beta: Improve error handling when installing mu-plugins (#46115)

If the `WP_Filesystem` instance doesn't report any errors on a failure
(e.g. `WP_Filesystem_Direct` doesn't record any at all), check PHP's
`error_get_last()` before reporting "unknown error".

Author: anomiex

projects/plugins/beta/changelog/fix-beta-plugin-report-php-errors-from-plugin-upgrade

@@ -0,0 +1,4 @@
+Significance: patch
+Type: changed
+
+Check `error_get_last()` if the `WP_Filesystem` doesn't report any error on mu-plugin install failure.

projects/plugins/beta/src/class-plugin.php

@@ -581,9 +581,17 @@ private function update_mu_plugin_loader( $which ) {
 			}
 			$contents .= " */\n";
 		} else {
+			error_clear_last();
 			$contents = $wp_filesystem->get_contents( $file );
 			if ( $contents === false ) {
-				return is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->has_errors() ? $wp_filesystem->errors : new WP_Error( 'fs_error', "Unknown error while reading {$this->plugin_slug()}-loader.php" );
+				if ( is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->has_errors() ) {
+					return $wp_filesystem->errors;
+				}
+				$err = error_get_last();
+				if ( ! empty( $err['message'] ) ) {
+					return new WP_Error( 'fs_error', "PHP error while reading {$this->plugin_slug()}-loader.php: " . esc_html( $err['message'] ) );
+				}
+				return new WP_Error( 'fs_error', "Unknown error while reading {$this->plugin_slug()}-loader.php" );
 			}
 			if ( ! str_starts_with( $contents, '<?php' ) ) {
 				return new WP_Error( 'bad_mu_loader', "Mu-plugin loader {$this->plugin_slug()}-loader.php is unparseable" );
@@ -600,16 +608,25 @@ private function update_mu_plugin_loader( $which ) {
 		// Write the file, or delete it if it's our auto-generated file and turns out to be empty.
 		if ( preg_match( '#^<\?php\s*/\*\*\n \* Loader generated by Jetpack Beta plugin\n\n(?: \* Plugin Name: .*\n)?(?: \* Description: .*\n)? \*/\s*$#', $new_contents ) ) {
 			$what = 'deleting';
-			$ok   = $wp_filesystem->delete( $file );
+			error_clear_last();
+			$ok = $wp_filesystem->delete( $file );
 		} elseif ( $contents !== $new_contents ) {
 			$what = 'updating';
-			$ok   = $wp_filesystem->put_contents( $file, $new_contents );
+			error_clear_last();
+			$ok = $wp_filesystem->put_contents( $file, $new_contents );
 		} else {
 			$what = 'nothing';
 			$ok   = true;
 		}
 		if ( ! $ok ) {
-			return is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->has_errors() ? $wp_filesystem->errors : new WP_Error( 'fs_error', "Unknown error while $what {$this->plugin_slug()}-loader.php" );
+			if ( is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->has_errors() ) {
+				return $wp_filesystem->errors;
+			}
+			$err = error_get_last();
+			if ( ! empty( $err['message'] ) ) {
+				return new WP_Error( 'fs_error', "PHP error while $what {$this->plugin_slug()}-loader.php: " . esc_html( $err['message'] ) );
+			}
+			return new WP_Error( 'fs_error', "Unknown error while $what {$this->plugin_slug()}-loader.php" );
 		}
 		return null;
 	}