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| <section id="ffi-framing-internals"> |
| <span id="ffi-internals"></span><h1>FFI framing internals<a class="headerlink" href="#ffi-framing-internals" title="Link to this heading">#</a></h1> |
| <p>Read this before changing how datafusion-python frames FFI components. You do |
| <strong>not</strong> need it to write an extension library — that is the |
| <a class="reference internal" href="../extension-guide/index.html#extension-guide"><span class="std std-ref">Extension Guide</span></a>.</p> |
| <p>The invariants here are the reasons the extension-facing rules are shaped the |
| way they are. Each one has an “obvious” simplification that does not work, and |
| the point of this page is to record why, so the next person does not spend a |
| week rediscovering it.</p> |
| <p>For the wire format itself — how a codec id is stored alongside a payload, |
| routed back on decode, and which two cases stay unframed — see the module |
| documentation in <code class="docutils literal notranslate"><span class="pre">crates/core/src/codec.rs</span></code>. It is the authority; this page |
| does not restate it.</p> |
| <section id="one-session-one-arc-sessioncontext"> |
| <span id="ffi-internals-one-arc"></span><h2>One session, one <code class="docutils literal notranslate"><span class="pre">Arc<SessionContext></span></code><a class="headerlink" href="#one-session-one-arc-sessioncontext" title="Link to this heading">#</a></h2> |
| <p>Every codec handed to a foreign object carries an <code class="docutils literal notranslate"><span class="pre">FFI_TaskContextProvider</span></code>, |
| and that type holds its provider <strong>weakly</strong>. A registered catalog provider |
| upgrades the handle on every <code class="docutils literal notranslate"><span class="pre">supports_filters_pushdown</span></code> and every <code class="docutils literal notranslate"><span class="pre">scan</span></code>. |
| Those handles are bound to one particular <code class="docutils literal notranslate"><span class="pre">Arc<SessionContext></span></code> allocation, not |
| to the logical session, so anything that replaces the allocation orphans all of |
| them and the next query fails with <code class="docutils literal notranslate"><span class="pre">TaskContextProvider</span> <span class="pre">went</span> <span class="pre">out</span> <span class="pre">of</span> <span class="pre">scope</span> <span class="pre">over</span> <span class="pre">FFI</span> <span class="pre">boundary</span></code>.</p> |
| <p>So a <code class="docutils literal notranslate"><span class="pre">PySessionContext</span></code> keeps the <code class="docutils literal notranslate"><span class="pre">Arc<SessionContext></span></code> it was created with for |
| its whole life. Installing a query planner writes the new <code class="docutils literal notranslate"><span class="pre">SessionState</span></code> back |
| through <code class="docutils literal notranslate"><span class="pre">state_ref()</span></code>, exactly as <code class="docutils literal notranslate"><span class="pre">add_physical_optimizer_rule</span></code> does, rather |
| than deriving a replacement context. The session id is carried across that |
| rewrite — <code class="docutils literal notranslate"><span class="pre">SessionStateBuilder</span></code> mints a fresh one otherwise — so <code class="docutils literal notranslate"><span class="pre">session_id()</span></code> |
| and every <code class="docutils literal notranslate"><span class="pre">TaskContext</span></code> the session hands out keep agreeing.</p> |
| <p>Repairing the damage instead of avoiding it does not work in general. A context |
| can rebuild the codecs it holds in its own fields, but a codec already embedded |
| in a registered <code class="docutils literal notranslate"><span class="pre">FFI_CatalogProvider</span></code> — and in every <code class="docutils literal notranslate"><span class="pre">FFI_SchemaProvider</span></code> and |
| <code class="docutils literal notranslate"><span class="pre">FFI_TableProvider</span></code> minted from it — is not reachable from Python at all. Nor |
| can the codec simply retain the session that built it: a codec handed to a |
| provider is routinely registered straight back into that same session, which |
| would close the cycle |
| <code class="docutils literal notranslate"><span class="pre">SessionContext</span> <span class="pre">-></span> <span class="pre">catalog</span> <span class="pre">-></span> <span class="pre">FFI</span> <span class="pre">provider</span> <span class="pre">-></span> <span class="pre">FFI</span> <span class="pre">codec</span> <span class="pre">-></span> <span class="pre">SessionContext</span></code> and |
| leak it.</p> |
| <p><code class="docutils literal notranslate"><span class="pre">SessionContext.enable_url_table</span></code> is the one exception. It clones the |
| underlying <code class="docutils literal notranslate"><span class="pre">SessionContext</span></code>, so the returned context has an allocation of its |
| own and must not outlive the receiver. It also forks the session’s state while |
