From e6c12f7474bcbaa18e227780805977c784129997 Mon Sep 17 00:00:00 2001
From: Elvis Pranskevichus <elvis@edgedb.com>
Date: Tue, 21 Jan 2025 13:49:41 -0800
Subject: [PATCH] Allow dropping isolation level to REPEATABLE READ in a READ
 ONLY tx (#8237)

This re-enabled REPEATABLE READ isolation in explicit transaction
blocks, but only in those that are READ ONLY.  Anomalies observed in
read statements are normal non-serializability anomalies, so with an
explicit opt-in it is a reasonable tradeoff between lessening the
transaction serialization load and consistency.
---
 edb/edgeql-parser/src/keywords.rs       |  1 +
 edb/edgeql/parser/grammar/statements.py |  4 ++++
 edb/server/compiler/compiler.py         | 13 +++++++++++
 tests/test_server_proto.py              | 30 +++++++++++++++++++++++++
 4 files changed, 48 insertions(+)

diff --git a/edb/edgeql-parser/src/keywords.rs b/edb/edgeql-parser/src/keywords.rs
index e79393ea8c3..eb416dbd2fc 100644
--- a/edb/edgeql-parser/src/keywords.rs
+++ b/edb/edgeql-parser/src/keywords.rs
@@ -79,6 +79,7 @@ pub const UNRESERVED_KEYWORDS: phf::Set<&str> = phf_set!(
     "reject",
     "release",
     "rename",
+    "repeatable",
     "required",
     "reset",
     "restrict",
diff --git a/edb/edgeql/parser/grammar/statements.py b/edb/edgeql/parser/grammar/statements.py
index dd3cbef702e..3994896a155 100644
--- a/edb/edgeql/parser/grammar/statements.py
+++ b/edb/edgeql/parser/grammar/statements.py
@@ -66,6 +66,10 @@ def reduce_ISOLATION_SERIALIZABLE(self, *kids):
         self.val = (qltypes.TransactionIsolationLevel.SERIALIZABLE,
                     kids[0].span)
 
+    def reduce_ISOLATION_REPEATABLE_READ(self, *kids):
+        self.val = (qltypes.TransactionIsolationLevel.REPEATABLE_READ,
+                    kids[0].span)
+
     def reduce_READ_WRITE(self, *kids):
         self.val = (qltypes.TransactionAccessMode.READ_WRITE,
                     kids[0].span)
diff --git a/edb/server/compiler/compiler.py b/edb/server/compiler/compiler.py
index 1206bbcba82..922d7ecb445 100644
--- a/edb/server/compiler/compiler.py
+++ b/edb/server/compiler/compiler.py
@@ -2153,6 +2153,19 @@ def _compile_ql_transaction(
         ctx.state.start_tx()
 
         sqls = 'START TRANSACTION'
+        iso = ql.isolation
+        if iso is not None:
+            if (
+                iso is not qltypes.TransactionIsolationLevel.SERIALIZABLE
+                and ql.access is not qltypes.TransactionAccessMode.READ_ONLY
+            ):
+                raise errors.TransactionError(
+                    f"{iso.value} transaction isolation level is only "
+                    "supported in read-only transactions",
+                    span=ql.span,
+                    hint=f"specify READ ONLY access mode",
+                )
+            sqls += f' ISOLATION LEVEL {iso.value}'
         if ql.access is not None:
             sqls += f' {ql.access.value}'
         if ql.deferrable is not None:
diff --git a/tests/test_server_proto.py b/tests/test_server_proto.py
index 42f20abab3a..49e5abd1071 100644
--- a/tests/test_server_proto.py
+++ b/tests/test_server_proto.py
@@ -1577,6 +1577,36 @@ async def test_server_proto_tx_07(self):
             await self.con.query('SELECT 42'),
             [42])
 
+    async def test_server_proto_tx_08(self):
+        try:
+            await self.con.query('''
+                START TRANSACTION ISOLATION REPEATABLE READ, READ ONLY;
+            ''')
+
+            self.assertEqual(
+                await self.con.query(
+                    'select <str>sys::get_transaction_isolation();',
+                ),
+                ["RepeatableRead"],
+            )
+        finally:
+            await self.con.query(f'''
+                ROLLBACK;
+            ''')
+
+    async def test_server_proto_tx_09(self):
+        try:
+            with self.assertRaisesRegex(
+                    edgedb.TransactionError,
+                    'only supported in read-only transactions'):
+                await self.con.query('''
+                    START TRANSACTION ISOLATION REPEATABLE READ;
+                ''')
+        finally:
+            await self.con.query(f'''
+                ROLLBACK;
+            ''')
+
     async def test_server_proto_tx_10(self):
         # Basic test that ROLLBACK works on SET ALIAS changes.