chromium: Log target stdout and stderr on REPRL execution failures

Currently, when Fuzzilli fails to execute a script via the REPRL
protocol (e.g., if the target fails to launch and send the HELO message,
or crashes unexpectedly), the resulting error message is opaque (e.g.,
"Did not receive HELO message from child: Bad file descriptor").

Fuzzilli already captures the target's stdout and stderr in
memory-mapped files by default, this change extracts those buffers and
appends them to the Fuzzilli warning and error logs whenever
`reprl_execute` fails. This surfaces the actual crash dump, missing
dependencies, or startup errors directly in the logs, making debugging
broken targets locally or on bots significantly easier.

Bug: 492209808
Change-Id: If94fc9eadc97645ab240f648b7e6cf42378d091e
Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/9095283
Auto-Submit: Giovanni Ortuño Urquidi <ortuno@google.com>
Reviewed-by: Matthias Liedtke <mliedtke@google.com>
Commit-Queue: Matthias Liedtke <mliedtke@google.com>

Author: g-ortuno

Sources/Fuzzilli/Execution/REPRL.swift

@@ -113,7 +113,8 @@ public class REPRL: ComponentBase, ScriptRunner {
             // If we fail, we retry after a short timeout and with a fresh instance. If we still fail, we give up trying
             // to execute this program. If we repeatedly fail to execute any program, we abort.
             if status < 0 {
-                logger.warning("Script execution failed: \(String(cString: reprl_get_last_error(reprlContext))). Retrying in 1 second...")
+                let errorMsg = String(cString: reprl_get_last_error(reprlContext))
+                logger.warning("Script execution failed: \(errorMsg). Retrying in 1 second...")
                 if fuzzer.config.enableDiagnostics {
                     fuzzer.dispatchEvent(fuzzer.events.DiagnosticsEvent, data: (name: "REPRLFail", content: scriptBuffer.data(using: .utf8)!))
                 }
@@ -123,7 +124,16 @@ public class REPRL: ComponentBase, ScriptRunner {
         }
 
         if status < 0 {
-            logger.error("Script execution failed again: \(String(cString: reprl_get_last_error(reprlContext))). Giving up")
+            let errorMsg = String(cString: reprl_get_last_error(reprlContext))
+            let stderr = String(cString: reprl_fetch_stderr(reprlContext)).trimmingCharacters(in: .whitespacesAndNewlines)
+            let stdout = String(cString: reprl_fetch_stdout(reprlContext)).trimmingCharacters(in: .whitespacesAndNewlines)
+            let childOutput = """
+                stderr:
+                \(stderr.isEmpty ? "<no output>" : stderr)
+                stdout:
+                \(stdout.isEmpty ? "<no output>" : stdout)
+                """
+            logger.error("Script execution failed again: \(errorMsg). Giving up\n\(childOutput)")
             // If we weren't able to successfully execute a script in the last N attempts, abort now...
             recentlyFailedExecutions += 1
             if recentlyFailedExecutions >= 10 {