JavaScriptExecutor: Support large outputs

This is needed for a tool that uses the JavaScriptExecutor and produces
a large amount of output (the list of all builtins available in the
global scope).

Bug: 487347678
Change-Id: Ib83ee2ae33a609e5b8ce1598b14892a8cedfd0a4
Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/9047637
Reviewed-by: Danylo Mocherniuk <mdanylo@google.com>
Commit-Queue: Matthias Liedtke <mliedtke@google.com>

Author: Liedtke

Sources/Fuzzilli/Util/JavaScriptExecutor.swift

@@ -19,6 +19,30 @@ import WinSDK
 #endif /* os(Windows) */
 
 public class JavaScriptExecutor {
+    // helper to support program output larger than the Pipe()'s buffer.
+    private final class OutputBuffer: @unchecked Sendable {
+        private var data = Data()
+        private let lock = NSLock()
+
+        func append(_ newData: Data) {
+            lock.lock()
+            defer { lock.unlock() }
+            data.append(newData)
+        }
+
+        var count: Int {
+            lock.lock()
+            defer { lock.unlock() }
+            return data.count
+        }
+
+        var currentData: Data {
+            lock.lock()
+            defer { lock.unlock() }
+            return data
+        }
+    }
+
     /// Path to the js shell binary.
     let executablePath: String
 
@@ -84,6 +108,11 @@ public class JavaScriptExecutor {
         let outputPipe = Pipe()
         let errorPipe = Pipe()
 
+        let outputData = OutputBuffer()
+        outputPipe.fileHandleForReading.readabilityHandler = { handle in
+            outputData.append(handle.availableData)
+        }
+
         // Write input into file.
         let url = FileManager.default.temporaryDirectory
                .appendingPathComponent(UUID().uuidString)
@@ -138,19 +167,11 @@ public class JavaScriptExecutor {
         try FileManager.default.removeItem(at: url)
 
         // Fetch and return the output.
-        var output = ""
-        if let data = String(data: outputPipe.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8) {
-            output = data
-        } else {
-            output = "Process output is not valid UTF-8"
-        }
-        var error = ""
-        if let data = String(data: errorPipe.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8) {
-            error = data
-        } else {
-            error = "Process stderr is not valid UTF-8"
-        }
-        var outcome: Result.Outcome
+        var output = String(data: outputData.currentData, encoding: .utf8)
+            ?? "Process output is not valid UTF-8"
+        let error = String(data: errorPipe.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8)
+            ?? "Process stderr is not valid UTF-8"
+        let outcome: Result.Outcome
         if timedOut {
             outcome = .timedOut
             output += "\nError: Timed out"

Tests/FuzzilliTests/TestUtils.swift

@@ -85,3 +85,17 @@ func buildAndLiftProgram(buildFunc: (ProgramBuilder) -> ()) -> String {
     return buildAndLiftProgram(withLiftingOptions: [], buildFunc: buildFunc)
 }
 
+class TestUtilsTests: XCTestCase {
+
+    // Test that running a program via the JavaScriptExecutor that produces a large output succeeds.
+    // (This test case exists because when using a raw Pipe() as stdout without any further
+    // handling, the child process will be interrupted once the buffer size (64KB on Linux
+    // apparently) is filled.)
+    func testJavaScriptExecutorLargeOutput() throws {
+        let jsProg = "console.log(JSON.stringify(Array(50000).fill(1)))"
+        let runner = try GetJavaScriptExecutorOrSkipTest()
+        let result = try runner.executeScript(jsProg, withTimeout: 10)
+        XCTAssert(result.isSuccess, "\(result.output)\n\(result.error)")
+        XCTAssertGreaterThan(result.output.count, 100_000)
+    }
+}