Improve testing and error handling around operating before processing

include/qpdf/qpdf-c.h

@@ -113,7 +113,9 @@ extern "C" {
     char const* qpdf_get_qpdf_version();
 
     /* Returns dynamically allocated qpdf_data pointer; must be freed
-     * by calling qpdf_cleanup.
+     * by calling qpdf_cleanup. You must call qpdf_read or one of the
+     * other qpdf_read_* functions before calling any function that
+     * would need to operate on the PDF file.
      */
     QPDF_DLL
     qpdf_data qpdf_init();

libqpdf/QPDF.cc

@@ -85,8 +85,10 @@ class InvalidInputSource: public InputSource
   private:
     void throwException()
     {
-        throw std::runtime_error(
-            "QPDF operation attempted after closing input source");
+        throw std::logic_error(
+            "QPDF operation attempted on a QPDF object with no input source."
+            " QPDF operations are invalid before processFile (or another"
+            " process method) or after closeInputSource");
     }
 };
 

qpdf/qtest/qpdf/test73.out

@@ -1,2 +1,3 @@
-WARNING: closed input source: object 1/0: error reading object: QPDF operation attempted after closing input source
+getRoot: attempted to dereference an uninitialized QPDFObjectHandle
+WARNING: closed input source: object 1/0: error reading object: QPDF operation attempted on a QPDF object with no input source. QPDF operations are invalid before processFile (or another process method) or after closeInputSource
 closed input source: unable to find /Root dictionary

qpdf/test_driver.cc

@@ -2715,6 +2715,16 @@ void runtest(int n, char const* filename1, char const* arg2)
     }
     else if (n == 73)
     {
+        try
+        {
+            QPDF pdf2;
+            pdf2.getRoot();
+        }
+        catch (std::exception& e)
+        {
+            std::cerr << "getRoot: " << e.what() << std::endl;
+        }
+
         pdf.closeInputSource();
         pdf.getRoot().getKey("/Pages").unparseResolved();
     }