Catch exceptions in REPL to preserve session state on errors (#83)

* Initial plan

* Add exception handling to REPL to prevent session crashes

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

* Improve error message format for better user experience

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

* Update features/shell.feature

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix test failures: use 'try' instead of 'run' for STDERR output

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

* Fix variable name after merge and code style

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

* Suppress PHP warnings in eval to show clean error messages

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

* Remove error suppression operator - keep warnings visible

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

* Update assertion

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Lint fix

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>
Co-authored-by: Pascal Birchler <pascal.birchler@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Pascal Birchler <pascalb@google.com>

Author: 3 people

features/shell.feature

@@ -147,6 +147,44 @@ Feature: WordPress REPL
       history: -1: invalid option
       """
 
+  Scenario: Exception handling preserves session state
+    Given a WP install
+    And a session file:
+      """
+      $foo = 'test_value';
+      require 'nonexistent_file.txt';
+      echo $foo;
+      """
+
+    When I try `wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      test_value
+      """
+    And STDERR should contain:
+      """
+      Failed opening required 'nonexistent_file.txt'
+      """
+
+  Scenario: Exception handling for expression errors
+    Given a WP install
+    And a session file:
+      """
+      $bar = 'preserved';
+      nonexistent_function();
+      $bar;
+      """
+
+    When I try `wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      string(9) "preserved"
+      """
+    And STDERR should contain:
+      """
+      Error: Call to undefined function nonexistent_function()
+      """
+
   Scenario: User can define variable named $line
     Given a WP install
     And a session file:

src/WP_CLI/Shell/REPL.php

@@ -68,8 +68,13 @@ public function start() {
 
 			if ( self::starts_with( self::non_expressions(), $__repl_input_line ) ) {
 				ob_start();
-				// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
-				eval( $__repl_input_line );
+				try {
+					// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
+					eval( $__repl_input_line );
+				} catch ( \Throwable $e ) {
+					// Display the error message but continue the session
+					fwrite( STDERR, get_class( $e ) . ': ' . $e->getMessage() . "\n" );
+				}
 				$__repl_output = (string) ob_get_clean();
 				if ( 0 < strlen( $__repl_output ) ) {
 					$__repl_output = rtrim( $__repl_output, "\n" ) . "\n";
@@ -82,12 +87,23 @@ public function start() {
 
 				// Write directly to STDOUT, to sidestep any output buffers created by plugins
 				ob_start();
-				// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
-				$__repl_eval_result = eval( $__repl_input_line );
-				$__repl_output      = (string) ob_get_clean();
+				$__repl_eval_had_error = false;
+				try {
+					// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
+					$__repl_eval_result = eval( $__repl_input_line );
+				} catch ( \Throwable $e ) {
+					// Display the error message but continue the session
+					fwrite( STDERR, get_class( $e ) . ': ' . $e->getMessage() . "\n" );
+					$__repl_eval_had_error = true;
+					$__repl_eval_result    = null;
+				}
+				$__repl_output = (string) ob_get_clean();
 				if ( 0 < strlen( $__repl_output ) ) {
 					echo rtrim( $__repl_output, "\n" ) . "\n";
 				}
+				if ( $__repl_eval_had_error ) {
+					continue;
+				}
 				ob_start();
 				if ( ! $this->quiet ) {
 					echo '=> ';