Update pgvector to v0.6.0, third attempt

This includes a compatibility patch that is needed because pgvector
now skips WAL-logging during the index build, and WAL-logs the index
only in one go at the end. That's how GIN, GiST and SP-GIST index
builds work in core PostgreSQL too, but we need some Neon-specific
calls to mark the beginning and end of those build phases.

pgvector is the first index AM that does that with parallel workers,
so I had to modify those functions in the Neon extension to be aware
of parallel workers. Only the leader needs to create the underlying
file and perform the WAL-logging. (In principle, the parallel workers
could participate in the WAL-logging too, but pgvector doesn't do
that. This will need some further work if that changes).

The previous attempt at this (#6592) missed that parallel workers
needed those changes, and segfaulted in parallel build that spilled to
disk.

Testing
-------

We don't have a place for regression tests of extensions at the
moment. I tested this manually with the following script:

```
CREATE EXTENSION IF NOT EXISTS vector;

DROP TABLE IF EXISTS tst;
CREATE TABLE tst (i serial, v vector(3));

INSERT INTO tst (v) SELECT ARRAY[random(), random(), random()] FROM generate_series(1, 15000) g;

-- Serial build, in memory
ALTER TABLE tst SET (parallel_workers=0);
SET maintenance_work_mem='50 MB';
CREATE INDEX idx ON tst USING hnsw (v vector_l2_ops);

-- Test that the index works. (The table contents are random, and the
-- search is approximate anyway, so we cannot check the exact values.
-- For now, just eyeball that they look reasonable)
set enable_seqscan=off;
explain SELECT * FROM tst ORDER BY v <-> ARRAY[0, 0, 0]::vector LIMIT 5;
SELECT * FROM tst ORDER BY v <-> ARRAY[0, 0, 0]::vector LIMIT 5;

DROP INDEX idx;

-- Serial build, spills to on disk

ALTER TABLE tst SET (parallel_workers=0);
SET maintenance_work_mem='5 MB';
CREATE INDEX idx ON tst USING hnsw (v vector_l2_ops);
SELECT * FROM tst ORDER BY v <-> ARRAY[0, 0, 0]::vector LIMIT 5;
DROP INDEX idx;

-- Parallel build, in memory

ALTER TABLE tst SET (parallel_workers=4);
SET maintenance_work_mem='50 MB';
CREATE INDEX idx ON tst USING hnsw (v vector_l2_ops);
SELECT * FROM tst ORDER BY v <-> ARRAY[0, 0, 0]::vector LIMIT 5;
DROP INDEX idx;

-- Parallel build, spills to disk

ALTER TABLE tst SET (parallel_workers=4);
SET maintenance_work_mem='5 MB';
CREATE INDEX idx ON tst USING hnsw (v vector_l2_ops);
SELECT * FROM tst ORDER BY v <-> ARRAY[0, 0, 0]::vector LIMIT 5;
DROP INDEX idx;
```

.dockerignore

@@ -17,6 +17,7 @@
 !libs/
 !neon_local/
 !pageserver/
+!patches/
 !pgxn/
 !proxy/
 !s3_scrubber/

Dockerfile.compute-node

@@ -241,9 +241,12 @@ RUN wget https://github.com/df7cb/postgresql-unit/archive/refs/tags/7.7.tar.gz -
 FROM build-deps AS vector-pg-build
 COPY --from=pg-build /usr/local/pgsql/ /usr/local/pgsql/
 
-RUN wget https://github.com/pgvector/pgvector/archive/refs/tags/v0.5.1.tar.gz -O pgvector.tar.gz && \
-    echo "cc7a8e034a96e30a819911ac79d32f6bc47bdd1aa2de4d7d4904e26b83209dc8 pgvector.tar.gz" | sha256sum --check && \
+COPY patches/pgvector.patch /pgvector.patch
+
+RUN wget https://github.com/pgvector/pgvector/archive/refs/tags/v0.6.0.tar.gz -O pgvector.tar.gz && \
+    echo "b0cf4ba1ab016335ac8fb1cada0d2106235889a194fffeece217c5bda90b2f19 pgvector.tar.gz" | sha256sum --check && \
     mkdir pgvector-src && cd pgvector-src && tar xvzf ../pgvector.tar.gz --strip-components=1 -C . && \
+    patch -p1 < /pgvector.patch && \
     make -j $(getconf _NPROCESSORS_ONLN) PG_CONFIG=/usr/local/pgsql/bin/pg_config && \
     make -j $(getconf _NPROCESSORS_ONLN) install PG_CONFIG=/usr/local/pgsql/bin/pg_config && \
     echo 'trusted = true' >> /usr/local/pgsql/share/extension/vector.control

