qml: surface debug log open failures in the UI

DebugLogModel::openLogFile() already detects when the debug.log cannot be
opened (a missing file, or no application associated with the file type):
it sets an openError message, emits openErrorChanged, and returns false.
But SettingsDebugLog discarded the return value and never read openError,
so a failed open gave the user no feedback at all.

Bind the model's openError to an error label on the debug log page, the
same way the wallet backup page surfaces its open failure, so the user
sees why the log did not open.

Cover it both ways: a QML test (with a debug log model stub) checks the
error label shows on a failed open and clears on a later success, and a
C++ unit test checks the model's missing-file path returns false and sets
openError.

Author: epicleafies

qml/pages/settings/SettingsDebugLog.qml

@@ -101,6 +101,20 @@ Page {
         }
         spacing: 0
 
+        CoreText {
+            objectName: "debugLogOpenErrorText"
+            Layout.fillWidth: true
+            Layout.bottomMargin: text.length > 0 ? 10 : 0
+            visible: text.length > 0
+            text: debugLogModel.openError
+            color: Theme.color.red
+            font.pixelSize: 13
+            wrapMode: Text.WordWrap
+            horizontalAlignment: Text.AlignHCenter
+            Accessible.role: Accessible.StaticText
+            Accessible.name: text
+        }
+
         RowLayout {
             id: searchRow
             objectName: "debugLogSearchRow"

test/qml/qml_tests_main.cpp

@@ -3159,7 +3159,7 @@ class MockDebugLogModel : public QAbstractListModel
         m_filter = filter;
         Q_EMIT filterChanged();
     }
-    QString openError() const { return {}; }
+    QString openError() const { return m_open_error; }
     int loadMoreCalls() const { return m_load_more_calls; }
 
     Q_INVOKABLE void refresh(bool = false) {}
@@ -3170,7 +3170,29 @@ class MockDebugLogModel : public QAbstractListModel
         appendRowsForTest(20);
         setHasMoreLinesForTest(false);
     }
-    Q_INVOKABLE bool openLogFile() { return true; }
+    Q_INVOKABLE bool openLogFile()
+    {
+        const QString next = m_open_result ? QString{} : m_pending_error;
+        if (m_open_error != next) {
+            m_open_error = next;
+            Q_EMIT openErrorChanged();
+        }
+        return m_open_result;
+    }
+    Q_INVOKABLE void setOpenLogFileResult(bool ok, const QString& error)
+    {
+        m_open_result = ok;
+        m_pending_error = error;
+    }
+    Q_INVOKABLE void reset()
+    {
+        m_open_result = true;
+        m_pending_error.clear();
+        if (!m_open_error.isEmpty()) {
+            m_open_error.clear();
+            Q_EMIT openErrorChanged();
+        }
+    }
     Q_INVOKABLE void updateRelativeTimes() {}
 
     Q_INVOKABLE void resetForTest(int count, bool has_more_lines)
@@ -3308,6 +3330,9 @@ class MockDebugLogModel : public QAbstractListModel
     int m_load_more_calls{0};
     int m_next_new_row{0};
     int m_next_old_row{0};
+    QString m_open_error;
+    QString m_pending_error;
+    bool m_open_result{true};
 };
 
 class MockClipboard : public QObject
@@ -3401,6 +3426,7 @@ public Q_SLOTS:
         engine->rootContext()->setContextProperty(QStringLiteral("optionsModel"), &options_model);
         engine->rootContext()->setContextProperty(QStringLiteral("chainModel"), &chain_model);
         engine->rootContext()->setContextProperty(QStringLiteral("nodeModel"), &node_model);
+        engine->rootContext()->setContextProperty(QStringLiteral("debugLogModel"), &debug_log_model);
         engine->rootContext()->setContextProperty(QStringLiteral("peerTableModel"), &peer_table_model);
         engine->rootContext()->setContextProperty(QStringLiteral("networkTrafficTower"), &network_traffic_tower);
         engine->rootContext()->setContextProperty(QStringLiteral("testNetworkTrafficTower"), &network_traffic_tower);
@@ -3420,7 +3446,6 @@ public Q_SLOTS:
         engine->rootContext()->setContextProperty(QStringLiteral("testCoinsListModel"), &coins_list_model);
         engine->rootContext()->setContextProperty(QStringLiteral("testBumpModel"), &bump_model);
         engine->rootContext()->setContextProperty(QStringLiteral("desktopWindowBehaviorModel"), &desktop_window_behavior_model);
-        engine->rootContext()->setContextProperty(QStringLiteral("debugLogModel"), &debug_log_model);
         engine->rootContext()->setContextProperty(QStringLiteral("testDebugLogModel"), &debug_log_model);
         engine->addImportPath(QStringLiteral(BITCOINQML_QML_SOURCE_DIR));
     }

