From 21e9fc5aa7cd1d71de93b50fa1883c1ee056fe07 Mon Sep 17 00:00:00 2001
From: David Robertson <davidr@element.io>
Date: Fri, 8 Jul 2022 14:52:27 +0100
Subject: [PATCH 1/2] Ensure portdb selects rows with negative rowids

The `backward_select` query uses the `backward_chunk` variable as an
inclusive upper bound on the rowids it selects. It is initially 0 (see
`setup_table`). It is then set to

```
backward_chunk = min(row[0] for row in brows) - 1
```

where `brows` is the result of running the `backwards_select` query.
For this to make sense, we need to ensure that `backwards_select` picks
rows in descending order. Otherwise we'll jump right to the bottom of
the rowids, pick out the lowest batch only and discard everything we
skipped over. This is a Bad Thing.

I've tested this locally with the reproduction case reported in #13191.
Without the patch, I could reproduce the reported failure; with the
patch, the portdb script completes successfully.
---
 synapse/_scripts/synapse_port_db.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/synapse/_scripts/synapse_port_db.py b/synapse/_scripts/synapse_port_db.py
index 642fd41629b1..26834a437e9c 100755
--- a/synapse/_scripts/synapse_port_db.py
+++ b/synapse/_scripts/synapse_port_db.py
@@ -418,12 +418,15 @@ async def handle_table(
             self.progress.update(table, table_size)  # Mark table as done
             return
 
+        # We sweep over rowids in two directions: one forwards (rowids 1, 2, 3, ...)
+        # and another backwards (rowids 0, -1, -2, ...).
         forward_select = (
             "SELECT rowid, * FROM %s WHERE rowid >= ? ORDER BY rowid LIMIT ?" % (table,)
         )
 
         backward_select = (
-            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid LIMIT ?" % (table,)
+            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid DESC LIMIT ?"
+            % (table,)
         )
 
         do_forward = [True]

From d1decf65b051d4f57cd2d2fd372e5261c4a6f31e Mon Sep 17 00:00:00 2001
From: David Robertson <davidr@element.io>
Date: Fri, 8 Jul 2022 15:18:28 +0100
Subject: [PATCH 2/2] Changelog

---
 changelog.d/13226.bugfix | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/13226.bugfix

diff --git a/changelog.d/13226.bugfix b/changelog.d/13226.bugfix
new file mode 100644
index 000000000000..df96d41f3779
--- /dev/null
+++ b/changelog.d/13226.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where the `synapse_port_db` script could fail to copy rows with negative row ids.