Document which versions of Synapse have compatible schema versions. (#16661)

6 files changed, 216 insertions, 1 deletions

diff --git a/.github/workflows/docs-pr.yaml b/.github/workflows/docs-pr.yaml
index 3704bd66e2..9cf3d340a4 100644
--- a/.github/workflows/docs-pr.yaml
+++ b/.github/workflows/docs-pr.yaml
@@ -6,6 +6,7 @@ on:
       - docs/**
       - book.toml
       - .github/workflows/docs-pr.yaml
+      - scripts-dev/schema_versions.py
 
 jobs:
   pages:
@@ -13,12 +14,22 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          # Fetch all history so that the schema_versions script works.
+          fetch-depth: 0
 
       - name: Setup mdbook
         uses: peaceiris/actions-mdbook@adeb05db28a0c0004681db83893d56c0388ea9ea # v1.2.0
         with:
           mdbook-version: '0.4.17'
 
+      - name: Setup python
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.x"
+
+      - run: "pip install 'packaging>=20.0' 'GitPython>=3.1.20'"
+
       - name: Build the documentation
         # mdbook will only create an index.html if we're including docs/README.md in SUMMARY.md.
         # However, we're using docs/README.md for other purposes and need to pick a new page
diff --git a/.github/workflows/docs.yaml b/.github/workflows/docs.yaml
index c7cb2d78e5..31b9dbe3fe 100644
--- a/.github/workflows/docs.yaml
+++ b/.github/workflows/docs.yaml
@@ -51,12 +51,22 @@ jobs:
       - pre
     steps:
       - uses: actions/checkout@v4
+        with:
+          # Fetch all history so that the schema_versions script works.
+          fetch-depth: 0
 
       - name: Setup mdbook
         uses: peaceiris/actions-mdbook@adeb05db28a0c0004681db83893d56c0388ea9ea # v1.2.0
         with:
           mdbook-version: '0.4.17'
 
+      - name: Setup python
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.x"
+
+      - run: "pip install 'packaging>=20.0' 'GitPython>=3.1.20'"
+
       - name: Build the documentation
         # mdbook will only create an index.html if we're including docs/README.md in SUMMARY.md.
         # However, we're using docs/README.md for other purposes and need to pick a new page
diff --git a/book.toml b/book.toml
index fa83d86ffc..ed3f6151e0 100644
--- a/book.toml
+++ b/book.toml
@@ -36,4 +36,7 @@ additional-css = [
     "docs/website_files/indent-section-headers.css",
 ]
 additional-js = ["docs/website_files/table-of-contents.js"]
-theme = "docs/website_files/theme"
\ No newline at end of file
+theme = "docs/website_files/theme"
+
+[preprocessor.schema_versions]
+command = "./scripts-dev/schema_versions.py"
diff --git a/changelog.d/16661.doc b/changelog.d/16661.doc
new file mode 100644
index 0000000000..74f8fc84b8
--- /dev/null
+++ b/changelog.d/16661.doc
@@ -0,0 +1 @@
+Add schema rollback information to documentation.
diff --git a/docs/upgrade.md b/docs/upgrade.md
index ba2f7703bc..329c9c7787 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -88,6 +88,15 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+Generally Synapse database schemas are compatible across multiple versions, once
+a version of Synapse is deployed you may not be able to rollback automatically.
+The following table gives the version ranges and the earliest version they can
+be rolled back to. E.g. Synapse versions v1.58.0 through v1.61.1 can be rolled
+back safely to v1.57.0, but starting with v1.62.0 it is only safe to rollback to
+v1.61.0.
+
+<!-- REPLACE_WITH_SCHEMA_VERSIONS -->
+
 # Upgrading to v1.93.0
 
