/` unit file hierarchy
+  when executing the `install-systemd` Make target.
+
+### Fixed
+
+- Only unmount bind mounts if they have the expected source, and only unmount
+  volatile paths if they are OverlayFS mountpoints.
+- Ensure that `asd.conf`'s parent directory exists before attempt to copy
+  `asd.conf` into it.
+- Deduplicate sync targets (entries in the `WHATTOSYNC` array).  This means
+  testing for string equality on said targets' canonicalized representations,
+  as determined by `realpath -m` or `readlink -m`, if those commands are
+  available and respect the `-m` ("canonicalize missing") option, or otherwise
+  by a native shell routine that simply removes trailing slashes.
+  `anything-sync-daemon` issues a non-fatal warning if it detects duplicate
+  sync targets.
+- Ensure that all intended mount points are mounted as expected when performing
+  the "ungraceful state" check.  This means checking that bind mounts exist and
+  have the expected source (e.g. `/foo/bar/.baz-backup_asd-old` has the source
+  `/foo/bar/baz`) and that (when `USE_OVERLAYFS` is in effect) overlay mount
+  targets have the same mount options as their mount sources.  This makes it
+  possible for `asd` to recover from partial mount states -- that is, to
+  sensibly re-mount intended mount points before performing re-synchronization.
+- In the "ungraceful state" check, only unmount intended mountpoints if they
+  are mounted as expected (in the manner described immediately above).  This
+  means that `asd` is no longer liable to unmount other filesystems that may
+  happen to be mounted in the relevant locations.
+- Install the `asd.conf` parent directory before attempting to copy `asd.conf`
+  into place.
+
+### Changed
+
+- **BREAKING CHANGE**: Load `asd.conf` in a subprocess. This is a breaking
+  change for configurations that attempt to inspect or modify the
+  `anything-sync-daemon` script's internal state.
+- **BREAKING CHANGE**: Enforce a timeout of configurable
+  duration on loading `asd.conf` operation.  This is a breaking change for
+  configurations that do not load within said timeout, though this can be
+  addressed by specifying a longer timeout in the `ASDCONFTIMEOUT` environment
+  variable.
+- **BREAKING CHANGE**: Use the portable shebang `#!/usr/bin/env bash` rather
+  than using `#!/bin/bash`.  This is a breaking change for any setups that (a)
+  have a `bash` under an entry in `PATH` that takes precedence over
+  `/bin/bash`, and (b) are compatible with `/bin/bash` but not the
+  higher-precedence `bash`.  It is also incompatible with setups that lack
+  `/bin` in `PATH`.
+- Respect [the `NO_COLOR` environment variable](https://no-color.org); that is,
+  do not colorize output if `NO_COLOR` is set to a non-empty value.
+- **BREAKING CHANGE**: Attempt to obtain an exclusive lock in-process (via
+  `flock -n <file-descriptor>`) rather than by re-executing when the `FLOCKER`
+  environment variable is unset or empty.  This breaks setups that bypass
+  locking by setting `FLOCKER` to a non-empty value.
+- **BREAKING CHANGE**: detect the presence of `asd.service` by running
+  `systemd list-unit-files asd.service`, in addition to checking for the unit
+  file at `/usr/lib/systemd/system/asd.service` (or, if running `asd` as a
+  non-root user, `/usr/lib/systemd/user/asd.service`).  This makes `asd` liable
+  to conclude that it is being run under systemd when previously it would have
+  concluded otherwise.
+- **BREAKING CHANGE**: detect the `INVOCATION_ID` environment variable and, if
+  it is present, determine the name of the corresponding systemd service, if
+  one exists.  If a service is detected, assume that `asd` is running under
+  systemd.  As with the item just above, this makes `asd` liable to conclude
+  that it is being run under systemd when previously it would have concluded
+  otherwise.
+
+### Removed
+
+- Removed the `asd.conf` "syntax check".  Now, `anything-sync-daemon` accepts
+  any valid Bash code in `asd.conf`.