Only set the default expiration date on share creation

Fixes #19685

The default expiration date should only be set when we create a new
share. So if a share is created and the expiration date is unset. And
after that the password is updated the expiration date should remain
unset.

lib/private/share20/manager.php

@@ -261,7 +261,7 @@ class Manager implements IManager {
 		}
 
 		// If expiredate is empty set a default one if there is a default
-		if ($expirationDate === null && $this->shareApiLinkDefaultExpireDate()) {
+		if ($share->getFullId() === null && $expirationDate === null && $this->shareApiLinkDefaultExpireDate()) {
 			$expirationDate = new \DateTime();
 			$expirationDate->setTime(0,0,0);
 			$expirationDate->add(new \DateInterval('P'.$this->shareApiLinkDefaultExpireDays().'D'));

lib/private/share20/share.php

@@ -90,6 +90,9 @@ class Share implements \OCP\Share\IShare {
 	 * @inheritdoc
 	 */
 	public function getFullId() {
+		if ($this->providerId === null || $this->id === null) {
+			return null;
+		}
 		return $this->providerId . ':' . $this->id;
 	}
 

lib/public/share/ishare.php

@@ -46,7 +46,7 @@ interface IShare {
 	 * Get the full share id. This is the <providerid>:<internalid>.
 	 * The full id is unique in the system.
 	 *
-	 * @return string
+	 * @return string|null null is returned when the fullId can't be constructed
 	 * @since 9.0.0
 	 */
 	public function getFullId();

tests/lib/share20/managertest.php

@@ -890,6 +890,22 @@ class ManagerTest extends \Test\TestCase {
 		$this->invokePrivate($this->manager, 'validateExpirationDate', [$share]);
 	}
 
+	public function testValidateExpirationDateExistingShareNoDefault() {
+		$share = $this->manager->newShare();
+
+		$share->setId('42')->setProviderId('foo');
+
+		$this->config->method('getAppValue')
+			->will($this->returnValueMap([
+				['core', 'shareapi_default_expire_date', 'no', 'yes'],
+				['core', 'shareapi_expire_after_n_days', '7', '6'],
+			]));
+
+		$this->invokePrivate($this->manager, 'validateExpirationDate', [$share]);
+
+		$this->assertEquals(null, $share->getExpirationDate());
+	}
+
 	/**
 	 * @expectedException Exception
 	 * @expectedExceptionMessage Only sharing with group members is allowed