 ## Minimum supported Rust version
diff --git a/scripts-dev/schema_versions.py b/scripts-dev/schema_versions.py
new file mode 100755
index 0000000000..5fd73251cd
--- /dev/null
+++ b/scripts-dev/schema_versions.py
@@ -0,0 +1,181 @@
+#!/usr/bin/env python
+# Copyright 2023 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""A script to calculate which versions of Synapse have backwards-compatible
+database schemas. It creates a Markdown table of Synapse versions and the earliest
+compatible version.
+
+It is compatible with the mdbook protocol for preprocessors (see
+https://rust-lang.github.io/mdBook/for_developers/preprocessors.html#implementing-a-preprocessor-with-a-different-language):
+
+Exit 0 to denote support for all renderers:
+
+    ./scripts-dev/schema_versions.py supports <mdbook renderer>
+
+Parse a JSON list from stdin and add the table to the proper documetnation page:
+
+    ./scripts-dev/schema_versions.py
+
+Additionally, the script supports dumping the table to stdout for debugging:
+
+    ./scripts-dev/schema_versions.py dump
+"""
+
+import io
+import json
+import sys
+from collections import defaultdict
+from typing import Any, Dict, Iterator, Optional, Tuple
+
+import git
+from packaging import version
+
+# The schema version has moved around over the years.
+SCHEMA_VERSION_FILES = (
+    "synapse/storage/schema/__init__.py",
+    "synapse/storage/prepare_database.py",
+    "synapse/storage/__init__.py",
+    "synapse/app/homeserver.py",
+)
+
+
+# Skip versions of Synapse < v1.0, they're old and essentially not
+# compatible with today's federation.
+OLDEST_SHOWN_VERSION = version.parse("v1.0")
+
+
+def get_schema_versions(tag: git.Tag) -> Tuple[Optional[int], Optional[int]]:
+    """Get the schema and schema compat versions for a tag."""
+    schema_version = None
+    schema_compat_version = None
+
+    for file in SCHEMA_VERSION_FILES:
+        try:
+            schema_file = tag.commit.tree / file
+        except KeyError:
+            continue
+
+        # We (usually) can't execute the code since it might have unknown imports.
+        if file != "synapse/storage/schema/__init__.py":
+            with io.BytesIO(schema_file.data_stream.read()) as f:
+                for line in f.readlines():
+                    if line.startswith(b"SCHEMA_VERSION"):
+                        schema_version = int(line.split()[2])
+
+                        # Bail early.
+                        break
+        else:
+            # SCHEMA_COMPAT_VERSION is sometimes across multiple lines, the easist
+            # thing to do is exec the code. Luckily it has only ever existed in
+            # a file which imports nothing else from Synapse.
+            locals: Dict[str, Any] = {}
+            exec(schema_file.data_stream.read().decode("utf-8"), {}, locals)
+            schema_version = locals["SCHEMA_VERSION"]
+            schema_compat_version = locals.get("SCHEMA_COMPAT_VERSION")
+
+    return schema_version, schema_compat_version
+
+
+def get_tags(repo: git.Repo) -> Iterator[git.Tag]:
+    """Return an iterator of tags sorted by version."""
+    tags = []
+    for tag in repo.tags:
+        # All "real" Synapse tags are of the form vX.Y.Z.
+        if not tag.name.startswith("v"):
+            continue
+
+        # There's a weird tag from the initial react UI.
+        if tag.name == "v0.1":
+            continue
+
+        try:
+            tag_version = version.parse(tag.name)
+        except version.InvalidVersion:
+            # Skip invalid versions.
+            continue
+
+        # Skip pre- and post-release versions.
+        if tag_version.is_prerelease or tag_version.is_postrelease or tag_version.local:
+            continue
+
+        # Skip old versions.
+        if tag_version < OLDEST_SHOWN_VERSION:
+            continue
+
+        tags.append((tag_version, tag))
+
+    # Sort based on the version number (not lexically).
+    return (tag for _, tag in sorted(tags, key=lambda t: t[0]))
+
+
+def calculate_version_chart() -> str:
+    repo = git.Repo(path=".")
+
+    # Map of schema version -> Synapse versions which are at that schema version.
+    schema_versions = defaultdict(list)
+    # Map of schema version -> Synapse versions which are compatible with that
+    # schema version.
+    schema_compat_versions = defaultdict(list)
+
+    # Find ranges of versions which are compatible with a schema version.
+    #
+    # There are two modes of operation:
+    #
+    # 1. Pre-schema_compat_version (i.e. schema_compat_version of None), then
+    #    Synapse is compatible up/downgrading to a version with
+    #    schema_version >= its current version.
+    #
+    # 2. Post-schema_compat_version (i.e. schema_compat_version is *not* None),
+    #    then Synapse is compatible up/downgrading to a version with
+    #    schema version >= schema_compat_version.
+    #
+    #    This is more generous and avoids versions that cannot be rolled back.
+    #
+    # See https://github.com/matrix-org/synapse/pull/9933 which was included in v1.37.0.
+    for tag in get_tags(repo):
+        schema_version, schema_compat_version = get_schema_versions(tag)
+
+        # If a schema compat version is given, prefer that over the schema version.
+        schema_versions[schema_version].append(tag.name)
+        schema_compat_versions[schema_compat_version or schema_version].append(tag.name)
+
+    # Generate a table which maps the latest Synapse version compatible with each
+    # schema version.
+    result = f"| {'Versions': ^19} | Compatible version |\n"
+    result += f"|{'-' * (19 + 2)}|{'-' * (18 + 2)}|\n"
+    for schema_version, synapse_versions in schema_compat_versions.items():
+        result += f"| {synapse_versions[0] + ' – ' + synapse_versions[-1]: ^19} | {schema_versions[schema_version][0]: ^18} |\n"
+
+    return result
+
+
+if __name__ == "__main__":
+    if len(sys.argv) == 3 and sys.argv[1] == "supports":
+        # We don't care about the renderer which is being used, which is the second argument.
+        sys.exit(0)
+    elif len(sys.argv) == 2 and sys.argv[1] == "dump":
+        print(calculate_version_chart())
+    else:
+        # Expect JSON data on stdin.
+        context, book = json.load(sys.stdin)
+
+        for section in book["sections"]:
+            if "Chapter" in section and section["Chapter"]["path"] == "upgrade.md":
+                section["Chapter"]["content"] = section["Chapter"]["content"].replace(
+                    "<!-- REPLACE_WITH_SCHEMA_VERSIONS -->", calculate_version_chart()
+                )
+
+        # Print the result back out to stdout.
+        print(json.dumps(book))