| keeping its id, so two handles report one <code class="docutils literal notranslate"><span class="pre">session_id()</span></code> with divergent |
| configuration. That is a bug rather than a design, tracked in |
| <a class="reference external" href="https://github.com/apache/datafusion-python/issues/1708">apache/datafusion-python#1708</a>; |
| do not copy the pattern.</p> |
| </section> |
| <section id="why-planner-codec-rebinding-is-one-level-deep"> |
| <span id="ffi-internals-rebinding"></span><h2>Why planner codec rebinding is one level deep<a class="headerlink" href="#why-planner-codec-rebinding-is-one-level-deep" title="Link to this heading">#</a></h2> |
| <p>Installing a codec on a session that already has a foreign planner rebuilds |
| that planner against the new chain. The rebuild swaps the codecs on the |
| installed <code class="docutils literal notranslate"><span class="pre">ForeignQueryPlanner</span></code> handle, and only that handle. A planner that |
| wraps a fallback resolved that fallback when <em>it</em> was installed, and holds the |
| result inside its own library’s private data — behind a <code class="docutils literal notranslate"><span class="pre">create_physical_plan</span></code> |
| function pointer, with no Python-side handle. A codec installed afterwards |
| therefore reaches the outer planner and not the fallback, which keeps whichever |
| codecs were in force when it was imported.</p> |
| <p>Neither side can repair that:</p> |
| <ul class="simple"> |
| <li><p><strong>The host cannot reach it.</strong> <code class="docutils literal notranslate"><span class="pre">FFI_QueryPlanner::new_with_ffi_codecs</span></code> unwraps |
| exactly one <code class="docutils literal notranslate"><span class="pre">ForeignQueryPlanner</span></code> layer. There is no deeper handle to unwrap |
| — the same situation as a codec embedded in a registered |
| <code class="docutils literal notranslate"><span class="pre">FFI_CatalogProvider</span></code>.</p></li> |
| <li><p><strong>The planner library cannot re-derive it.</strong> <code class="docutils literal notranslate"><span class="pre">FFI_QueryPlanner</span></code> holds its |
| codecs by value, and <code class="docutils literal notranslate"><span class="pre">Session</span></code> exposes no accessor for the ones the host |
| currently has, so <code class="docutils literal notranslate"><span class="pre">create_physical_plan</span></code> cannot pick them up from the session |
| it is handed. The rebuild has to be eager, and an eager rebuild only sees the |
| top layer.</p></li> |
| </ul> |
| <p>A fix has to come from upstream, and is tracked in |
| <a class="reference external" href="https://github.com/apache/datafusion/issues/24762">apache/datafusion#24762</a>.</p> |
| <p>The stale codecs stay usable rather than dangling — they hold weak handles to |
| the one <code class="docutils literal notranslate"><span class="pre">Arc<SessionContext></span></code> that the previous section keeps alive — so the |
| effect is a fallback hop serializing with an older codec, not a failure. It is |
| also invisible to the examples in this repository, which use one fallback in |
| the same cdylib as its wrapper; <code class="docutils literal notranslate"><span class="pre">datafusion-ffi</span></code> short-circuits a same-library |
| hop rather than serializing, so no codec runs. A fallback in a <em>different</em> |
| library would serialize, and would do it with the codecs it was imported with.</p> |
| <p>The extension-facing consequence — install codecs before a layered planner, and |
| prefer <code class="docutils literal notranslate"><span class="pre">with_extensions</span></code> — is documented at <a class="reference internal" href="../extension-guide/query-planners.html#planner-codec-rebinding"><span class="std std-ref">Install codecs before a layered planner</span></a>.</p> |
| </section> |
| <section id="why-with-extensions-commits-last"> |
| <span id="ffi-internals-commit-order"></span><h2>Why <code class="docutils literal notranslate"><span class="pre">with_extensions</span></code> commits last<a class="headerlink" href="#why-with-extensions-commits-last" title="Link to this heading">#</a></h2> |
| <p><code class="docutils literal notranslate"><span class="pre">with_extensions</span></code> promises that a bundle which raises leaves the session as it |
| was, apart from anything a hook writes to the context it is handed |
| (<a class="reference internal" href="../extension-guide/bundles.html#extension-bundles-transaction"><span class="std std-ref">Failure and rollback</span></a>). Keeping that promise is an ordering constraint on the implementation, not |
| a property of any one step, because the planner is bound on the shared |
| <code class="docutils literal notranslate"><span class="pre">SessionState</span></code> rather than on the returned handle.</p> |
| <p>A call therefore splits into four steps, of which only the last writes:</p> |
| <ol class="arabic simple"> |
| <li><p><strong>Collect.</strong> Every <code class="docutils literal notranslate"><span class="pre">__datafusion_session_components__</span></code> runs and its codecs |
