More mutex locking for SCH_FIELDs.

Fixes: lp:1822678
* https://bugs.launchpad.net/kicad/+bug/1822678

eeschema/sch_field.cpp

@@ -79,6 +79,10 @@ EDA_ITEM* SCH_FIELD::Clone() const
 
 const wxString SCH_FIELD::GetFullyQualifiedText() const
 {
+    // When in UTF-8 mode, wxString puts string iterators in a linked list, and
+    // that linked list is not thread-safe.
+    std::lock_guard<std::mutex> guard( m_mutex );
+
     wxString text = m_Text;
 
     /* For more than one part per package, we must add the part selection
@@ -98,6 +102,36 @@ const wxString SCH_FIELD::GetFullyQualifiedText() const
 }
 
 
+const wxString SCH_FIELD::GetText() const
+{
+    // When in UTF-8 mode, wxString puts string iterators in a linked list, and
+    // that linked list is not thread-safe.
+    std::lock_guard<std::mutex> guard( m_mutex );
+
+    return m_Text;
+}
+
+
+wxString SCH_FIELD::GetShownText() const
+{
+    // When in UTF-8 mode, wxString puts string iterators in a linked list, and
+    // that linked list is not thread-safe.
+    std::lock_guard<std::mutex> guard( m_mutex );
+
+    return m_Text;
+}
+
+
+void SCH_FIELD::SetText( const wxString& aText )
+{
+    // When in UTF-8 mode, wxString puts string iterators in a linked list, and
+    // that linked list is not thread-safe.
+    std::lock_guard<std::mutex> guard( m_mutex );
+
+    m_Text = aText;
+}
+
+
 int SCH_FIELD::GetPenSize() const
 {
     int pensize = GetThickness();

eeschema/sch_field.h

@@ -90,6 +90,10 @@ public:
 
     void SetId( int aId ) { m_id = aId; }
 
+    const wxString GetText() const override;
+    wxString GetShownText() const override;
+    void SetText( const wxString& aText ) override;
+
     /**
      * Function GetFullyQualifiedText
      * returns the fully qualified field text by allowing for the part suffix to be added

include/eda_text.h

@@ -139,8 +139,12 @@ public:
      * returns the string associated with the text object.
      *
      * @return a const wxString reference containing the string of the item.
+     *
+     * WARNING: this routine MUST NOT return a reference!
+     * While it would be faster, it would also defeat the use of RAII mutex locks
+     * in subclasses to get around wxString's multi-threading bugs in UTF-8 mode.
      */
-    const wxString& GetText() const { return m_Text; }
+    virtual const wxString/* & forbidden */ GetText() const { return m_Text; }
 
     /**
      * Returns the string actually shown after processing of the base