add SystemErrno to Error (#740)

* adding SystemErrno to Error, and fixing error logic when open fails

* fix for old versions of libsqlite3 that do not have sqlite3_system_errno defined

* fixing pre-processor logic

sqlite3.go

@@ -183,6 +183,12 @@ static int _sqlite3_limit(sqlite3* db, int limitId, int newLimit) {
   return sqlite3_limit(db, limitId, newLimit);
 #endif
 }
+
+#if SQLITE_VERSION_NUMBER < 3012000
+static int sqlite3_system_errno(sqlite3 *db) {
+  return 0;
+}
+#endif
 */
 import "C"
 import (
@@ -198,6 +204,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 	"unsafe"
 )
@@ -749,15 +756,28 @@ func (c *SQLiteConn) lastError() error {
 	return lastError(c.db)
 }
 
+// Note: may be called with db == nil
 func lastError(db *C.sqlite3) error {
-	rv := C.sqlite3_errcode(db)
+	rv := C.sqlite3_errcode(db) // returns SQLITE_NOMEM if db == nil
 	if rv == C.SQLITE_OK {
 		return nil
 	}
+	extrv := C.sqlite3_extended_errcode(db)    // returns SQLITE_NOMEM if db == nil
+	errStr := C.GoString(C.sqlite3_errmsg(db)) // returns "out of memory" if db == nil
+
+	// https://www.sqlite.org/c3ref/system_errno.html
+	// sqlite3_system_errno is only meaningful if the error code was SQLITE_CANTOPEN,
+	// or it was SQLITE_IOERR and the extended code was not SQLITE_IOERR_NOMEM
+	var systemErrno syscall.Errno
+	if rv == C.SQLITE_CANTOPEN || (rv == C.SQLITE_IOERR && extrv != C.SQLITE_IOERR_NOMEM) {
+		systemErrno = syscall.Errno(C.sqlite3_system_errno(db))
+	}
+
 	return Error{
 		Code:         ErrNo(rv),
-		ExtendedCode: ErrNoExtended(C.sqlite3_extended_errcode(db)),
-		err:          C.GoString(C.sqlite3_errmsg(db)),
+		ExtendedCode: ErrNoExtended(extrv),
+		SystemErrno:  systemErrno,
+		err:          errStr,
 	}
 }
 
@@ -869,10 +889,6 @@ func (c *SQLiteConn) begin(ctx context.Context) (driver.Tx, error) {
 	return &SQLiteTx{c}, nil
 }
 
-func errorString(err Error) string {
-	return C.GoString(C.sqlite3_errstr(C.int(err.Code)))
-}
-
 // Open database and return a new connection.
 //
 // A pragma can take either zero or one argument.
@@ -1342,10 +1358,13 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 		mutex|C.SQLITE_OPEN_READWRITE|C.SQLITE_OPEN_CREATE,
 		nil)
 	if rv != 0 {
+		// Save off the error _before_ closing the database.
+		// This is safe even if db is nil.
+		err := lastError(db)
 		if db != nil {
 			C.sqlite3_close_v2(db)
 		}
-		return nil, Error{Code: ErrNo(rv)}
+		return nil, err
 	}
 	if db == nil {
 		return nil, errors.New("sqlite succeeded without returning a database")