patches/pgvector.patch

@@ -0,0 +1,78 @@
+From 0b0194a57bd0f3598bd57dbedd0df3932330169d Mon Sep 17 00:00:00 2001
+From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
+Date: Fri, 2 Feb 2024 22:26:45 +0200
+Subject: [PATCH 1/1] Make v0.6.0 work with Neon
+
+Now that the WAL-logging happens as a separate step at the end of the
+build, we need a few neon-specific hints to make it work.
+---
+ src/hnswbuild.c | 36 ++++++++++++++++++++++++++++++++++++
+ 1 file changed, 36 insertions(+)
+
+diff --git a/src/hnswbuild.c b/src/hnswbuild.c
+index 680789b..ec54dea 100644
+--- a/src/hnswbuild.c
++++ b/src/hnswbuild.c
+@@ -840,9 +840,17 @@ HnswParallelBuildMain(dsm_segment *seg, shm_toc *toc)
+ 
+ 	hnswarea = shm_toc_lookup(toc, PARALLEL_KEY_HNSW_AREA, false);
+ 
++#ifdef NEON_SMGR
++	smgr_start_unlogged_build(RelationGetSmgr(indexRel));
++#endif
++
+ 	/* Perform inserts */
+ 	HnswParallelScanAndInsert(heapRel, indexRel, hnswshared, hnswarea, false);
+ 
++#ifdef NEON_SMGR
++	smgr_finish_unlogged_build_phase_1(RelationGetSmgr(indexRel));
++#endif
++
+ 	/* Close relations within worker */
+ 	index_close(indexRel, indexLockmode);
+ 	table_close(heapRel, heapLockmode);
+@@ -1089,13 +1097,41 @@ BuildIndex(Relation heap, Relation index, IndexInfo *indexInfo,
+ 	SeedRandom(42);
+ #endif
+ 
++#ifdef NEON_SMGR
++	smgr_start_unlogged_build(RelationGetSmgr(index));
++#endif
++
+ 	InitBuildState(buildstate, heap, index, indexInfo, forkNum);
+ 
+ 	BuildGraph(buildstate, forkNum);
+ 
++#ifdef NEON_SMGR
++	smgr_finish_unlogged_build_phase_1(RelationGetSmgr(index));
++#endif
++
+ 	if (RelationNeedsWAL(index))
++	{
+ 		log_newpage_range(index, forkNum, 0, RelationGetNumberOfBlocks(index), true);
+ 
++#ifdef NEON_SMGR
++		{
++#if PG_VERSION_NUM >= 160000
++			RelFileLocator rlocator = RelationGetSmgr(index)->smgr_rlocator.locator;
++#else
++			RelFileNode rlocator = RelationGetSmgr(index)->smgr_rnode.node;
++#endif
++
++			SetLastWrittenLSNForBlockRange(XactLastRecEnd, rlocator,
++										   MAIN_FORKNUM, 0, RelationGetNumberOfBlocks(index));
++			SetLastWrittenLSNForRelation(XactLastRecEnd, rlocator, MAIN_FORKNUM);
++		}
++#endif
++	}
++
++#ifdef NEON_SMGR
++	smgr_end_unlogged_build(RelationGetSmgr(index));
++#endif
++
+ 	FreeBuildState(buildstate);
+ }
+ 
+-- 
+2.39.2
+

pgxn/neon/pagestore_smgr.c

@@ -45,6 +45,7 @@
  */
 #include "postgres.h"
 
+#include "access/parallel.h"
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "access/xlogdefs.h"
@@ -2712,10 +2713,14 @@ neon_start_unlogged_build(SMgrRelation reln)
 	reln->smgr_relpersistence = RELPERSISTENCE_UNLOGGED;
 
 	/*
+	 * Create the local file. In a parallel build, the leader is expected to
+	 * call this first and do it.
+	 *
 	 * FIXME: should we pass isRedo true to create the tablespace dir if it
 	 * doesn't exist? Is it needed?
 	 */
-	mdcreate(reln, MAIN_FORKNUM, false);
+	if (!IsParallelWorker())
+		mdcreate(reln, MAIN_FORKNUM, false);
 }
 
 /*
@@ -2739,7 +2744,17 @@ neon_finish_unlogged_build_phase_1(SMgrRelation reln)
 	Assert(unlogged_build_phase == UNLOGGED_BUILD_PHASE_1);
 	Assert(reln->smgr_relpersistence == RELPERSISTENCE_UNLOGGED);
 
-	unlogged_build_phase = UNLOGGED_BUILD_PHASE_2;
+	/*
+	 * In a parallel build, (only) the leader process performs the 2nd
+	 * phase.
+	 */
+	if (IsParallelWorker())
+	{
+		unlogged_build_rel = NULL;
+		unlogged_build_phase = UNLOGGED_BUILD_NOT_IN_PROGRESS;
+	}
+	else
+		unlogged_build_phase = UNLOGGED_BUILD_PHASE_2;
 }
 
 /*