database/sql: fix panic with concurrent Conn and Close

The current implementation has a panic when the database is closed
concurrently with a new connection attempt.

connRequestSet.CloseAndRemoveAll sets connRequestSet.s to a nil slice.
If this happens between calls to connRequestSet.Add and
connRequestSet.Delete, there is a panic when trying to write to the nil
slice. This is sequence is likely to occur in DB.conn, where the mutex
is released between calls to db.connRequests.Add and
db.connRequests.Delete

This change updates connRequestSet.CloseAndRemoveAll to set the curIdx
to -1 for all pending requests before setting its internal slice to nil.
CloseAndRemoveAll already iterates the full slice to close all the request
channels. It seems appropriate to set curIdx to -1 before deleting the
slice for 3 reasons:
1. connRequestSet.deleteIndex also sets curIdx to -1
2. curIdx will not be relevant to anything after the slice is set to nil
3. connRequestSet.Delete already checks for negative indices

Fixes #68949

Author: nklaassen

src/database/sql/sql.go

@@ -1368,8 +1368,8 @@ func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn
 
 			db.waitDuration.Add(int64(time.Since(waitStart)))
 
-			// If we failed to delete it, that means something else
-			// grabbed it and is about to send on it.
+			// If we failed to delete it, that means either the DB was closed or
+			// something else grabbed it and is about to send on it.
 			if !deleted {
 				// TODO(bradfitz): rather than this best effort select, we
 				// should probably start a goroutine to read from req. This best
@@ -3594,6 +3594,7 @@ type connRequestAndIndex struct {
 // and clears the set.
 func (s *connRequestSet) CloseAndRemoveAll() {
 	for _, v := range s.s {
+		*v.curIdx = -1
 		close(v.req)
 	}
 	s.s = nil

src/database/sql/sql_test.go

@@ -4920,6 +4920,17 @@ func TestConnRequestSet(t *testing.T) {
 			t.Error("wasn't random")
 		}
 	})
+	t.Run("close-delete", func(t *testing.T) {
+		reset()
+		ch := make(chan connRequest)
+		dh := s.Add(ch)
+		wantLen(1)
+		s.CloseAndRemoveAll()
+		wantLen(0)
+		if s.Delete(dh) {
+			t.Error("unexpected delete after CloseAndRemoveAll")
+		}
+	})
 }
 
 func BenchmarkConnRequestSet(b *testing.B) {