Detect SSI conflicts before reporting constraint violations

While prior to this patch the user-visible effect on the database
of any set of successfully committed serializable transactions was
always consistent with some one-at-a-time order of execution of
those transactions, the presence of declarative constraints could
allow errors to occur which were not possible in any such ordering,
and developers had no good workarounds to prevent user-facing
errors where they were not necessary or desired.  This patch adds
a check for serialization failure ahead of duplicate key checking
so that if a developer explicitly (redundantly) checks for the
pre-existing value they will get the desired serialization failure
where the problem is caused by a concurrent serializable
transaction; otherwise they will get a duplicate key error.

While it would be better if the reads performed by the constraints
could count as part of the work of the transaction for
serialization failure checking, and we will hopefully get there
some day, this patch allows a clean and reliable way for developers
to work around the issue.  In many cases existing code will already
be doing the right thing for this to "just work".

Author: Thomas Munro, with minor editing of docs by me
Reviewed-by: Marko Tiikkaja, Kevin Grittner

doc/src/sgml/mvcc.sgml

@@ -644,7 +644,7 @@ ERROR:  could not serialize access due to read/write dependencies among transact
     first.  In <productname>PostgreSQL</productname> these locks do not
     cause any blocking and therefore can <emphasis>not</> play any part in
     causing a deadlock.  They are used to identify and flag dependencies
-    among concurrent serializable transactions which in certain combinations
+    among concurrent Serializable transactions which in certain combinations
     can lead to serialization anomalies.  In contrast, a Read Committed or
     Repeatable Read transaction which wants to ensure data consistency may
     need to take out a lock on an entire table, which could block other
@@ -679,12 +679,13 @@ ERROR:  could not serialize access due to read/write dependencies among transact
 
    <para>
     Consistent use of Serializable transactions can simplify development.
-    The guarantee that any set of concurrent serializable transactions will
-    have the same effect as if they were run one at a time means that if
-    you can demonstrate that a single transaction, as written, will do the
-    right thing when run by itself, you can have confidence that it will
-    do the right thing in any mix of serializable transactions, even without
-    any information about what those other transactions might do.  It is
+    The guarantee that any set of successfully committed concurrent
+    Serializable transactions will have the same effect as if they were run
+    one at a time means that if you can demonstrate that a single transaction,
+    as written, will do the right thing when run by itself, you can have
+    confidence that it will do the right thing in any mix of Serializable
+    transactions, even without any information about what those other
+    transactions might do, or it will not successfully commit.  It is
     important that an environment which uses this technique have a
     generalized way of handling serialization failures (which always return
     with a SQLSTATE value of '40001'), because it will be very hard to
@@ -698,6 +699,26 @@ ERROR:  could not serialize access due to read/write dependencies among transact
     for some environments.
    </para>
 
+   <para>
+    While <productname>PostgreSQL</>'s Serializable transaction isolation
+    level only allows concurrent transactions to commit if it can prove there
+    is a serial order of execution that would produce the same effect, it
+    doesn't always prevent errors from being raised that would not occur in
+    true serial execution.  In particular, it is possible to see unique
+    constraint violations caused by conflicts with overlapping Serializable
+    transactions even after explicitly checking that the key isn't present
+    before attempting to insert it.  This can be avoided by making sure
+    that <emphasis>all</> Serializable transactions that insert potentially
+    conflicting keys explicitly check if they can do so first.  For example,
+    imagine an application that asks the user for a new key and then checks
+    that it doesn't exist already by trying to select it first, or generates
+    a new key by selecting the maximum existing key and adding one.  If some
+    Serializable transactions insert new keys directly without following this
+    protocol, unique constraints violations might be reported even in cases
+    where they could not occur in a serial execution of the concurrent
+    transactions.
+   </para>
+
    <para>
     For optimal performance when relying on Serializable transactions for
     concurrency control, these issues should be considered:

src/backend/access/nbtree/nbtinsert.c

@@ -391,6 +391,14 @@ _bt_check_unique(Relation rel, IndexTuple itup, Relation heapRel,
 						break;
 					}
 