test/qml/tst_debuglog.qml

@@ -0,0 +1,63 @@
+// Copyright (c) 2026 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+import QtQuick 2.15
+import QtTest 1.2
+import "../../qml/pages/settings"
+
+TestCase {
+    name: "DebugLog"
+    when: windowShown
+    width: 520
+    height: 720
+
+    Component {
+        id: debugLogComponent
+
+        SettingsDebugLog {
+            width: 480
+            height: 680
+        }
+    }
+
+    function init() {
+        debugLogModel.reset()
+    }
+
+    function createPage() {
+        const page = createTemporaryObject(debugLogComponent, this)
+        verify(page !== null)
+        return page
+    }
+
+    // A failed open surfaces the model's error message on the page. The label
+    // is hidden (text bound to the empty openError) until the open fails.
+    function test_open_failure_shows_error() {
+        const page = createPage()
+        const errorText = findChild(page, "debugLogOpenErrorText")
+        verify(errorText !== null)
+        compare(errorText.text, "")
+
+        debugLogModel.setOpenLogFileResult(false, "Could not open debug log file.")
+        debugLogModel.openLogFile()
+
+        compare(errorText.text, "Could not open debug log file.")
+    }
+
+    // A later successful open clears the previously shown error.
+    function test_successful_open_clears_error() {
+        const page = createPage()
+        const errorText = findChild(page, "debugLogOpenErrorText")
+        verify(errorText !== null)
+
+        debugLogModel.setOpenLogFileResult(false, "Could not open debug log file.")
+        debugLogModel.openLogFile()
+        compare(errorText.text, "Could not open debug log file.")
+
+        debugLogModel.setOpenLogFileResult(true, "")
+        debugLogModel.openLogFile()
+
+        compare(errorText.text, "")
+    }
+}

test/test_debuglogmodel.cpp

@@ -69,6 +69,8 @@ private Q_SLOTS:
     void filter_updatesIncrementallyAndWhileInactive();
     void rotation_fallsBackToFullSnapshot();
     void loadLimit_changesKeepRetainedCacheBounded();
+    void openErrorEmptyByDefault();
+    void openMissingFileReportsError();
 };
 
 void DebugLogModelTests::inactiveModel_ignoresRefreshUntilActivated()
@@ -607,6 +609,35 @@ void DebugLogModelTests::loadLimit_changesKeepRetainedCacheBounded()
     QCOMPARE(insert_spy.at(0).at(2).toInt(), 3);
 }
 
+void DebugLogModelTests::openErrorEmptyByDefault()
+{
+    QTemporaryDir temp_dir;
+    QVERIFY(temp_dir.isValid());
+    const fs::path log_path = fs::PathFromString(temp_dir.filePath("debug.log").toStdString());
+
+    DebugLogModel model(log_path);
+    QVERIFY(model.openError().isEmpty());
+}
+
+// openLogFile() must report the failure rather than silently returning when the
+// log file cannot be opened, so the UI can surface it. A missing file is the
+// deterministic failure path; the "no associated application" case (which needs
+// a desktop environment) goes through the same set-error/return-false branch.
+void DebugLogModelTests::openMissingFileReportsError()
+{
+    QTemporaryDir temp_dir;
+    QVERIFY(temp_dir.isValid());
+    // A file that does not exist inside an otherwise-empty temporary directory.
+    const fs::path missing = fs::PathFromString(temp_dir.filePath("does-not-exist.log").toStdString());
+
+    DebugLogModel model(missing);
+    QSignalSpy error_spy(&model, &DebugLogModel::openErrorChanged);
+
+    QVERIFY(!model.openLogFile());
+    QVERIFY(!model.openError().isEmpty());
+    QCOMPARE(error_spy.count(), 1);
+}
+
 #ifdef BITCOINQML_NO_TEST_MAIN
 #include <test/qt_test_registry.h>
 BITCOINQML_REGISTER_QT_TEST(DebugLogModelTests)