[fsresizer] Document configuration

- Add some notes on configuration of fsresizer
 - Convenience methods for checking validity.

src/modules/fsresizer/ResizeFSJob.cpp

@@ -85,6 +85,11 @@ ResizeFSJob::prettyName() const
 Calamares::JobResult
 ResizeFSJob::exec()
 {
+    if ( !isValid() )
+        return Calamares::JobResult::error(
+            tr( "Invalid configuration" ),
+            tr( "The file-system resize job has an invalid configuration "
+                "and will not run." ) );
     return Calamares::JobResult::ok();
 }
 

src/modules/fsresizer/ResizeFSJob.h

@@ -33,6 +33,12 @@ class PLUGINDLLEXPORT ResizeFSJob : public Calamares::CppJob
     Q_OBJECT
 
 public:
+    /** @brief Size expressions
+     * 
+     * Sizes can be specified in MiB or percent (of the device they
+     * are on). This class handles parsing of such strings from the
+     * config file.
+     */
     class RelativeSize
     {
     public:
@@ -49,6 +55,11 @@ public:
         int value() const { return m_value; }
         Unit unit() const { return m_unit; }
 
+        constexpr bool isValid() const 
+        {
+            return ( unit() != None ) && ( value() > 0 );
+        }
+        
     private:
         int m_value;
         Unit m_unit;
@@ -63,6 +74,12 @@ public:
 
     void setConfigurationMap( const QVariantMap& configurationMap ) override;
 
+    constexpr bool isValid() const
+    {
+        return ( !m_fsname.isEmpty() || !m_devicename.isEmpty() ) &&
+               m_size.isValid();
+    }
+    
 private:
     RelativeSize m_size;
     RelativeSize m_atleast;

src/modules/fsresizer/fsresizer.conf

@@ -21,10 +21,13 @@ fs: /
 # in percent, so set it to 100. Perhaps a fixed size is
 # needed (that would be weird though, since you don't know
 # how big the card is), use MiB as suffix in that case.
+# If missing, then it's assumed to be 0, and no resizing
+# will happen.
 size: 100%
 
 # Resizing might not be worth it, though. Set the minimum
 # that it must grow; if it cannot grow that much, the
 # resizing is skipped. Can be in percentage or absolute
-# size, as above.
+# size, as above. If missing, then it's assumed to be 0,
+# which means resizing is always worthwhile.
 atleast: 1000MiB