ci: introduce towncrier to generate changelog from fragments

.changelog/.gitignore

@@ -0,0 +1 @@
+!.gitignore

.github/workflows/changelog.yml

@@ -1,6 +1,6 @@
-# This action requires that any PR targeting the main branch should touch at
-# least one CHANGELOG file. If a CHANGELOG entry is not required, add the "Skip
-# Changelog" label to disable this action.
+# This action requires that any PR targeting the main branch should add a
+# changelog fragment file in the .changelog/ directory. If a changelog entry
+# is not required, add the "Skip Changelog" label to disable this action.
 
 name: changelog
 
@@ -22,18 +22,40 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
-      - name: Check for CHANGELOG changes
+      - name: Fetch base branch
+        run: git fetch origin ${{ github.base_ref }} --depth=1
+
+      - name: Ensure no direct changes to CHANGELOG.md
         run: |
-          # Only the latest commit of the feature branch is available
-          # automatically. To diff with the base branch, we need to
-          # fetch that too (and we only need its latest commit).
-          git fetch origin ${{ github.base_ref }} --depth=1
-          if [[ $(git diff --name-only FETCH_HEAD | grep CHANGELOG) ]]
+          if [[ $(git diff --name-only FETCH_HEAD -- 'CHANGELOG.md') ]]
           then
-            echo "A CHANGELOG was modified. Looks good!"
-          else
-            echo "No CHANGELOG was modified."
-            echo "Please add a CHANGELOG entry, or add the \"Skip Changelog\" label if not required."
+            echo "CHANGELOG.md should not be directly modified."
+            echo "Please add a changelog fragment file to the .changelog/ directory instead."
+            echo ""
+            echo "Create a fragment with:"
+            echo "  tox -e new-changelog -- ${{ github.event.pull_request.number }} TYPE \"Description\""
+            echo "where TYPE is one of: added, changed, deprecated, removed, fixed"
+            echo ""
+            echo "Or add the \"Skip Changelog\" label if this job should be skipped."
+            false
+          fi
+
+      - name: Install towncrier
+        run: pip install towncrier
+
+      - name: Check for changelog fragment
+        run: |
+          if ! towncrier check --compare-with origin/${{ github.base_ref }}; then
+            echo ""
+            echo "No changelog fragment found for this PR."
+            echo ""
+            echo "Create a fragment with:"
+            echo "  tox -e new-changelog -- ${{ github.event.pull_request.number }} TYPE \"Description\""
+            echo "where TYPE is one of: added, changed, deprecated, removed, fixed"
+            echo ""
+            echo "Or add the \"Skip Changelog\" label if this job should be skipped."
             false
           fi

.github/workflows/prepare-patch-release.yml

@@ -14,20 +14,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install toml
-        run: pip install toml
+      - name: Install dependencies
+        run: pip install toml towncrier
 
       - run: |
           if [[ ! $GITHUB_REF_NAME =~ ^release/v[0-9]+\.[0-9]+\.x-0\.[0-9]+bx$ ]]; then
             echo this workflow should only be run against long-term release branches
             exit 1
           fi
 
-          if ! grep --quiet "^## Unreleased$" CHANGELOG.md; then
-            echo the change log is missing an \"Unreleased\" section
-            exit 1
-          fi
-
       - name: Set environment variables
         run: |
           stable_version=$(./scripts/eachdist.py version --mode stable)
@@ -62,10 +57,8 @@ jobs:
       - name: Update version
         run: .github/scripts/update-version-patch.sh $STABLE_VERSION $UNSTABLE_VERSION $STABLE_VERSION_PREV $UNSTABLE_VERSION_PREV
 
-      - name: Update the change log with the approximate release date
-        run: |
-          date=$(date "+%Y-%m-%d")
-          sed -Ei "s/^## Unreleased$/## Version ${STABLE_VERSION}\/${UNSTABLE_VERSION} ($date)/" CHANGELOG.md
+      - name: Generate changelog
+        run: towncrier build --yes --version "$STABLE_VERSION/$UNSTABLE_VERSION"
 