+					/*
+					 * Check for a conflict-in as we would if we were going to
+					 * write to this page.  We aren't actually going to write,
+					 * but we want a chance to report SSI conflicts that would
+					 * otherwise be masked by this unique constraint violation.
+					 */
+					CheckForSerializableConflictIn(rel, NULL, buf);
+
 					/*
 					 * This is a definite conflict.  Break the tuple down into
 					 * datums and report the error.  But first, make sure we

src/test/isolation/expected/read-write-unique-2.out

+Parsed test spec with 2 sessions
+
+starting permutation: r1 r2 w1 w2 c1 c2
+step r1: SELECT * FROM test WHERE i = 42;
+i              
+
+step r2: SELECT * FROM test WHERE i = 42;
+i              
+
+step w1: INSERT INTO test VALUES (42);
+step w2: INSERT INTO test VALUES (42); <waiting ...>
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR:  could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
+
+starting permutation: r1 w1 c1 r2 w2 c2
+step r1: SELECT * FROM test WHERE i = 42;
+i              
+
+step w1: INSERT INTO test VALUES (42);
+step c1: COMMIT;
+step r2: SELECT * FROM test WHERE i = 42;
+i              
+
+42             
+step w2: INSERT INTO test VALUES (42);
+ERROR:  duplicate key value violates unique constraint "test_pkey"
+step c2: COMMIT;

src/test/isolation/expected/read-write-unique-3.out

+Parsed test spec with 2 sessions
+
+starting permutation: rw1 rw2 c1 c2
+step rw1: SELECT insert_unique(1, '1');
+insert_unique  
+
+               
+step rw2: SELECT insert_unique(1, '2'); <waiting ...>
+step c1: COMMIT;
+step rw2: <... completed>
+error in steps c1 rw2: ERROR:  could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;

src/test/isolation/expected/read-write-unique-4.out

+Parsed test spec with 2 sessions
+
+starting permutation: r1 r2 w1 w2 c1 c2
+step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce       
+
+3              
+step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce       
+
+3              
+step w1: INSERT INTO invoice VALUES (2016, 3);
+step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR:  could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
+
+starting permutation: r1 w1 w2 c1 c2
+step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce       
+
+3              
+step w1: INSERT INTO invoice VALUES (2016, 3);
+step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR:  duplicate key value violates unique constraint "invoice_pkey"
+step c2: COMMIT;
+
+starting permutation: r2 w1 w2 c1 c2
+step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce       
+
+3              
+step w1: INSERT INTO invoice VALUES (2016, 3);
+step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR:  duplicate key value violates unique constraint "invoice_pkey"
+step c2: COMMIT;

src/test/isolation/expected/read-write-unique.out

+Parsed test spec with 2 sessions
+
+starting permutation: r1 r2 w1 w2 c1 c2
+step r1: SELECT * FROM test;
+i              
+
+step r2: SELECT * FROM test;
+i              
+
+step w1: INSERT INTO test VALUES (42);
+step w2: INSERT INTO test VALUES (42); <waiting ...>
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR:  could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
+
+starting permutation: r1 w1 c1 r2 w2 c2
+step r1: SELECT * FROM test;
+i              
+
+step w1: INSERT INTO test VALUES (42);
+step c1: COMMIT;
+step r2: SELECT * FROM test;
+i              
+
+42             
+step w2: INSERT INTO test VALUES (42);
+ERROR:  duplicate key value violates unique constraint "test_pkey"
+step c2: COMMIT;

src/test/isolation/isolation_schedule

+test: read-write-unique
+test: read-write-unique-2
+test: read-write-unique-3
+test: read-write-unique-4
 test: simple-write-skew
 test: receipt-report
 test: temporal-range-integrity

