MySQL: treat the MYSQL_FIELD as read-only

The MariaDB-connector-c version 3.2 and MariaDB server version 10.6
cooperate to avoid re-transferring the query metadata, so the fact that
we were modifying it was causing it to improperly decode the DATETIME
data types into string, as we had asked. We ended up with a 7-byte
string that was actually the date binary-encoded.

References:
 - https://jira.mariadb.org/browse/MDEV-26271
 - https://github.com/MythTV/mythtv/issues/373
 - https://bugs.kde.org/show_bug.cgi?id=440296

Fixes: QTBUG-95639
Change-Id: I4a40ccbd3321467a8429fffd169afdbb26cf29f6
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
(cherry picked from commit 549ee216fd5bf2b3810e940bcbd4bbd8c64ac73f)
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 17 insertions, 22 deletions

diff --git a/src/plugins/sqldrivers/mysql/qsql_mysql.cpp b/src/plugins/sqldrivers/mysql/qsql_mysql.cpp
index f2ae3fbc47..e43c196c10 100644
--- a/src/plugins/sqldrivers/mysql/qsql_mysql.cpp
+++ b/src/plugins/sqldrivers/mysql/qsql_mysql.cpp
@@ -234,14 +234,14 @@ public:
     struct QMyField
     {
         QMyField()
-            : outField(0), nullIndicator(false), bufLength(0ul),
-              myField(0), type(QVariant::Invalid)
+            : outField(0), myField(Q_NULLPTR), bufLength(0ul),
+              type(QVariant::Invalid), nullIndicator(false)
         {}
         char *outField;
-        my_bool nullIndicator;
+        const MYSQL_FIELD *myField;
         ulong bufLength;
-        MYSQL_FIELD *myField;
         QVariant::Type type;
+        my_bool nullIndicator;
     };
 
     QVector<QMyField> fields;
@@ -378,12 +378,10 @@ static bool qIsInteger(int t)
 
 void QMYSQLResultPrivate::bindBlobs()
 {
-    int i;
-    MYSQL_FIELD *fieldInfo;
     MYSQL_BIND *bind;
 
-    for(i = 0; i < fields.count(); ++i) {
-        fieldInfo = fields.at(i).myField;
+    for (int i = 0; i < fields.count(); ++i) {
+        const MYSQL_FIELD *fieldInfo = fields.at(i).myField;
         if (qIsBlob(inBinds[i].buffer_type) && meta && fieldInfo) {
             bind = &inBinds[i];
             bind->buffer_length = fieldInfo->max_length;
@@ -396,8 +394,6 @@ void QMYSQLResultPrivate::bindBlobs()
 
 bool QMYSQLResultPrivate::bindInValues()
 {
-    MYSQL_BIND *bind;
-    char *field;
     int i = 0;
 
     if (!meta)
@@ -410,35 +406,34 @@ bool QMYSQLResultPrivate::bindInValues()
     inBinds = new MYSQL_BIND[fields.size()];
     memset(inBinds, 0, fields.size() * sizeof(MYSQL_BIND));
 
-    MYSQL_FIELD *fieldInfo;
+    const MYSQL_FIELD *fieldInfo;
 
     while((fieldInfo = mysql_fetch_field(meta))) {
+        MYSQL_BIND *bind = &inBinds[i];
+
         QMyField &f = fields[i];
         f.myField = fieldInfo;
-
+        bind->buffer_length = f.bufLength = fieldInfo->length + 1;
+        bind->buffer_type = fieldInfo->type;
         f.type = qDecodeMYSQLType(fieldInfo->type, fieldInfo->flags);
         if (qIsBlob(fieldInfo->type)) {
             // the size of a blob-field is available as soon as we call
             // mysql_stmt_store_result()
             // after mysql_stmt_exec() in QMYSQLResult::exec()
-            fieldInfo->length = 0;
+            bind->buffer_length = f.bufLength = 0;
             hasBlobs = true;
         } else if (qIsInteger(f.type)) {
-            fieldInfo->length = 8;
+            bind->buffer_length = f.bufLength = 8;
         } else {
-            fieldInfo->type = MYSQL_TYPE_STRING;
+            bind->buffer_type = MYSQL_TYPE_STRING;
         }
-        bind = &inBinds[i];
-        field = new char[fieldInfo->length + 1];
-        memset(field, 0, fieldInfo->length + 1);
 
-        bind->buffer_type = fieldInfo->type;
-        bind->buffer = field;
-        bind->buffer_length = f.bufLength = fieldInfo->length + 1;
         bind->is_null = &f.nullIndicator;
         bind->length = &f.bufLength;
         bind->is_unsigned = fieldInfo->flags & UNSIGNED_FLAG ? 1 : 0;
-        f.outField=field;
+
+        char *field = new char[bind->buffer_length + 1]{};
+        bind->buffer = f.outField = field;
 
         ++i;
     }