sqlite: disable shared cache by default

The shared cache works well but is probably not the right default.
It has much stricter locking semantics than you get from SQLite
with a WAL database, reducing performance in some important cases.

This appears to be an implementation limit of the shared cache,
not something fundamental to its design. If it ever gets WAL-like
locking it should be turned on by default.

blob_test.go

@@ -107,11 +107,12 @@ func TestBlob(t *testing.T) {
 }
 
 func TestConcurrentBlobSpins(t *testing.T) {
-	c, err := sqlite.OpenConn("file::memory:?mode=memory", 0)
+	flags := sqlite.SQLITE_OPEN_READWRITE | sqlite.SQLITE_OPEN_CREATE | sqlite.SQLITE_OPEN_URI | sqlite.SQLITE_OPEN_NOMUTEX | sqlite.SQLITE_OPEN_SHAREDCACHE
+	c, err := sqlite.OpenConn("file::memory:?mode=memory", flags)
 	if err != nil {
 		t.Fatal(err)
 	}
-	c2, err := sqlite.OpenConn("file::memory:?mode=memory", 0)
+	c2, err := sqlite.OpenConn("file::memory:?mode=memory", flags)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -169,8 +170,10 @@ func TestConcurrentBlobSpins(t *testing.T) {
 // TestConcurrentBlobWrites looks for unexpected SQLITE_LOCKED errors
 // when using the (default) shared cache.
 func TestConcurrentBlobWrites(t *testing.T) {
+	flags := sqlite.SQLITE_OPEN_READWRITE | sqlite.SQLITE_OPEN_CREATE | sqlite.SQLITE_OPEN_URI | sqlite.SQLITE_OPEN_NOMUTEX | sqlite.SQLITE_OPEN_SHAREDCACHE
+
 	const numBlobs = 5
-	c, err := sqlite.OpenConn("file::memory:?mode=memory", 0)
+	c, err := sqlite.OpenConn("file::memory:?mode=memory", flags)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -207,7 +210,7 @@ func TestConcurrentBlobWrites(t *testing.T) {
 			b := make([]byte, 1024)
 			b[0] = byte(i)
 
-			c, err := sqlite.OpenConn("file::memory:?mode=memory", 0)
+			c, err := sqlite.OpenConn("file::memory:?mode=memory", flags)
 			if err != nil {
 				t.Fatal(err)
 			}

pool.go

@@ -54,17 +54,10 @@ type Pool struct {
 //
 //	SQLITE_OPEN_READWRITE
 //	SQLITE_OPEN_CREATE
-//	SQLITE_OPEN_SHAREDCACHE
 //	SQLITE_OPEN_WAL
 //	SQLITE_OPEN_URI
 //	SQLITE_OPEN_NOMUTEX
-//
-// The pool is always created with the shared cache enabled.
 func Open(uri string, flags OpenFlags, poolSize int) (*Pool, error) {
-	if flags == 0 {
-		flags = SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_WAL | SQLITE_OPEN_URI | SQLITE_OPEN_NOMUTEX
-	}
-	flags |= SQLITE_OPEN_SHAREDCACHE
 	if uri == ":memory:" {
 		return nil, strerror{msg: `sqlite: ":memory:" does not work with multiple connections, use "file::memory:?mode=memory"`}
 	}

pool_test.go

@@ -35,7 +35,8 @@ const poolSize = 20
 // any error is t.Fatal.
 func newMemPool(t *testing.T) *sqlite.Pool {
 	t.Helper()
-	dbpool, err := sqlite.Open("file::memory:?mode=memory", 0, poolSize)
+	flags := sqlite.SQLITE_OPEN_READWRITE | sqlite.SQLITE_OPEN_CREATE | sqlite.SQLITE_OPEN_URI | sqlite.SQLITE_OPEN_NOMUTEX | sqlite.SQLITE_OPEN_SHAREDCACHE
+	dbpool, err := sqlite.Open("file::memory:?mode=memory", flags, poolSize)
 	if err != nil {
 		t.Fatal(err)
 	}

sqlite.go

@@ -103,7 +103,6 @@ const (
 //
 //	SQLITE_OPEN_READWRITE
 //	SQLITE_OPEN_CREATE
-//	SQLITE_OPEN_SHAREDCACHE
 //	SQLITE_OPEN_WAL
 //	SQLITE_OPEN_URI
 //	SQLITE_OPEN_NOMUTEX
@@ -116,7 +115,7 @@ func OpenConn(path string, flags OpenFlags) (*Conn, error) {
 func openConn(path string, flags OpenFlags) (*Conn, error) {
 	sqliteInit.Do(sqliteInitFn)
 	if flags == 0 {
-		flags = SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_SHAREDCACHE | SQLITE_OPEN_WAL | SQLITE_OPEN_URI | SQLITE_OPEN_NOMUTEX
+		flags = SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_WAL | SQLITE_OPEN_URI | SQLITE_OPEN_NOMUTEX
 	}
 	conn := &Conn{
 		stmts: make(map[string]*Stmt),