add: fix revert for dry-run and interrupts

This change correctly reverts pyproject.toml changes for dry runs as 
well as keyboard interrupts.

Resolves: #2933 #318

poetry/console/commands/add.py

@@ -100,7 +100,6 @@ def handle(self):
         packages = [name for name in packages if name not in existing_packages]
 
         if not packages:
-            self.poetry.file.write(content)
             self.line("Nothing to add.")
             return 0
 
@@ -152,29 +151,29 @@ def handle(self):
 
             poetry_content[section][_constraint["name"]] = constraint
 
-        # Write new content
-        self.poetry.file.write(content)
+        try:
+            # Write new content
+            self.poetry.file.write(content)
 
-        # Cosmetic new line
-        self.line("")
+            # Cosmetic new line
+            self.line("")
 
-        # Update packages
-        self.reset_poetry()
+            # Update packages
+            self.reset_poetry()
 
-        self._installer.set_package(self.poetry.package)
-        self._installer.dry_run(self.option("dry-run"))
-        self._installer.verbose(self._io.is_verbose())
-        self._installer.update(True)
-        if self.option("lock"):
-            self._installer.lock()
+            self._installer.set_package(self.poetry.package)
+            self._installer.dry_run(self.option("dry-run"))
+            self._installer.verbose(self._io.is_verbose())
+            self._installer.update(True)
+            if self.option("lock"):
+                self._installer.lock()
 
-        self._installer.whitelist([r["name"] for r in requirements])
+            self._installer.whitelist([r["name"] for r in requirements])
 
-        try:
             status = self._installer.run()
-        except Exception:
+        except BaseException:
+            # Using BaseException here as some exceptions, eg: KeyboardInterrupt, do not inherit from Exception
             self.poetry.file.write(original_content)
-
             raise
 
         if status != 0 or self.option("dry-run"):

tests/console/commands/test_add.py

@@ -1630,3 +1630,26 @@ def test_add_with_lock_old_installer(app, repo, installer, old_tester):
 """
 
     assert expected == old_tester.io.fetch_output()
+
+
+def test_add_keyboard_interrupt_restore_content(app, repo, installer, tester, mocker):
+    mocker.patch(
+        "poetry.installation.installer.Installer.run", side_effect=KeyboardInterrupt()
+    )
+    original_content = app.poetry.file.read()
+
+    repo.add_package(get_package("cachy", "0.2.0"))
+
+    tester.execute("cachy --dry-run")
+
+    assert original_content == app.poetry.file.read()
+
+
+def test_dry_run_restore_original_content(app, repo, installer, tester):
+    original_content = app.poetry.file.read()
+
+    repo.add_package(get_package("cachy", "0.2.0"))
+
+    tester.execute("cachy --dry-run")
+
+    assert original_content == app.poetry.file.read()