GUI/Intro: Rework UI flow to let the user set prune size in GBs

src/qt/forms/intro.ui

@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>674</width>
-    <height>415</height>
+    <height>447</height>
    </rect>
   </property>
   <property name="windowTitle">
@@ -200,6 +200,40 @@
      </item>
     </layout>
    </item>
+   <item>
+    <layout class="QHBoxLayout" name="pruneOptLayout">
+     <item>
+      <widget class="QCheckBox" name="prune">
+       <property name="text">
+        <string>Limit block chain storage to</string>
+       </property>
+       <property name="toolTip">
+        <string>Reverting this setting requires re-downloading the entire blockchain. It is faster to download the full chain first and prune it later. Disables some advanced features.</string>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <widget class="QSpinBox" name="pruneGB">
+       <property name="suffix">
+        <string> GB</string>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <spacer name="horizontalSpacer_2">
+       <property name="orientation">
+        <enum>Qt::Horizontal</enum>
+       </property>
+       <property name="sizeHint" stdset="0">
+        <size>
+         <width>40</width>
+         <height>20</height>
+        </size>
+       </property>
+      </spacer>
+     </item>
+    </layout>
+   </item>
    <item>
     <widget class="QLabel" name="lblExplanation1">
      <property name="text">
@@ -210,16 +244,6 @@
      </property>
     </widget>
    </item>
-   <item>
-    <widget class="QCheckBox" name="prune">
-     <property name="toolTip">
-      <string>Reverting this setting requires re-downloading the entire blockchain. It is faster to download the full chain first and prune it later. Disables some advanced features.</string>
-     </property>
-     <property name="text">
-      <string></string>
-     </property>
-    </widget>
-   </item>
    <item>
     <widget class="QLabel" name="lblExplanation2">
      <property name="text">

src/qt/intro.cpp

@@ -17,6 +17,7 @@
 
 #include <interfaces/node.h>
 #include <util/system.h>
+#include <validation.h>
 
 #include <QFileDialog>
 #include <QSettings>
@@ -139,17 +140,25 @@ Intro::Intro(QWidget *parent, int64_t blockchain_size_gb, int64_t chain_state_si
     );
     ui->lblExplanation2->setText(ui->lblExplanation2->text().arg(PACKAGE_NAME));
 
+    const int min_prune_target_GB = std::ceil(MIN_DISK_SPACE_FOR_BLOCK_FILES / 1e9);
+    ui->pruneGB->setRange(min_prune_target_GB, std::numeric_limits<int>::max());
     if (gArgs.GetArg("-prune", 0) > 1) { // -prune=1 means enabled, above that it's a size in MiB
         ui->prune->setChecked(true);
         ui->prune->setEnabled(false);
     }
-    ui->prune->setText(tr("Discard blocks after verification, except most recent %1 GB (prune)").arg(m_prune_target_gb));
+    ui->pruneGB->setValue(m_prune_target_gb);
+    ui->pruneGB->setToolTip(ui->prune->toolTip());
     UpdatePruneLabels(ui->prune->isChecked());
 
     connect(ui->prune, &QCheckBox::toggled, [this](bool prune_checked) {
         UpdatePruneLabels(prune_checked);
         UpdateFreeSpaceLabel();
     });
+    connect(ui->pruneGB, QOverload<int>::of(&QSpinBox::valueChanged), [this](int prune_GB) {
+        m_prune_target_gb = prune_GB;
+        UpdatePruneLabels(ui->prune->isChecked());
+        UpdateFreeSpaceLabel();
+    });
 
     startThread();
 }
@@ -371,6 +380,7 @@ void Intro::UpdatePruneLabels(bool prune_checked)
         storageRequiresMsg = tr("Approximately %1 GB of data will be stored in this directory.");
     }
     ui->lblExplanation3->setVisible(prune_checked);
+    ui->pruneGB->setEnabled(prune_checked);
     ui->sizeWarningLabel->setText(
         tr("%1 will download and store a copy of the Bitcoin block chain.").arg(PACKAGE_NAME) + " " +
         storageRequiresMsg.arg(m_required_space_gb) + " " +

src/qt/intro.h

@@ -73,7 +73,7 @@ private:
     //! Total required space (in GB) depending on user choice (prune or not prune).
     int64_t m_required_space_gb{0};
     uint64_t m_bytes_available{0};
-    const int64_t m_prune_target_gb;
+    int64_t m_prune_target_gb;
 
     void startThread();
     void checkPath(const QString &dataDir);