test: show QueryContext cancel/Scan race condition

This test case shows a race condition we see in the test suite of our
application. We already created an issue with the [Go
project](golang/go#23519), but the conclusion was
that the [db drivers should fix
it](golang/go#23519 (comment)).

I was able to reproduce this issue with the following go versions:

- `go version go1.9.3 darwin/amd64` (output shown below)
- `go version devel +5af1e7d78e Sun Jan 28 19:54:48 2018 +0000 darwin/amd64` (latest master)

Please let me know if I can provide more information and/or if you'd
like me to attempt to create a patch to fix the issue.

```
$ while go test -v -race -run 'TestContextCancelQueryWhileScan'; do :; done
=== RUN   TestContextCancelQueryWhileScan
--- PASS: TestContextCancelQueryWhileScan (0.13s)
PASS
ok  	github.com/lib/pq	1.160s
=== RUN   TestContextCancelQueryWhileScan
==================
WARNING: DATA RACE
Read at 0x00c4201ae5ae by goroutine 23:
  runtime.slicebytetostring()
      /usr/local/Cellar/go/1.9.3/libexec/src/runtime/string.go:72 +0x0
  database/sql.convertAssign()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/convert.go:244 +0x2333
  database/sql.(*Rows).Scan()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:2727 +0x295
  github.com/lib/pq.TestContextCancelQueryWhileScan.func1()
      /Users/felixge/code/go/src/github.com/lib/pq/go18_test.go:188 +0x38a

Previous write at 0x00c4201ae5a8 by goroutine 50:
  runtime.slicecopy()
      /usr/local/Cellar/go/1.9.3/libexec/src/runtime/slice.go:160 +0x0
  bufio.(*Reader).Read()
      /usr/local/Cellar/go/1.9.3/libexec/src/bufio/bufio.go:224 +0x240
  io.ReadAtLeast()
      /usr/local/Cellar/go/1.9.3/libexec/src/io/io.go:309 +0x93
  io.ReadFull()
      /usr/local/Cellar/go/1.9.3/libexec/src/io/io.go:327 +0x72
  github.com/lib/pq.(*conn).recvMessage()
      /Users/felixge/code/go/src/github.com/lib/pq/conn.go:971 +0x38a
  github.com/lib/pq.(*conn).recv1Buf()
      /Users/felixge/code/go/src/github.com/lib/pq/conn.go:1007 +0x46
  github.com/lib/pq.(*rows).Next()
      /Users/felixge/code/go/src/github.com/lib/pq/conn.go:1399 +0x13b
  github.com/lib/pq.(*rows).Close()
      /Users/felixge/code/go/src/github.com/lib/pq/conn.go:1356 +0x8f
  database/sql.(*Rows).close.func1()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:2761 +0x6e
  database/sql.withLock()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:2873 +0x76
  database/sql.(*Rows).close()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:2760 +0x1ca
  database/sql.(*Rows).awaitDone()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:2436 +0x14c

Goroutine 23 (running) created at:
  github.com/lib/pq.TestContextCancelQueryWhileScan()
      /Users/felixge/code/go/src/github.com/lib/pq/go18_test.go:174 +0x163
  testing.tRunner()
      /usr/local/Cellar/go/1.9.3/libexec/src/testing/testing.go:746 +0x16c

Goroutine 50 (running) created at:
  database/sql.(*Rows).initContextClose()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:2420 +0xdd
  database/sql.(*DB).queryDC()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:1334 +0xb7f
  database/sql.(*DB).query()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:1304 +0x193
  database/sql.(*DB).QueryContext()
      /usr/local/Cellar/go/1.9.3/libexec/src/database/sql/sql.go:1281 +0xe2
  github.com/lib/pq.TestContextCancelQueryWhileScan.func1()
      /Users/felixge/code/go/src/github.com/lib/pq/go18_test.go:183 +0x22f
==================
--- FAIL: TestContextCancelQueryWhileScan (0.12s)
	testing.go:699: race detected during execution of test
FAIL
exit status 1
FAIL	github.com/lib/pq	0.153s
```

Author: felixge

go18_test.go

@@ -5,8 +5,10 @@ package pq
 import (
 	"context"
 	"database/sql"
+	"math/rand"
 	"runtime"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
@@ -157,6 +159,48 @@ func TestContextCancelQuery(t *testing.T) {
 	}
 }
 
+// TestContextCancelQueryWhileScan checks for race conditions that arise when
+// a query context is canceled while a user is calling rows.Scan(). The code
+// is based on database/sql TestIssue18429.
+// See https://github.com/golang/go/issues/23519
+func TestContextCancelQueryWhileScan(t *testing.T) {
+	db := openTestConn(t)
+	defer db.Close()
+
+	const milliWait = 30
+	sem := make(chan bool, 20)
+
+	// This query seems to work well for triggering race conditions between
+	// the rows.Scan() call below, and the implicit rows.Close() call triggered
+	// by ctx timing out.
+	sql := `SELECT (g/10)::int, json_agg(g) FROM generate_series(1, 1000) g, pg_sleep($1) GROUP BY 1;`
+	var wg sync.WaitGroup
+	for i := 0; i < contextRaceIterations; i++ {
+		sem <- true
+		wg.Add(1)
+		go func() {
+			defer func() {
+				<-sem
+				wg.Done()
+			}()
+			qwait := float64(time.Duration(rand.Intn(milliWait))*time.Millisecond) / 1000
+
+			ctx, cancel := context.WithTimeout(context.Background(), time.Duration(rand.Intn(milliWait))*time.Millisecond)
+			defer cancel()
+			rows, _ := db.QueryContext(ctx, sql, qwait)
+			if rows != nil {
+				var d int
+				var n string
+				for rows.Next() {
+					rows.Scan(&d, &n)
+				}
+				rows.Close()
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 // TestIssue617 tests that a failed query in QueryContext doesn't lead to a
 // goroutine leak.
 func TestIssue617(t *testing.T) {