       - name: Use CLA approved github bot
         run: .github/scripts/use-cla-approved-github-bot.sh
@@ -99,3 +92,31 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           gh pr edit ${{ steps.create_pr.outputs.pr_url }}  --add-label "prepare-release"
+
+      - uses: actions/checkout@v4
+        with:
+          ref: main
+
+      - name: Use CLA approved github bot
+        run: .github/scripts/use-cla-approved-github-bot.sh
+
+      - name: Backport patch release changelog to main
+        env:
+          GITHUB_TOKEN: ${{ steps.otelbot-token.outputs.token }}
+        run: |
+          release_branch="otelbot/prepare-release-${STABLE_VERSION}-${UNSTABLE_VERSION}"
+          message="Backport ${STABLE_VERSION}/${UNSTABLE_VERSION} changelog"
+          body="Backport \`${STABLE_VERSION}/${UNSTABLE_VERSION}\` changelog"
+          branch="otelbot/backport-changelog-from-${STABLE_VERSION}-${UNSTABLE_VERSION}"
+
+          git fetch origin $release_branch
+
+          # Copy the updated CHANGELOG.md from the release branch
+          git checkout $release_branch -- CHANGELOG.md
+          git commit -m "$message"
+
+          git push origin HEAD:$branch
+          gh pr create --title "$message" \
+                       --body "$body" \
+                       --head $branch \
+                       --base main

.github/workflows/prepare-release-branch.yml

@@ -27,11 +27,6 @@ jobs:
             exit 1
           fi
 
-          if ! grep --quiet "^## Unreleased$" CHANGELOG.md; then
-            echo the change log is missing an \"Unreleased\" section
-            exit 1
-          fi
-
           if [[ ! -z $PRERELEASE_VERSION ]]; then
             stable_version=$(./scripts/eachdist.py version --mode stable)
             stable_version=${stable_version//.dev/}
@@ -50,8 +45,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install toml
-        run: pip install toml
+      - name: Install dependencies
+        run: pip install toml towncrier
 
       - name: Create release branch
         env:
@@ -88,10 +83,8 @@ jobs:
       - name: Update version
         run: .github/scripts/update-version.sh $STABLE_VERSION $UNSTABLE_VERSION
 
-      - name: Update the change log with the approximate release date
-        run: |
-          date=$(date "+%Y-%m-%d")
-          sed -Ei "s/^## Unreleased$/## Version ${STABLE_VERSION}\/${UNSTABLE_VERSION} ($date)/" CHANGELOG.md
+      - name: Generate changelog
+        run: towncrier build --yes --version "$STABLE_VERSION/$UNSTABLE_VERSION"
 
       - name: Use CLA approved github bot
         run: .github/scripts/use-cla-approved-github-bot.sh
@@ -135,8 +128,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install toml
-        run: pip install toml
+      - name: Install dependencies
+        run: pip install toml towncrier
 
       - name: Set environment variables
         env:
@@ -184,11 +177,8 @@ jobs:
       - name: Update version
         run: .github/scripts/update-version.sh $STABLE_NEXT_VERSION $UNSTABLE_NEXT_VERSION
 
-      - name: Update the change log on main
-        run: |
-          # the actual release date on main will be updated at the end of the release workflow
-          date=$(date "+%Y-%m-%d")
-          sed -Ei "s/^## Unreleased$/## Unreleased\n\n## Version ${STABLE_VERSION}\/${UNSTABLE_VERSION} ($date)/" CHANGELOG.md
+      - name: Generate changelog
+        run: towncrier build --yes --version "$STABLE_VERSION/$UNSTABLE_VERSION"
 
       - name: Use CLA approved github bot
         run: .github/scripts/use-cla-approved-github-bot.sh

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 All notable changes to this project will be documented in this file.
 
+<!--
+Do *NOT* add changelog entries here!
+
+This changelog is managed by towncrier and is compiled at release time.
+
+See https://github.com/open-telemetry/opentelemetry-python/blob/main/CONTRIBUTING.md#changelog for details.
+-->
+
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
@@ -10,6 +18,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 > [!IMPORTANT]
 > We are working on stabilizing the Log signal that would require making deprecations and breaking changes. We will try to reduce the releases that may require an update to your code, especially for instrumentations or for sdk developers.
 
