Revert "Revert "[SPARK-21093][R] Terminate R's worker processes in the parent of R's daemon to prevent a leak""

This reverts commit 9f86d2f.

Author: HyukjinKwon

R/pkg/inst/worker/daemon.R

@@ -30,8 +30,55 @@ port <- as.integer(Sys.getenv("SPARKR_WORKER_PORT"))
 inputCon <- socketConnection(
     port = port, open = "rb", blocking = TRUE, timeout = connectionTimeout)
 
+# Waits indefinitely for a socket connecion by default.
+selectTimeout <- NULL
+
+# Exit code that children send to the parent to indicate they exited.
+exitCode <- 1
+
 while (TRUE) {
-  ready <- socketSelect(list(inputCon))
+  ready <- socketSelect(list(inputCon), timeout = selectTimeout)
+
+  # Note that the children should be terminated in the parent. If each child terminates
+  # itself, it appears that the resource is not released properly, that causes an unexpected
+  # termination of this daemon due to, for example, running out of file descriptors
+  # (see SPARK-21093). Therefore, the current implementation tries to retrieve children
+  # that are exited (but not terminated) and then sends a kill signal to terminate them properly
+  # in the parent.
+  #
+  # There are two paths that it attempts to send a signal to terminate the children in the parent.
+  #
+  #   1. Every second if any socket connection is not available and if there are child workers
+  #     running.
+  #   2. Right after a socket connection is available.
+  #
+  # In other words, the parent attempts to send the signal to the children every second if
+  # any worker is running or right before launching other worker children from the following
+  # new socket connection.
+
+  # Only the process IDs of children sent data to the parent are returned below. The children
+  # send a custom exit code to the parent after being exited and the parent tries
+  # to terminate them only if they sent the exit code.
+  children <- parallel:::selectChildren(timeout = 0)
+
+  if (is.integer(children)) {
+    lapply(children, function(child) {
+      # This data should be raw bytes if any data was sent from this child.
+      # Otherwise, this returns the PID.
+      data <- parallel:::readChild(child)
+      if (is.raw(data)) {
+        # This checks if the data from this child is the exit code that indicates an exited child.
+        if (unserialize(data) == exitCode) {
+          # If so, we terminate this child.
+          tools::pskill(child, tools::SIGUSR1)
+        }
+      }
+    })
+  } else if (is.null(children)) {
+    # If it is NULL, there are no children. Waits indefinitely for a socket connecion.
+    selectTimeout <- NULL
+  }
+
   if (ready) {
     port <- SparkR:::readInt(inputCon)
     # There is a small chance that it could be interrupted by signal, retry one time
@@ -44,12 +91,16 @@ while (TRUE) {
     }
     p <- parallel:::mcfork()
     if (inherits(p, "masterProcess")) {
+      # Reach here because this is a child process.
       close(inputCon)
       Sys.setenv(SPARKR_WORKER_PORT = port)
       try(source(script))
-      # Set SIGUSR1 so that child can exit
-      tools::pskill(Sys.getpid(), tools::SIGUSR1)
-      parallel:::mcexit(0L)
+      # Note that this mcexit does not fully terminate this child. So, this writes back
+      # a custom exit code so that the parent can read and terminate this child.
+      parallel:::mcexit(0L, send = exitCode)
+    } else {
+      # Forking succeeded and we need to check if they finished their jobs every second.
+      selectTimeout <- 1
     }
   }
 }