kv: assert txn unused in SetFixedTimestamp

[nvanbenschoten]

This commit asserts that a transaction has not been used to read or to
write by the time that SetFixedTimestamp is called on it.

This was extracted from #68194 and modified to return an error from
SetFixedTimestamp on misuse instead of fatal-ing. This provides a
sufficient, temporary backstop for #68216 until the conn executor logic
is fixed:

[email protected]:26257/movr> create table t (x int);
CREATE TABLE

[email protected]:26257/movr> insert into t values (1);
INSERT 1

[email protected]:26257/movr> select crdb_internal_mvcc_timestamp, * from t;
   crdb_internal_mvcc_timestamp  | x
---------------------------------+----
  1628094563935439000.0000000000 | 1
(1 row)

[email protected]:26257/movr> begin as of system time (1628094563935439000.0000000000-1)::string;
BEGIN

[email protected]:26257/movr  OPEN> select * from t;
  x
-----
(0 rows)

[email protected]:26257/movr  OPEN> prepare y as select * from t as of system time 1628094563935439000.0000000000;
ERROR: internal error: cannot set fixed timestamp, txn "sql txn" meta={id=e5e81c19 pri=0.01517572 epo=0 ts=1628094563.935438999,0 min=1628094563.935438999,0 seq=0} lock=false stat=PENDING rts=1628094563.935438999,0 wto=false gul=1628094563.935438999,0 already performed reads
SQLSTATE: XX000
DETAIL: stack trace:
github.com/cockroachdb/cockroach/pkg/kv/kvclient/kvcoord/txn_coord_sender.go:1016: SetFixedTimestamp()
github.com/cockroachdb/cockroach/pkg/kv/txn.go:1200: SetFixedTimestamp()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:278: populatePrepared()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:220: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:226: prepare()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_prepare.go:112: addPreparedStmt()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:570: execStmtInOpenState()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:126: execStmt()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1626: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1628: execCmd()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1550: run()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:627: ServeConn()
github.com/cockroachdb/cockroach/pkg/sql/pgwire/conn.go:645: func1()
runtime/asm_amd64.s:1371: goexit()

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[email protected]:26257/? ERROR>

[cockroach-teamcity]

This change is 

[knz]

modulo a clarification on an edge case

Reviewed 31 of 31 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @AlexTalks, @jordanlewis, @nvanbenschoten, and @shermanCRL)

---

pkg/sql/txn_state.go, line 272 at r1 (raw file):

	ts.mu.Lock()
	defer ts.mu.Unlock()
	ts.isHistorical = true

do you want to set isHistorical even if there's an error returned?

[nvanbenschoten]

Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @AlexTalks, @jordanlewis, and @knz)

---

pkg/sql/txn_state.go, line 272 at r1 (raw file):

Previously, knz (kena) wrote…

do you want to set isHistorical even if there's an error returned?

Good point, done.

[jordanlewis]

cc @ajwerner

Reviewable status: complete! 1 of 0 LGTMs obtained (and 1 stale) (waiting on @AlexTalks and @knz)

[nvanbenschoten]

TFTRs!

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)