Mx offset bug (#4775)

Fix mx_offset_to_flags_offset() function

Fixes issue #4774

Postgres `MXOffsetToFlagsOffset` was not correctly converted to Rust
because cast to u16 is done before division by modulo. It is possible
only if divider is power of two.

Add a small rust unit test to check that the function produces same results
as the PostgreSQL macro, and extend the existing python test to cover
this bug.

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

libs/postgres_ffi/src/nonrelfile_utils.rs

@@ -57,9 +57,9 @@ pub fn slru_may_delete_clogsegment(segpage: u32, cutoff_page: u32) -> bool {
 // Multixact utils
 
 pub fn mx_offset_to_flags_offset(xid: MultiXactId) -> usize {
-    ((xid / pg_constants::MULTIXACT_MEMBERS_PER_MEMBERGROUP as u32) as u16
-        % pg_constants::MULTIXACT_MEMBERGROUPS_PER_PAGE
-        * pg_constants::MULTIXACT_MEMBERGROUP_SIZE) as usize
+    ((xid / pg_constants::MULTIXACT_MEMBERS_PER_MEMBERGROUP as u32)
+        % pg_constants::MULTIXACT_MEMBERGROUPS_PER_PAGE as u32
+        * pg_constants::MULTIXACT_MEMBERGROUP_SIZE as u32) as usize
 }
 
 pub fn mx_offset_to_flags_bitshift(xid: MultiXactId) -> u16 {
@@ -81,3 +81,41 @@ fn mx_offset_to_member_page(xid: u32) -> u32 {
 pub fn mx_offset_to_member_segment(xid: u32) -> i32 {
     (mx_offset_to_member_page(xid) / pg_constants::SLRU_PAGES_PER_SEGMENT) as i32
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_multixid_calc() {
+        // Check that the mx_offset_* functions produce the same values as the
+        // corresponding PostgreSQL C macros (MXOffsetTo*). These test values
+        // were generated by calling the PostgreSQL macros with a little C
+        // program.
+        assert_eq!(mx_offset_to_member_segment(0), 0);
+        assert_eq!(mx_offset_to_member_page(0), 0);
+        assert_eq!(mx_offset_to_flags_offset(0), 0);
+        assert_eq!(mx_offset_to_flags_bitshift(0), 0);
+        assert_eq!(mx_offset_to_member_offset(0), 4);
+        assert_eq!(mx_offset_to_member_segment(1), 0);
+        assert_eq!(mx_offset_to_member_page(1), 0);
+        assert_eq!(mx_offset_to_flags_offset(1), 0);
+        assert_eq!(mx_offset_to_flags_bitshift(1), 8);
+        assert_eq!(mx_offset_to_member_offset(1), 8);
+        assert_eq!(mx_offset_to_member_segment(123456789), 2358);
+        assert_eq!(mx_offset_to_member_page(123456789), 75462);
+        assert_eq!(mx_offset_to_flags_offset(123456789), 4780);
+        assert_eq!(mx_offset_to_flags_bitshift(123456789), 8);
+        assert_eq!(mx_offset_to_member_offset(123456789), 4788);
+        assert_eq!(mx_offset_to_member_segment(u32::MAX - 1), 82040);
+        assert_eq!(mx_offset_to_member_page(u32::MAX - 1), 2625285);
+        assert_eq!(mx_offset_to_flags_offset(u32::MAX - 1), 5160);
+        assert_eq!(mx_offset_to_flags_bitshift(u32::MAX - 1), 16);
+        assert_eq!(mx_offset_to_member_offset(u32::MAX - 1), 5172);
+        assert_eq!(mx_offset_to_member_segment(u32::MAX), 82040);
+        assert_eq!(mx_offset_to_member_page(u32::MAX), 2625285);
+        assert_eq!(mx_offset_to_flags_offset(u32::MAX), 5160);
+        assert_eq!(mx_offset_to_flags_bitshift(u32::MAX), 24);
+        assert_eq!(mx_offset_to_member_offset(u32::MAX), 5176);
+    }
+}

test_runner/regress/test_multixact.py

@@ -8,6 +8,10 @@ from fixtures.utils import query_scalar
 # Now this test is very minimalistic -
 # it only checks next_multixact_id field in restored pg_control,
 # since we don't have functions to check multixact internals.
+# We do check that the datadir contents exported from the
+# pageserver match what the running PostgreSQL produced. This
+# is enough to verify that the WAL records are handled correctly
+# in the pageserver.
 #
 def test_multixact(neon_simple_env: NeonEnv, test_output_dir):
     env = neon_simple_env
@@ -18,8 +22,8 @@ def test_multixact(neon_simple_env: NeonEnv, test_output_dir):
     cur = endpoint.connect().cursor()
     cur.execute(
         """
-        CREATE TABLE t1(i int primary key);
-        INSERT INTO t1 select * from generate_series(1, 100);
+        CREATE TABLE t1(i int primary key, n_updated int);
+        INSERT INTO t1 select g, 0 from generate_series(1, 50) g;
     """
     )
 
@@ -29,6 +33,7 @@ def test_multixact(neon_simple_env: NeonEnv, test_output_dir):
 
     # Lock entries using parallel connections in a round-robin fashion.
     nclients = 20
+    update_every = 97
     connections = []
     for _ in range(nclients):
         # Do not turn on autocommit. We want to hold the key-share locks.
@@ -36,14 +41,20 @@ def test_multixact(neon_simple_env: NeonEnv, test_output_dir):
         connections.append(conn)
 
     # On each iteration, we commit the previous transaction on a connection,
-    # and issue antoher select. Each SELECT generates a new multixact that
+    # and issue another select. Each SELECT generates a new multixact that
     # includes the new XID, and the XIDs of all the other parallel transactions.
     # This generates enough traffic on both multixact offsets and members SLRUs
     # to cross page boundaries.
-    for i in range(5000):
+    for i in range(20000):
         conn = connections[i % nclients]
         conn.commit()
-        conn.cursor().execute("select * from t1 for key share")
+
+        # Perform some non-key UPDATEs too, to exercise different multixact
+        # member statuses.
+        if i % update_every == 0:
+            conn.cursor().execute(f"update t1 set n_updated = n_updated + 1 where i = {i % 50}")
+        else:
+            conn.cursor().execute("select * from t1 for key share")
 
     # We have multixacts now. We can close the connections.
     for c in connections: