Move `FreespaceChecker` class into its own module #881

For some reason, the MOC compiler in older versions of Qt 6 fails to parse qt/intro.cpp, as noted in this comment.

This PR proposes a move-only refactoring to simplify the source structure by eliminating the need for the inline #include <qt/intro.moc>, thereby effectively working around the issue.

Required for https://github.com/bitcoin/bitcoin/pull/32998.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin-core/gui/runs/46777090693} _{LLM reason (✨ experimental): The CI failure is caused by multiple linting errors including a circular dependency, missing include guards, and other linting checks.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

ACK dd392a64bb0608847f771f8b1f09c2fcae146923

Shouldn't the copyright dates in these files be updated? git blame has some lines from 2022 in intro.h and 2024/25 in intro.cpp.

You could avoid the circularity with something like:

diff --git a/src/qt/freespacechecker.h b/src/qt/freespacechecker.h
index d3a61a11571..214324be7c8 100644
--- a/src/qt/freespacechecker.h
+++ b/src/qt/freespacechecker.h
@@ -9,8 +9,6 @@
 #include <QString>
 #include <QtGlobal>

-class Intro;
-
 /* Check free space asynchronously to prevent hanging the UI thread.

    Up to one request to check a path is in flight to this thread; when the check()
@@ -26,7 +24,12 @@ class FreespaceChecker : public QObject
     Q_OBJECT

 public:
-    explicit FreespaceChecker(Intro *intro);
+    class PathQuery {
+    public:
+        virtual QString getPathToCheck() = 0;
+    };
+
+    explicit FreespaceChecker(PathQuery* intro) : intro{intro} { }

     enum Status {
         ST_OK,
@@ -40,7 +43,7 @@ Q_SIGNALS:
     void reply(int status, const QString &message, quint64 available);

 private:
-    Intro *intro;
+    PathQuery* intro;
 };

 #endif // BITCOIN_QT_FREESPACECHECKER_H
diff --git a/src/qt/freespacechecker.cpp b/src/qt/freespacechecker.cpp
index 28c55baf7d0..c259facb782 100644
--- a/src/qt/freespacechecker.cpp
+++ b/src/qt/freespacechecker.cpp
@@ -5,7 +5,6 @@
 #include <qt/freespacechecker.h>

 #include <qt/guiutil.h>
-#include <qt/intro.h>
 #include <util/fs.h>

 #include <QDir>
@@ -13,11 +12,6 @@

 #include <cstdint>

-FreespaceChecker::FreespaceChecker(Intro *_intro)
-{
-    this->intro = _intro;
-}
-
 void FreespaceChecker::check()
 {
     QString dataDirStr = intro->getPathToCheck();
diff --git a/src/qt/intro.h b/src/qt/intro.h
index 7b34c73b025..409ecea16d3 100644
--- a/src/qt/intro.h
+++ b/src/qt/intro.h
@@ -5,14 +5,14 @@
 #ifndef BITCOIN_QT_INTRO_H
 #define BITCOIN_QT_INTRO_H

+#include <qt/freespacechecker.h>
+
 #include <QDialog>
 #include <QMutex>
 #include <QThread>

 static const bool DEFAULT_CHOOSE_DATADIR = false;

-class FreespaceChecker;
-
 namespace interfaces {
     class Node;
 }
@@ -25,7 +25,7 @@ namespace Ui {
   Allows the user to choose a data directory,
   in which the wallet and block chain will be stored.
  */
-class Intro : public QDialog
+class Intro : public QDialog, public FreespaceChecker::PathQuery
 {
     Q_OBJECT

@@ -78,7 +78,7 @@ private:

     void startThread();
     void checkPath(const QString &dataDir);
-    QString getPathToCheck();
+    QString getPathToCheck() override;
     void UpdatePruneLabels(bool prune_checked);
     void UpdateFreeSpaceLabel();

diff --git a/test/lint/lint-circular-dependencies.py b/test/lint/lint-circular-dependencies.py
index 2ec79724d10..9554e560650 100755
--- a/test/lint/lint-circular-dependencies.py
+++ b/test/lint/lint-circular-dependencies.py
@@ -16,7 +16,6 @@ EXPECTED_CIRCULAR_DEPENDENCIES = (
     "node/blockstorage -> validation -> node/blockstorage",
     "node/utxo_snapshot -> validation -> node/utxo_snapshot",
     "qt/addresstablemodel -> qt/walletmodel -> qt/addresstablemodel",
-    "qt/freespacechecker -> qt/intro -> qt/freespacechecker",
     "qt/recentrequeststablemodel -> qt/walletmodel -> qt/recentrequeststablemodel",
     "qt/sendcoinsdialog -> qt/walletmodel -> qt/sendcoinsdialog",
     "qt/transactiontablemodel -> qt/walletmodel -> qt/transactiontablemodel",

@ajtowns

Thank you for the review! Your feedback has been addressed.

ACK 4542412ae063903654fc1cd95f77889bae76d61c

nit: qt/intro.h forward decl of class FreespaceChecker is redundant since the header is included now.

nit: qt/intro.h forward decl of class FreespaceChecker is redundant since the header is included now.

Thanks! Fixed.

ACK 3a03f075606b19e411b8bd19870242e0e0b58fcb