Add API/ABI checker (using LVC)

v2 (by azat):
- drop package installations
- use local .abi-check over $HOME/abi-check for build dir
- drop regex check, simply use default values (too complex otherwise)
- use sub-shell to avoid cd back
- add missing quotes
- make the style uniq across the whole file (no tabs for indent, copy-paste?)
- drop `set -x`, use `bash -x abi-check.sh` over
- drop EVENT_ABI_CHECK
- use /usr/bin/env bash as shebang
- use `find | xargs` over `cp $(grep -v)`
- adjust markdown syntax in abi-check/README.md
- adjust link to the publicly available documentation

v3 (by azat):
- docker image
- git check-ignore
- make -j8
- allow to change defaults

Refs: #887
(cherry picked from commit 735c891e71dc7de737dc09cdef26683eea394e66)

extra/abi-check/README.md

@@ -0,0 +1,35 @@
+## libevent ABI/API changes
+
+
+This script is used to generate information about changes in libevent ABI/API
+between various versions using [LVC tools](https://github.com/lvc). Such an
+overview can help developers migrate from one version to another.
+
+Here is the `abi_check.sh`, which is used to generate ABI/API timeline for
+libevent.
+
+You can limit the number of included libevent versions via a number given
+as a parameter to the script. For example
+
+```shell
+$ ./abi_check.sh 3
+```
+
+generates overview for the last 3 versions and the current version.
+If no parameter given, it will generate overview for the last 2 versions and
+the current version by default.
+
+But this script requires some tools that are available in the following docker image:
+
+```
+docker.pkg.github.com/azat/docker-images/lvc-debian
+```
+
+And the full command looks like:
+
+```shell
+  docker run --rm -it -v $PWD:/src:ro -w /src -v tmp/le-abi-check-root:/abi-root -e ABI_CHECK_ROOT=/abi-root docker.pkg.github.com/azat/docker-images/lvc-debian /src/extra/abi-check/abi_check.sh
+```
+
+'timeline/libevent/index.html' is the final result and can be viewed
+[here](https://libevent.org/abi)

extra/abi-check/abi_check.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+
+# Requirements:
+# - wdiff
+# - rfcdiff
+# - universal-ctags
+# - abi-tracker
+# - abi-monitor
+# - git
+#
+# All of this are included in:
+#   docker.pkg.github.com/azat/docker-images/lvc-debian:latest
+#
+# TODO:
+# - move image into libevent namespace
+
+# verify backward compatibility of API/ABI changes
+
+set -e
+
+LIMIT=${1:-2}
+EVENT_SOURCE_DIR=${EVENT_SOURCE_DIR:-"$(cd "$(dirname "$0")"/../.. && pwd)"}
+ABI_CHECK_ROOT=${ABI_CHECK_ROOT:-$EVENT_SOURCE_DIR/.abi-check}
+ABI_CHECK_WORKSPACE=${ABI_CHECK_WORKSPACE:-"work/abi-check"}
+
+mkdir -p "$ABI_CHECK_ROOT/$ABI_CHECK_WORKSPACE"
+cd "$ABI_CHECK_ROOT/$ABI_CHECK_WORKSPACE"
+
+# copy current source code and profile into workspace
+mkdir -p src/libevent/current
+mkdir -p installed/libevent/current
+( # to avoid cd back
+  cd "$EVENT_SOURCE_DIR"
+  # XXX: not `git archive` since it will not copy changes that are not in index,
+  # and maybe some issues on CI (since it does not contain full clone)
+  find . -maxdepth 1 -mindepth 1 | {
+    git check-ignore --no-index --verbose --non-matching --stdin
+  } | fgrep :: | cut -f2 | grep -v /.git/ | tee /dev/stderr | {
+    xargs cp -r -t "$ABI_CHECK_ROOT/$ABI_CHECK_WORKSPACE/src/libevent/current/"
+  }
+  cp extra/abi-check/libevent.json "$ABI_CHECK_ROOT/$ABI_CHECK_WORKSPACE/"
+)
+
+# run LVC tools
+abi-monitor -get -limit "$LIMIT" libevent.json
+abi-monitor -make -j8 -v current -build libevent.json
+abi-monitor -make -j8 -build libevent.json
+abi-tracker -build libevent.json
+
+# remove useless files
+rm -rf src installed build_logs libevent.json

extra/abi-check/libevent.json

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+{
+  "Name":           "libevent",
+  "Title":          "Libevent",
+  "SourceUrl":      "https://github.com/libevent/libevent/tags",
+  "Git":            "https://github.com/libevent/libevent.git",
+  "Maintainer":     "Nick Mathewson, Azat Khuzhin and Niels Provos",
+  "MaintainerUrl":  "https://libevent.org",
+  "BuildSystem":    "Autotools",
+  "HeadersDiff":    "On",
+  "Package":        "release-",
+  "ReleasePattern": "\A([\d\-\.\_]+)(|\-beta|\-rc|\-stable)\Z",
+}