Merge bitcoin-core/gui#125: Enable changing the autoprune block space size in intro dialog

415fb2e1abac189fcbe8eccf2ea065724d17460f GUI/Intro: Move prune setting below explanation (Luke Dashjr)
2a84c6bcf61429ac507b506a39247b3a66edbcb4 GUI/Intro: Estimate max age of backups that can be restored with pruning (Luke Dashjr)
e2dcd957fa206a28404ff824fb764b8889705fb2 GUI/Intro: Rework UI flow to let the user set prune size in GBs (Luke Dashjr)
f2e5a6b54fa38b10d822609939b8108bd8fe041b GUI/Intro: Abstract GUI-to-option into Intro::getPrune (Luke Dashjr)
62932cc686dc46ce14c026993deea8e561654177 GUI/Intro: Return actual prune setting from showIfNeeded (Luke Dashjr)

Pull request description:

  ![Screenshot_20200911_095102](https://user-images.githubusercontent.com/1095675/92933661-0c4cea00-f436-11ea-9853-2456091ffab3.png)

  Moved from https://github.com/bitcoin/bitcoin/pull/18728

ACKs for top commit:
  ryanofsky:
    Code review ACK 415fb2e1abac189fcbe8eccf2ea065724d17460f. Changes since last review: mb/gib suffixes, constexpr QOverload expected_backup_days tweaks, new moveonly layout commit
  jarolrod:
    Tested ACK 415fb2e.
  Talkless:
    tACK 415fb2e1abac189fcbe8eccf2ea065724d17460f, tested on Debian Sid with Qt 5.15.2.
  hebasto:
    ACK 415fb2e1abac189fcbe8eccf2ea065724d17460f, my unresolved comments are not blockers, and they could be resolved in follow ups.

Tree-SHA512: bd4882a9c08e6a6eb14b7fb6366983db8581425b4949fea212785d34d8fad9e32fb81ca8c8cdbfb2c05ea394aaf5a746ba2cf16623795c7252c3bdb61d455f00

1 files changed, 28 insertions, 3 deletions

diff --git a/src/qt/intro.cpp b/src/qt/intro.cpp
index aa6b2665fa..ed39307fd7 100644
--- a/src/qt/intro.cpp
+++ b/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 +140,26 @@ 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());
+    ui->lblPruneSuffix->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();
 }
@@ -182,7 +192,17 @@ void Intro::setDataDirectory(const QString &dataDir)
     }
 }
 
-bool Intro::showIfNeeded(bool& did_show_intro, bool& prune)
+int64_t Intro::getPruneMiB() const
+{
+    switch (ui->prune->checkState()) {
+    case Qt::Checked:
+        return PruneGBtoMiB(m_prune_target_gb);
+    case Qt::Unchecked: default:
+        return 0;
+    }
+}
+
+bool Intro::showIfNeeded(bool& did_show_intro, int64_t& prune_MiB)
 {
     did_show_intro = false;
 
@@ -233,7 +253,7 @@ bool Intro::showIfNeeded(bool& did_show_intro, bool& prune)
         }
 
         // Additional preferences:
-        prune = intro.ui->prune->isChecked();
+        prune_MiB = intro.getPruneMiB();
 
         settings.setValue("strDataDir", dataDir);
         settings.setValue("fReset", false);
@@ -361,6 +381,11 @@ 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);
+    static constexpr uint64_t nPowTargetSpacing = 10 * 60;  // from chainparams, which we don't have at this stage
+    static constexpr uint32_t expected_block_data_size = 2250000;  // includes undo data
+    const uint64_t expected_backup_days = m_prune_target_gb * 1e9 / (uint64_t(expected_block_data_size) * 86400 / nPowTargetSpacing);
+    ui->lblPruneSuffix->setText(tr("(sufficient to restore backups %n day(s) old)", "block chain pruning", expected_backup_days));
     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) + " " +