Keep hype render from overwriting its presentation or a half-written manifest

src/cli.cpp

@@ -249,12 +249,18 @@ int render(const QStringList &arguments) {
         output = name;
     else if (QFileInfo(output).isDir())
         output = QDir(output).filePath(name);
+    const QString target = QFileInfo(output).canonicalFilePath();
+    if (!target.isEmpty() && target == QFileInfo(deck.path()).canonicalFilePath())
+        return fail("Choose an output other than the presentation itself.");
     QDir().mkpath(QFileInfo(output).absolutePath());
     // A slide with problems still renders, with the problems on a banner, so it can be looked at.
     const auto problems = slideProblemsAt(deck, parsed, number - 1);
     QString warning;
     const QImage image = paint(deck, number - 1, width, &warning);
-    if (!image.save(output, "PNG"))
+    // Written beside and renamed over, so an existing file, or a hard link to the
+    // presentation, is replaced rather than overwritten in place.
+    QSaveFile file(output);
+    if (!file.open(QIODevice::WriteOnly) || !image.save(&file, "PNG") || !file.commit())
         return fail("Could not save " + output);
     int errors = 0;
     QJsonArray list;

src/deck.cpp

@@ -1607,10 +1607,12 @@ bool Deck::renderImages(const QString &directory, int width, bool powerPoint) {
         }
         slides.append(entry);
     }
-    QFile manifest(directory + "/slides.json");
-    if (!manifest.open(QIODevice::WriteOnly))
+    QSaveFile manifest(directory + "/slides.json");
+    const QByteArray json = QJsonDocument(QJsonObject{{"title", title()}, {"slides", slides}}).toJson();
+    if (!manifest.open(QIODevice::WriteOnly) || manifest.write(json) != json.size() || !manifest.commit()) {
+        setStatus("Could not save slides.json: " + manifest.errorString());
         return false;
-    manifest.write(QJsonDocument(QJsonObject{{"title", title()}, {"slides", slides}}).toJson());
+    }
     return true;
 }
 bool Deck::exportPptx(const QString &path) {

tests/test_cli.py

@@ -63,6 +63,21 @@ def test_save_applies_a_theme_and_exits(self):
         self.assertTrue(self.hype(self.deck, '--theme', 'paper', '--save', code=1).stderr.strip())
         self.assertEqual(self.deck.read_text(), unfinished)
 
+    def test_render_never_overwrites_the_presentation_and_reports_its_manifest(self):
+        self.write(DECK)
+        link = self.root / 'link.md'
+        link.symlink_to(self.deck)
+        for output in (self.deck, link):
+            self.assertIn('presentation itself', self.hype('render', self.deck, '--slide', 1, '-o', output, code=1).stderr)
+        self.assertEqual(self.deck.read_text(), DECK)
+        alias = self.root / 'alias.md'
+        os.link(self.deck, alias)
+        self.hype('render', self.deck, '--slide', 1, '-o', alias)
+        self.assertEqual(self.deck.read_text(), DECK)
+        self.png_size(alias)
+        (self.root / 'slides/slides.json').mkdir(parents=True)
+        self.assertIn('slides.json', self.hype('render', self.deck, '-o', self.root / 'slides', code=1).stderr)
+
     def test_check_reports_every_problem_with_slide_and_line(self):
         self.write(DECK + '\n---\n\n![](missing.png)\n\n---\n\n![bogus=1](gone.png)\n\n---\n')
         report = json.loads(self.hype('check', self.deck, '--json', code=1).stdout)