[machineid] Implement entropy-file creation

- read-urandom or copy-existing-file are implemented
 - fairly chatty on failure
 - needs tests (probably the implementation should be moved to
   a separate file and unit-tested)

src/modules/machineid/MachineIdJob.cpp

@@ -52,12 +52,29 @@ removeFile( const QString& rootMountPoint, const QString& fileName )
     QFile::remove( rootMountPoint + fileName );
 }
 
+/// @brief Copy @p fileName from host into target system at @p rootMountPoint
+static Calamares::JobResult
+copyFile( const QString& rootMountPoint, const QString& fileName )
+{
+    QFile f( fileName );
+    if ( !f.exists() )
+    {
+        return Calamares::JobResult::error( QObject::tr( "File not found" ), fileName );
+    }
+    if ( !f.copy( rootMountPoint + fileName ) )
+    {
+        return Calamares::JobResult::error( QObject::tr( "File not found" ), rootMountPoint + fileName );
+    }
+    return Calamares::JobResult::ok();
+}
+
 /// @brief How to generate entropy (bool-like)
 enum class EntropyGeneration
 {
     New,
     CopyFromHost
 };
+
 /// @brief How to create the DBus machine-id (bool-like)
 enum class DBusGeneration
 {
@@ -65,10 +82,92 @@ enum class DBusGeneration
     SymlinkFromSystemD
 };
 
+static int
+getUrandomPoolSize()
+{
+    QFile f( "/proc/sys/kernel/random/poolsize" );
+    constexpr const int minimumPoolSize = 512;
+    int poolSize = minimumPoolSize;
+
+    if ( f.exists() && f.open( QIODevice::ReadOnly | QIODevice::Text ) )
+    {
+        QByteArray v = f.read( 16 );
+        if ( v.length() > 2 )
+        {
+            bool ok = false;
+            poolSize = v.toInt( &ok );
+            if ( !ok )
+            {
+                poolSize = minimumPoolSize;
+            }
+        }
+    }
+    return poolSize >= minimumPoolSize ? poolSize : minimumPoolSize;
+}
+
+static Calamares::JobResult
+createNewEntropy( int poolSize, const QString& rootMountPoint, const QString& fileName )
+{
+    QFile urandom( "/dev/urandom" );
+    if ( urandom.exists() && urandom.open( QIODevice::ReadOnly ) )
+    {
+        QByteArray data = urandom.read( poolSize );
+        urandom.close();
+
+        QFile entropyFile( rootMountPoint + fileName );
+        if ( entropyFile.exists() )
+        {
+            cWarning() << "Entropy file" << ( rootMountPoint + fileName ) << "already exists.";
+            return Calamares::JobResult::ok();  // .. anyway
+        }
+        if ( !entropyFile.open( QIODevice::WriteOnly ) )
+        {
+            return Calamares::JobResult::error(
+                QObject::tr( "File not found" ),
+                QObject::tr( "Could not create new random file <pre>%1</pre>." ).arg( fileName ) );
+        }
+        entropyFile.write( data );
+        entropyFile.close();
+        if ( entropyFile.size() < data.length() )
+        {
+            cWarning() << "Entropy file is" << entropyFile.size() << "bytes, random data was" << data.length();
+        }
+        if ( data.length() < poolSize )
+        {
+            cWarning() << "Entropy data is" << data.length() << "bytes, rather than poolSize" << poolSize;
+        }
+    }
+    return Calamares::JobResult::error(
+        QObject::tr( "File not found" ),
+        QObject::tr( "Could not read random file <pre>%1</pre>." ).arg( QStringLiteral( "/dev/urandom" ) ) );
+}
+
+
 static Calamares::JobResult
 createEntropy( const EntropyGeneration kind, const QString& rootMountPoint, const QString& fileName )
 {
-    return Calamares::JobResult::internalError( QObject::tr( "Internal Error" ), QObject::tr( "Not implemented" ), 0 );
+    if ( kind == EntropyGeneration::CopyFromHost )
+    {
+        if ( QFile::exists( fileName ) )
+        {
+            auto r = copyFile( rootMountPoint, fileName );
+            if ( r )
+            {
+                return r;
+            }
+            else
+            {
+                cWarning() << "Could not copy" << fileName << "for entropy, generating new.";
+            }
+        }
+        else
+        {
+            cWarning() << "Host system entropy does not exist at" << fileName;
+        }
+    }
+
+    int poolSize = getUrandomPoolSize();
+    return createNewEntropy( poolSize, rootMountPoint, fileName );
 }
 
 static Calamares::JobResult