feat: extend stopchldtarget with syscall4 and syscall6

user/stopchldtarget.c

@@ -0,0 +1,68 @@
+#include "ulib.h"
+
+/*
+ * Only ever spawned by stopchldtest.c, with two shared pipe handles as
+ * argv[1] (the "ready" pipe, this process writes) and argv[2] (the "go"
+ * pipe, this process reads) -- identical shape to user/jobtarget.c, reused
+ * here so stopchldtest.c can stop/continue a target genuinely Blocked
+ * mid-syscall while ALSO watching for the new SIGCHLD-on-stop/
+ * SIGCHLD-on-continue notifications (sched.c's threadStop/threadContinue)
+ * against itself, something jobtest.c/jobtarget.c never needed since
+ * neither side there installs a sigChld handler. A distinct exit code (66,
+ * not jobtarget.c's 55) keeps the two pairs' needles/asserts from ever
+ * being confusable in the serial log.
+ */
+
+int main(int argc, char** argv)
+{
+    print("stopchldtarget: start, pid=");
+    printDec(userGetpid());
+    print("\n");
+
+    if (argc != 3) {
+        print("stopchldtarget: FAIL expected two argv (ready handle, go handle)\n");
+        return 1;
+    }
+
+    long readyHandle = parseDec(argv[1]);
+    long goHandle = parseDec(argv[2]);
+
+    long goFd = userPipeAttach(goHandle, 0);
+    if (goFd < 0) {
+        print("stopchldtarget: FAIL attach go pipe as reader\n");
+        return 1;
+    }
+
+    long readyFd = userPipeAttach(readyHandle, 1);
+    if (readyFd < 0) {
+        print("stopchldtarget: FAIL attach ready pipe as writer\n");
+        return 1;
+    }
+    if (userWrite((int)readyFd, "r", 1) != 1) {
+        print("stopchldtarget: FAIL write ready byte\n");
+        return 1;
+    }
+    userClose((int)readyFd);
+    print("stopchldtarget: ready, blocking on go pipe\n");
+
+    char goByte = 0;
+    /* Real blocking read: parks this thread (threadBlocked) until a byte is
+       actually available. stopchldtest.c raises SIGSTOP against this exact
+       blocking read, waits for its own SIGCHLD-on-stop notification, writes
+       the byte, raises SIGCONT, waits for its own SIGCHLD-on-continue
+       notification, and only then this read can return -- the same
+       restart-syscall retry contract user/jobtarget.c already relies on. */
+    if (userRead((int)goFd, &goByte, 1) != 1) {
+        print("stopchldtarget: FAIL read go byte\n");
+        return 1;
+    }
+    userClose((int)goFd);
+
+    if (goByte != 'g') {
+        print("stopchldtarget: FAIL wrong go byte\n");
+        return 1;
+    }
+    print("stopchldtarget: received go\n");
+
+    return 66;
+}