+<!-- changelog start -->
+
 ## Unreleased
 
 - `opentelemetry-sdk`: fix multi-processor `force_flush` skipping remaining processors when one returns `None`

CONTRIBUTING.md

@@ -73,6 +73,37 @@ You can run `tox` with the following arguments:
 - `tox -e tracecontext` to run integration tests for tracecontext.
 - `tox -e precommit` to run all `pre-commit` actions
 
+### Changelog
+
+This project uses [towncrier](https://towncrier.readthedocs.io/) to manage the changelog. Instead of editing `CHANGELOG.md` directly, each PR should include a changelog fragment file in the `.changelog/` directory.
+
+**Creating a changelog fragment:**
+
+Create a file named `.changelog/<PR_NUMBER>.<TYPE>` where `TYPE` is one of: `added`, `changed`, `deprecated`, `removed`, `fixed`.
+
+The file should contain a one-line description of the change. For example, `.changelog/1234.added`:
+
+```
+`opentelemetry-sdk`: add support for new feature
+```
+
+**Writing a good changelog entry:**
+
+- Write in imperative tone, as if completing the phrase "This change will..."
+- Keep entries concise — ideally under 80 characters
+- Prefix with the affected package name when applicable (e.g. `` `opentelemetry-sdk`: ... ``)
+- Don't include the PR number — towncrier adds it automatically
+
+**Preview the changelog:**
+
+```console
+towncrier build --draft --version Unreleased
+```
+
+The CI will verify that a changelog fragment exists and that `CHANGELOG.md` is not directly modified.
+
+If your change does not need a changelog entry, add the "Skip Changelog" label to the PR.
+
 `ruff check` and `ruff format` are executed when `tox -e ruff` is run. We strongly recommend you to configure [pre-commit](https://pre-commit.com/) locally to run `ruff` and `rstcheck` automatically before each commit by installing it as git hooks. You just need to [install pre-commit](https://pre-commit.com/#install) in your environment:
 
 ```console

pyproject.toml

@@ -165,11 +165,46 @@ target-python-version = "3.10"
 custom-template-dir = "opentelemetry-sdk/codegen"
 additional-imports = "typing.ClassVar,opentelemetry.sdk._configuration._common._additional_properties"
 
+[tool.towncrier]
+directory = ".changelog"
+filename = "CHANGELOG.md"
+start_string = "<!-- changelog start -->\n"
+template = "scripts/changelog_template.j2"
+issue_format = "[#{issue}](https://github.com/open-telemetry/opentelemetry-python/pull/{issue})"
+wrap = true # wrap fragments to 79 char line length
+issue_pattern = "^(\\d+)" # only PR numbers as fragment prefix (e.g. 1234.fixed)
+
+[[tool.towncrier.type]]
+directory = "added"
+name = "Added"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "changed"
+name = "Changed"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "deprecated"
+name = "Deprecated"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "removed"
+name = "Removed"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "fixed"
+name = "Fixed"
+showcontent = true
+
 [dependency-groups]
 dev = [
     "tox",
     "tox-uv>=1",
     "pre-commit",
     "datamodel-code-generator[http]",
     "datamodel-code-generator[ruff]",
+    "towncrier",
 ]

scripts/changelog_template.j2

@@ -0,0 +1,24 @@
+## Version {{ versiondata.version }} ({{ versiondata.date }})
+
+{% for section, _ in sections.items() %}
+{%- if section %}{{ section }}{% endif -%}
+
+{% if sections[section] %}
+{% for category, val in definitions.items() if category in sections[section] %}
+### {{ definitions[category]['name'] }}
+
+{% for text, values in sections[section][category].items() %}
+{% if "\n  - " in text or '\n  * ' in text %}
+{%- set main_text, sub_items = text.split('\n', 1) %}
+- {{ main_text }} ({{ values|join(', ') }})
+{% if sub_items %}
+  {{- sub_items }}
+{% endif %}
+{% else %}
+- {{ text }} ({{ values|join(', ') }})
+{% endif %}
+{% endfor %}
+
+{% endfor %}
+{% endif %}
+{% endfor %}