Enable Sentry's "releases" feature

The releases feature is useful in itself, and this should also stop the
"discarded session update because of missing release" messages that
`sentry_sdk` is logging all the time.

Author: seanh

.coveragerc

@@ -5,6 +5,8 @@ concurrency = gevent
 source =
     viahtml
     tests/unit
+omit =
+    viahtml/_version.py
 
 [report]
 show_missing = True

viahtml/_version.py

@@ -0,0 +1,77 @@
+import datetime
+import subprocess
+from subprocess import DEVNULL
+
+# git-archive substitution markers. When this file is written out by a `git
+# archive` command, these will be replaced by the short commit hash and the
+# commit date, respectively.
+VERSION_GIT_REF = "$Format:%h$"
+VERSION_GIT_DATE = "$Format:%ct$"
+
+# Fallback version in case we cannot derive the version.
+VERSION_UNKNOWN = "0+unknown"
+
+
+def fetch_git_ref():
+    return subprocess.check_output(  # noqa: S603
+        ["git", "rev-parse", "--short", "HEAD"],  # noqa: S607
+        stderr=DEVNULL,
+    ).strip()
+
+
+def fetch_git_date(ref):
+    output = subprocess.check_output(
+        ["git", "show", "-s", "--format=%ct", ref]
+    )  # noqa: S603, S607
+    return datetime.datetime.fromtimestamp(int(output))  # noqa: DTZ006
+
+
+def fetch_git_dirty():
+    # Ensure git index is up-to-date first. This usually isn't necessary, but
+    # can be needed inside a docker container where the index is out of date.
+    subprocess.call(["git", "update-index", "-q", "--refresh"])  # noqa: S603, S607
+    dirty_tree = bool(
+        subprocess.call(["git", "diff-files", "--quiet"])
+    )  # noqa: S603, S607
+    dirty_index = bool(
+        subprocess.call(
+            ["git", "diff-index", "--quiet", "--cached", "HEAD"]
+        )  # noqa: S603, S607
+    )
+    return dirty_tree or dirty_index
+
+
+def git_version():
+    ref = fetch_git_ref()
+    date = fetch_git_date(ref)
+    dirty = fetch_git_dirty()
+    return pep440_version(date, ref, dirty)
+
+
+def git_archive_version():  # pragma: no cover
+    ref = VERSION_GIT_REF
+    date = datetime.datetime.fromtimestamp(int(VERSION_GIT_DATE))  # noqa: DTZ006
+    return pep440_version(date, ref)
+
+
+def pep440_version(date, ref, dirty=False):  # noqa: FBT002
+    """Build a PEP440-compliant version number from the passed information."""
+    return f"{date.strftime('%Y%m%d')}+g{ref}{'.dirty' if dirty else ''}"
+
+
+def get_version():  # pragma: no cover
+    """Fetch the current application version."""
+    # First we try to retrieve the current application version from git.
+    try:
+        return git_version()
+    except subprocess.CalledProcessError:
+        pass
+
+    # We are not in a git checkout or extracting the version from git failed,
+    # so we attempt to read a version written into the header of this file by
+    # `git archive`.
+    if not VERSION_GIT_REF.startswith("$"):
+        return git_archive_version()
+
+    # If neither of these strategies work, we fall back to VERSION_UNKNOWN.
+    return VERSION_UNKNOWN

viahtml/wsgi.py

@@ -3,8 +3,7 @@
 import os
 import signal
 
-# Our job here is to leave this `application` attribute laying around as
-# it's what uWSGI expects to find.
+from viahtml._version import get_version
 from viahtml.app import Application
 
 
@@ -26,7 +25,12 @@ def _dump_stacks(_signal, frame):  # pylint:disable=unused-argument, # pragma: n
     from sentry_sdk.integrations.wsgi import SentryWsgiMiddleware
 
     # pylint: disable=redefined-variable-type
-    sentry_sdk.init(dsn=os.environ["SENTRY_DSN"])
+    sentry_sdk.init(
+        dsn=os.environ["SENTRY_DSN"],
+        # Enable Sentry's "Releases" feature, see:
+        # https://docs.sentry.io/platforms/python/configuration/options/#release
+        release=get_version(),
+    )
     application = SentryWsgiMiddleware(application)
 
 # Add a way to dump stacks so we can see what a uWSGI worker is doing if it