ipc: allow configuration of executor output buffer size

If an external sandbox program wraps the executor, it may include much
more log output than the standard executor, making 128k of logs
insufficient to get full context. Add a flag to allow increasing the
size of the buffer.

The current buffering mechanism isn't ideal, as it allocates the entire
size up front and may include as little as half of the buffer size of
output, but it works for now when you just need a bit more space.

ipc/ipc.go

@@ -67,6 +67,7 @@ var (
 	// so let's better wait than report false misleading crashes.
 	flagTimeout     = flag.Duration("timeout", 1*time.Minute, "execution timeout")
 	flagAbortSignal = flag.Int("abort_signal", int(syscall.SIGKILL), "initial signal to send to executor in error conditions; upgrades to SIGKILL if executor does not exit")
+	flagBufferSize  = flag.Uint64("buffer_size", 128<<10, "internal buffer size (in bytes) for executor output")
 )
 
 // ExecutorFailure is returned from MakeEnv or from env.Exec when executor terminates by calling fail function.
@@ -88,6 +89,9 @@ type Config struct {
 	// AbortSignal is the signal to send to the executor in error
 	// conditions.
 	AbortSignal syscall.Signal
+
+	// BufferSize is the size of the internal buffer for executor output.
+	BufferSize uint64
 }
 
 func DefaultConfig() (Config, error) {
@@ -115,6 +119,7 @@ func DefaultConfig() (Config, error) {
 	}
 	c.Timeout = *flagTimeout
 	c.AbortSignal = syscall.Signal(*flagAbortSignal)
+	c.BufferSize = *flagBufferSize
 	return c, nil
 }
 
@@ -438,16 +443,15 @@ func makeCommand(pid int, bin []string, config Config, inFile *os.File, outFile
 		cmd.Stderr = wp
 		go func(c *command) {
 			// Read out output in case executor constantly prints something.
-			const BufSize = 128 << 10
-			output := make([]byte, BufSize)
-			size := 0
+			output := make([]byte, c.config.BufferSize)
+			var size uint64
 			for {
 				n, err := rp.Read(output[size:])
 				if n > 0 {
-					size += n
-					if size >= BufSize*3/4 {
-						copy(output, output[size-BufSize/2:size])
-						size = BufSize / 2
+					size += uint64(n)
+					if size >= c.config.BufferSize*3/4 {
+						copy(output, output[size-c.config.BufferSize/2:size])
+						size = c.config.BufferSize / 2
 					}
 				}
 				if err != nil {