Add experimental SQLITE_FCNTL_EXTERNAL_READER file control.

FossilOrigin-Name: e16da5af822ef31d7e05992403cf9787fbb3d9abb0b5283aba55ea07e1830a72

test/external_reader.test

@@ -0,0 +1,74 @@
+# 2021 April 2
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+source $testdir/lock_common.tcl
+set testprefix external_reader
+
+ifcapable !wal {
+  finish_test 
+  return 
+}
+if {$::tcl_platform(platform)!="unix"} {
+  finish_test 
+  return
+}
+
+do_multiclient_test tn {
+
+  set bExternal 1
+  if {[info commands db3]!=""} { set bExternal 0 }
+
+  do_test 1.$tn.0 {
+    sql1 {
+      PRAGMA journal_mode = wal;
+      CREATE TABLE t1(a, b);
+      INSERT INTO t1 VALUES(1, 2);
+    }
+  } {wal}
+
+  do_test 1.$tn.1 {
+    sql2 { SELECT * FROM t1 }
+  } {1 2}
+
+  do_test 1.$tn.2 {
+    code1 {
+      file_control_external_reader db
+    } 
+  } {0}
+
+  do_test 1.$tn.3 {
+    sql2 { 
+      BEGIN;
+        SELECT * FROM t1;
+    }
+  } {1 2}
+
+  do_test 1.$tn.4 {
+    code1 {
+      file_control_external_reader db
+    } 
+  } $bExternal
+
+  do_test 1.$tn.5 {
+    sql2 { COMMIT }
+  } {}
+
+  do_test 1.$tn.6 {
+    code1 { file_control_external_reader db } 
+  } 0
+
+}
+
+
+finish_test