src/test/isolation/specs/read-write-unique-2.spec

+# Read-write-unique test.
+
+setup
+{
+  CREATE TABLE test (i integer PRIMARY KEY);
+}
+
+teardown
+{
+  DROP TABLE test;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r1" { SELECT * FROM test WHERE i = 42; }
+step "w1" { INSERT INTO test VALUES (42); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r2" { SELECT * FROM test WHERE i = 42; }
+step "w2" { INSERT INTO test VALUES (42); }
+step "c2" { COMMIT; }
+
+# Two SSI transactions see that there is no row with value 42
+# in the table, then try to insert that value; T1 inserts,
+# and then T2 blocks waiting for T1 to commit.  Finally,
+# T2 reports a serialization failure.
+
+permutation "r1" "r2" "w1" "w2" "c1" "c2"
+
+# If the value is already visible before T2 begins, then a
+# regular unique constraint violation should still be raised
+# by T2.
+
+permutation "r1" "w1" "c1" "r2" "w2" "c2"

src/test/isolation/specs/read-write-unique-3.spec

+# Read-write-unique test.
+# From bug report 9301.
+
+setup
+{
+  CREATE TABLE test (
+    key   integer UNIQUE,
+    val   text
+  );
+
+  CREATE OR REPLACE FUNCTION insert_unique(k integer, v text) RETURNS void
+  LANGUAGE SQL AS $$
+    INSERT INTO test (key, val) SELECT k, v WHERE NOT EXISTS (SELECT key FROM test WHERE key = k);
+  $$;
+}
+
+teardown
+{
+  DROP FUNCTION insert_unique(integer, text);
+  DROP TABLE test;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "rw1" { SELECT insert_unique(1, '1'); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "rw2" { SELECT insert_unique(1, '2'); }
+step "c2" { COMMIT; }
+
+permutation "rw1" "rw2" "c1" "c2"

src/test/isolation/specs/read-write-unique-4.spec

+# Read-write-unique test.
+# Implementing a gapless sequence of ID numbers for each year.
+
+setup
+{
+  CREATE TABLE invoice (
+    year int,
+    invoice_number int,
+    PRIMARY KEY (year, invoice_number)
+  );
+
+  INSERT INTO invoice VALUES (2016, 1), (2016, 2);
+}
+
+teardown
+{
+  DROP TABLE invoice;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r1" { SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016; }
+step "w1" { INSERT INTO invoice VALUES (2016, 3); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r2" { SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016; }
+step "w2" { INSERT INTO invoice VALUES (2016, 3); }
+step "c2" { COMMIT; }
+
+# if they both read first then there should be an SSI conflict
+permutation "r1" "r2" "w1" "w2" "c1" "c2"
+
+# cases where one session doesn't explicitly read before writing:
+
+# if s2 doesn't explicitly read, then trying to insert the value
+# generates a unique constraint violation after s1 commits, as if s2
+# ran after s1
+permutation "r1" "w1" "w2" "c1" "c2"
+
+# if s1 doesn't explicitly read, but s2 does, then s1 inserts and
+# commits first, should s2 experience an SSI failure instead of a
+# unique constraint violation?  there is no serial order of operations
+# (s1, s2) or (s2, s1) where s1 succeeds, and s2 doesn't see the row
+# in an explicit select but then fails to insert due to unique
+# constraint violation
+permutation "r2" "w1" "w2" "c1" "c2"

src/test/isolation/specs/read-write-unique.spec

+# Read-write-unique test.
+
+setup
+{
+  CREATE TABLE test (i integer PRIMARY KEY);
+}
+
+teardown
+{
+  DROP TABLE test;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r1" { SELECT * FROM test; }
+step "w1" { INSERT INTO test VALUES (42); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r2" { SELECT * FROM test; }
+step "w2" { INSERT INTO test VALUES (42); }
+step "c2" { COMMIT; }
+
+# Two SSI transactions see that there is no row with value 42
+# in the table, then try to insert that value; T1 inserts,
+# and then T2 blocks waiting for T1 to commit.  Finally,
+# T2 reports a serialization failure.
+#
+# (In an earlier version of Postgres, T2 would report a unique
+# constraint violation).
+
+permutation "r1" "r2" "w1" "w2" "c1" "c2"
+
+# If the value is already visible before T2 begins, then a
+# regular unique constraint violation should still be raised
+# by T2.
+
+permutation "r1" "w1" "c1" "r2" "w2" "c2"