Handle pages/under/overlay JSON file in begin

...since they have to be handled before other options. It was working
because, in both cases, `file` was alphabetically before the other
keys, but this implementation gives a stronger guarantee.

libqpdf/QPDFJob_json.cc

@@ -57,6 +57,8 @@ namespace
         bare_handler_t bindBare(void (Handlers::*f)());
         json_handler_t bindJSON(void (Handlers::*f)(JSON));
 
+        void beginUnderOverlay(JSON const& j);
+
         std::list<std::shared_ptr<JSONHandler>> json_handlers;
         bool partial;
         JSONHandler* jh{nullptr}; // points to last of json_handlers
@@ -225,6 +227,24 @@ Handlers::handle(JSON& j)
     this->json_handlers.back()->handle(".", j);
 }
 
+void
+Handlers::beginUnderOverlay(JSON const& j)
+{
+    // File has to be processed before items, so handle it here.
+    bool file_seen = false;
+    std::string file;
+    j.forEachDictItem([&](std::string const& key, JSON const& value) {
+        if (key == "file") {
+            file_seen = value.getString(file);
+        }
+    });
+    if (!file_seen) {
+        QTC::TC("qpdf", "QPDFJob json over/under no file");
+        usage("file is required in underlay/overlay specification");
+    }
+    c_uo->file(file);
+}
+
 void
 Handlers::setupInputFile()
 {
@@ -468,16 +488,17 @@ void
 Handlers::beginPages(JSON j)
 {
     bool file_seen = false;
+    std::string file;
     j.forEachDictItem([&](std::string const& key, JSON const& value) {
         if (key == "file") {
-            std::string v;
-            file_seen = value.getString(v);
+            file_seen = value.getString(file);
         }
     });
     if (!file_seen) {
         QTC::TC("qpdf", "QPDFJob json pages no file");
         usage("file is required in page specification");
     }
+    c_pages->file(file);
 }
 
 void
@@ -489,7 +510,8 @@ Handlers::endPages()
 void
 Handlers::setupPagesFile()
 {
-    addParameter([this](char const* p) { c_pages->file(p); });
+    // This is handled in beginPages since file() has to be called first.
+    ignoreItem();
 }
 
 void
@@ -499,9 +521,10 @@ Handlers::setupPagesPassword()
 }
 
 void
-Handlers::beginOverlay(JSON)
+Handlers::beginOverlay(JSON j)
 {
     this->c_uo = c_main->overlay();
+    beginUnderOverlay(j);
 }
 
 void
@@ -514,7 +537,8 @@ Handlers::endOverlay()
 void
 Handlers::setupOverlayFile()
 {
-    addParameter([this](char const* p) { c_uo->file(p); });
+    // This is handled in beginOverlay since file() has to be called first.
+    ignoreItem();
 }
 
 void
@@ -524,9 +548,10 @@ Handlers::setupOverlayPassword()
 }
 
 void
-Handlers::beginUnderlay(JSON)
+Handlers::beginUnderlay(JSON j)
 {
     this->c_uo = c_main->underlay();
+    beginUnderOverlay(j);
 }
 
 void
@@ -539,7 +564,8 @@ Handlers::endUnderlay()
 void
 Handlers::setupUnderlayFile()
 {
-    addParameter([this](char const* p) { c_uo->file(p); });
+    // This is handled in beginUnderlay since file() has to be called first.
+    ignoreItem();
 }
 
 void

qpdf/qpdf.testcov

@@ -691,3 +691,4 @@ QPDFPageDocumentHelper flatten resources missing or invalid 0
 QPDF recover xref stream 0
 QPDFJob misplaced page range 0
 QPDFJob duplicated range 0
+QPDFJob json over/under no file 0

qpdf/qtest/qpdf/bad-json-overlay-no-file.json

@@ -0,0 +1,5 @@
+{
+  "overlay": {
+    "to": "1-z"
+  }
+}

qpdf/qtest/qpdf/bad-overlay-no-file-json.out

@@ -0,0 +1,10 @@
+
+qpdf: error with job-json file bad-json-overlay-no-file.json: file is required in underlay/overlay specification
+Run qpdf --job-json-help for information on the file format.
+
+For help:
+  qpdf --help=usage       usage information
+  qpdf --help=topic       help on a topic
+  qpdf --help=--option    help on an option
+  qpdf --help             general help and a topic list
+

qpdf/qtest/qpdfjob.test

@@ -26,6 +26,7 @@ my @bad_json = (
     "encrypt-missing-password",
     "encrypt-no-key-length",
     "pages-no-file",
+    "overlay-no-file",
     "schema-error",
     "json-error"
     );