Source

File: wp-admin/includes/class-wp-automatic-updater.php

protected function send_debug_email() {
		$update_count = 0;
		foreach ( $this->update_results as $type => $updates ) {
			$update_count += count( $updates );
		}

		$body     = array();
		$failures = 0;

		/* translators: %s: Network home URL. */
		$body[] = sprintf( __( 'WordPress site: %s' ), network_home_url( '/' ) );

		// Core.
		if ( isset( $this->update_results['core'] ) ) {
			$result = $this->update_results['core'][0];
			if ( $result->result && ! is_wp_error( $result->result ) ) {
				/* translators: %s: WordPress version. */
				$body[] = sprintf( __( 'SUCCESS: WordPress was successfully updated to %s' ), $result->name );
			} else {
				/* translators: %s: WordPress version. */
				$body[] = sprintf( __( 'FAILED: WordPress failed to update to %s' ), $result->name );
				$failures++;
			}
			$body[] = '';
		}

		// Plugins, Themes, Translations.
		foreach ( array( 'plugin', 'theme', 'translation' ) as $type ) {
			if ( ! isset( $this->update_results[ $type ] ) ) {
				continue;
			}
			$success_items = wp_list_filter( $this->update_results[ $type ], array( 'result' => true ) );
			if ( $success_items ) {
				$messages = array(
					'plugin'      => __( 'The following plugins were successfully updated:' ),
					'theme'       => __( 'The following themes were successfully updated:' ),
					'translation' => __( 'The following translations were successfully updated:' ),
				);

				$body[] = $messages[ $type ];
				foreach ( wp_list_pluck( $success_items, 'name' ) as $name ) {
					/* translators: %s: Name of plugin / theme / translation. */
					$body[] = ' * ' . sprintf( __( 'SUCCESS: %s' ), $name );
				}
			}
			if ( $success_items != $this->update_results[ $type ] ) {
				// Failed updates.
				$messages = array(
					'plugin'      => __( 'The following plugins failed to update:' ),
					'theme'       => __( 'The following themes failed to update:' ),
					'translation' => __( 'The following translations failed to update:' ),
				);

				$body[] = $messages[ $type ];
				foreach ( $this->update_results[ $type ] as $item ) {
					if ( ! $item->result || is_wp_error( $item->result ) ) {
						/* translators: %s: Name of plugin / theme / translation. */
						$body[] = ' * ' . sprintf( __( 'FAILED: %s' ), $item->name );
						$failures++;
					}
				}
			}
			$body[] = '';
		}

		$site_title = wp_specialchars_decode( get_bloginfo( 'name' ), ENT_QUOTES );
		if ( $failures ) {
			$body[] = trim(
				__(
					"BETA TESTING?
=============

This debugging email is sent when you are using a development version of WordPress.

If you think these failures might be due to a bug in WordPress, could you report it?
 * Open a thread in the support forums: https://wordpress.org/support/forum/alphabeta
 * Or, if you're comfortable writing a bug report: https://core.trac.wordpress.org/

Thanks! -- The WordPress Team"
				)
			);
			$body[] = '';

			/* translators: Background update failed notification email subject. %s: Site title. */
			$subject = sprintf( __( '[%s] Background Update Failed' ), $site_title );
		} else {
			/* translators: Background update finished notification email subject. %s: Site title. */
			$subject = sprintf( __( '[%s] Background Update Finished' ), $site_title );
		}

		$body[] = trim(
			__(
				'UPDATE LOG
=========='
			)
		);
		$body[] = '';

		foreach ( array( 'core', 'plugin', 'theme', 'translation' ) as $type ) {
			if ( ! isset( $this->update_results[ $type ] ) ) {
				continue;
			}
			foreach ( $this->update_results[ $type ] as $update ) {
				$body[] = $update->name;
				$body[] = str_repeat( '-', strlen( $update->name ) );
				foreach ( $update->messages as $message ) {
					$body[] = '  ' . html_entity_decode( str_replace( '…', '...', $message ) );
				}
				if ( is_wp_error( $update->result ) ) {
					$results = array( 'update' => $update->result );
					// If we rolled back, we want to know an error that occurred then too.
					if ( 'rollback_was_required' === $update->result->get_error_code() ) {
						$results = (array) $update->result->get_error_data();
					}
					foreach ( $results as $result_type => $result ) {
						if ( ! is_wp_error( $result ) ) {
							continue;
						}

						if ( 'rollback' === $result_type ) {
							/* translators: 1: Error code, 2: Error message. */
							$body[] = '  ' . sprintf( __( 'Rollback Error: [%1$s] %2$s' ), $result->get_error_code(), $result->get_error_message() );
						} else {
							/* translators: 1: Error code, 2: Error message. */
							$body[] = '  ' . sprintf( __( 'Error: [%1$s] %2$s' ), $result->get_error_code(), $result->get_error_message() );
						}

						if ( $result->get_error_data() ) {
							$body[] = '         ' . implode( ', ', (array) $result->get_error_data() );
						}
					}
				}
				$body[] = '';
			}
		}

		$email = array(
			'to'      => get_site_option( 'admin_email' ),
			'subject' => $subject,
			'body'    => implode( "\n", $body ),
			'headers' => '',
		);

		/**
		 * Filters the debug email that can be sent following an automatic
		 * background core update.
		 *
		 * @since 3.8.0
		 *
		 * @param array $email {
		 *     Array of email arguments that will be passed to wp_mail().
		 *
		 *     @type string $to      The email recipient. An array of emails
		 *                           can be returned, as handled by wp_mail().
		 *     @type string $subject Email subject.
		 *     @type string $body    Email message body.
		 *     @type string $headers Any email headers. Default empty.
		 * }
		 * @param int   $failures The number of failures encountered while upgrading.
		 * @param mixed $results  The results of all attempted updates.
		 */
		$email = apply_filters( 'automatic_updates_debug_email', $email, $failures, $this->update_results );

		wp_mail( $email['to'], wp_specialchars_decode( $email['subject'] ), $email['body'], $email['headers'] );
	}