kvclient: fix read-your-own-writes bug in txnWriteBuffer

Previously, we were inadvertently mutating the overlap scan bounds
when merging Scan responses with the buffer. This meant that we could
miss keys that we should've otherwise seen. Put differently, it was
possible for us to miss reading our own writes. I'm surprised we didn't
see this before, but I think it's because the b-tree needs to be
large enough for us to hit this. Either way, this is now fixed.

Epic: none

Release note: None

Author: arulajmani

pkg/kv/kvclient/kvcoord/txn_interceptor_write_buffer.go

@@ -923,6 +923,10 @@ func (twb *txnWriteBuffer) mergeBufferAndResp(
 		it.FirstOverlap(seek)
 	}
 	bufferNext := func() {
+		// NB: we must reset seek before every use, as it's shared across multiple
+		// methods on the txnWriteBuffer. In particular, it's used by
+		// maybeServeRead, which we may call below.
+		seek = twb.seekItemForSpan(respIter.startKey(), respIter.endKey())
 		if reverse {
 			it.PrevOverlap(seek)
 		} else {

pkg/sql/logictest/testdata/logic_test/buffered_writes

@@ -319,3 +319,123 @@ Put /Table/114/3/3/2/0 -> /BYTES/
 
 statement ok
 RESET vectorize
+
+statement ok
+CREATE TABLE uvw (
+  u INT,
+  v INT,
+  w INT
+)
+
+statement ok
+BEGIN;
+INSERT INTO uvw SELECT u, v, w FROM
+  generate_series(0, 3) AS u,
+  generate_series(0, 3) AS v,
+  generate_series(0, 3) AS w;
+
+query I
+SELECT count(*) FROM uvw
+----
+64
+
+statement ok
+COMMIT
+
+statement ok
+DROP TABLE uvw
+
+statement ok
+CREATE TABLE uvw (
+  u INT,
+  v INT,
+  w INT,
+  INDEX (u,v,w)
+)
+
+statement ok
+BEGIN;
+INSERT INTO uvw SELECT u, v, w FROM
+  generate_series(0, 3) AS u,
+  generate_series(0, 3) AS v,
+  generate_series(0, 3) AS w;
+UPDATE uvw SET u = NULL WHERE u = 0;
+UPDATE uvw SET v = NULL WHERE v = 0;
+UPDATE uvw SET w = NULL WHERE w = 0
+
+query III colnames
+SELECT * FROM uvw ORDER BY u, v, w
+----
+u     v     w
+NULL  NULL  NULL
+NULL  NULL  1
+NULL  NULL  2
+NULL  NULL  3
+NULL  1     NULL
+NULL  1     1
+NULL  1     2
+NULL  1     3
+NULL  2     NULL
+NULL  2     1
+NULL  2     2
+NULL  2     3
+NULL  3     NULL
+NULL  3     1
+NULL  3     2
+NULL  3     3
+1     NULL  NULL
+1     NULL  1
+1     NULL  2
+1     NULL  3
+1     1     NULL
+1     1     1
+1     1     2
+1     1     3
+1     2     NULL
+1     2     1
+1     2     2
+1     2     3
+1     3     NULL
+1     3     1
+1     3     2
+1     3     3
+2     NULL  NULL
+2     NULL  1
+2     NULL  2
+2     NULL  3
+2     1     NULL
+2     1     1
+2     1     2
+2     1     3
+2     2     NULL
+2     2     1
+2     2     2
+2     2     3
+2     3     NULL
+2     3     1
+2     3     2
+2     3     3
+3     NULL  NULL
+3     NULL  1
+3     NULL  2
+3     NULL  3
+3     1     NULL
+3     1     1
+3     1     2
+3     1     3
+3     2     NULL
+3     2     1
+3     2     2
+3     2     3
+3     3     NULL
+3     3     1
+3     3     2
+3     3     3
+
+statement ok
+COMMIT
+
+query I
+SELECT count(*) FROM uvw
+----
+64