| are gathered. Nothing is installed yet, so a hook that raises here has |
| touched nothing.</p></li> |
| <li><p><strong>Chains.</strong> The codecs are assembled into the returned handle. Codec chains |
| live on that handle rather than on the session, so this step writes nothing |
| to the session even though it can fail on a bad capsule or a duplicate id.</p></li> |
| <li><p><strong>Resolve.</strong> Every <code class="docutils literal notranslate"><span class="pre">__datafusion_session_planner__</span></code> runs, in argument order, |
| against the completed chains, and each supplied planner is exported to a |
| capsule. Every hook that can raise has run by the end of this step.</p></li> |
| <li><p><strong>Commit.</strong> The accumulated planner is re-imported from its capsule and |
| bound, in a single <code class="docutils literal notranslate"><span class="pre">SessionState</span></code> rebuild. The bind is skipped entirely when |
| the call installed nothing, so an empty call does not drag a planner sitting |
| on another handle’s codecs onto this one’s.</p></li> |
| </ol> |
| <p>Only step 4 touches the session, and it is not itself infallible: |
| <code class="docutils literal notranslate"><span class="pre">_install_extension_planner</span></code> runs <code class="docutils literal notranslate"><span class="pre">ffi_query_planner_from_pycapsule</span></code> before it |
| calls <code class="docutils literal notranslate"><span class="pre">set_session_query_planner</span></code>, which cannot fail. The property that keeps |
| the promise is therefore about order, not about any step being incapable of |
| raising — every fallible operation, including the ones inside the commit, |
| completes before the first write.</p> |
| <p>That is the rule for the next field added to <code class="docutils literal notranslate"><span class="pre">SessionExtensionComponents</span></code>, not |
| only a description of the current code: a new kind of component must do its |
| fallible work — importing a capsule, resolving a name — before anything is |
| written, so no failure can leave the session half-updated.</p> |
| <p>There would be nothing to roll back to if one did. The returned handle shares one |
| session with the receiver, so the damage is visible from every other handle; |
| and undoing a registration is not the same as restoring what it displaced, |
| because deregistering a function that shadowed a built-in removes the built-in |
| too. The split is cheaper than an undo log that cannot be written correctly.</p> |
| <p>The extension-facing statement of this is |
| <a class="reference internal" href="../extension-guide/bundles.html#extension-bundles-transaction"><span class="std std-ref">Failure and rollback</span></a>, which says only that declaring a |
| component is safe where registering one during the hook is not.</p> |
| </section> |
| <section id="two-argument-kinds-for-one-convention"> |
| <h2>Two argument kinds for one convention<a class="headerlink" href="#two-argument-kinds-for-one-convention" title="Link to this heading">#</a></h2> |
| <p><code class="docutils literal notranslate"><span class="pre">CapsuleGetterArg</span></code> in <code class="docutils literal notranslate"><span class="pre">crates/util/src/lib.rs</span></code> distinguishes three cases: no |
| argument, the session, and the host’s logical extension codec as a bare |
| capsule. Provider and catalog registration passes the codec; the extension |
| codecs, the query planner, and table functions get the session.</p> |
| <p>The distinction exists so that a <code class="docutils literal notranslate"><span class="pre">TypeError</span></code> from a getter that refused its |
| argument can be rewritten into an <code class="docutils literal notranslate"><span class="pre">ImportError</span></code> naming what the getter <em>should</em> |
| accept. Getting that message wrong sends an extension author to fix the wrong |
| signature, which is why every capsule getter routes through |
| <code class="docutils literal notranslate"><span class="pre">call_capsule_getter</span></code> rather than calling <code class="docutils literal notranslate"><span class="pre">getattr</span></code> directly. Three importers |
| previously each had their own copy of that logic and each missed later |
| corrections to it.</p> |
| <p>When adding a hook, decide which arm it needs by what its FFI constructor |
| requires: a task-context provider means it must have the session, since you |
| cannot get a provider off a capsule. A codec alone means either works, and |
| passing the codec keeps the host from handing out session handles it does not |
| need to.</p> |
| <p>The extension-facing statement of this is <a class="reference internal" href="../extension-guide/capsule-protocol.html#extension-getter-argument"><span class="std std-ref">What your getter receives</span></a>, |
| which deliberately describes the argument by capability — “something you can |
| read the host’s logical codec off” — rather than by type.</p> |
| </section> |
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