Add a test that stdin is always handed-off as blocking

No functional change here, just closing a gap in our coverage.

src/fish_test_helper.cpp

@@ -156,6 +156,22 @@ static void sigkill_self() {
     abort();
 }
 
+static void stdin_make_nonblocking() {
+    const int fd = STDIN_FILENO;
+    // Catch SIGCONT so pause() wakes us up.
+    signal(SIGCONT, [](int) {});
+
+    for (;;) {
+        int flags = fcntl(fd, F_GETFL, 0);
+        fprintf(stdout, "stdin was %sblocking\n", (flags & O_NONBLOCK) ? "non" : "");
+        if (fcntl(fd, F_SETFL, flags | O_NONBLOCK)) {
+            perror("fcntl");
+            exit(EXIT_FAILURE);
+        }
+        pause();
+    }
+}
+
 static void show_help();
 
 /// A thing that fish_test_helper can do.
@@ -188,6 +204,8 @@ static fth_command_t s_commands[] = {
      "Print to stdout the name(s) of ignored signals"},
     {"print_stop_cont", print_stop_cont, "Print when we get SIGTSTP and SIGCONT, exiting on input"},
     {"sigkill_self", sigkill_self, "Send SIGKILL to self"},
+    {"stdin_make_nonblocking", stdin_make_nonblocking,
+     "Print if stdin is blocking and then make it nonblocking"},
     {"help", show_help, "Print list of fish_test_helper commands"},
 };
 

tests/pexpects/stdin_nonblocking.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+# Verify that stdin is properly set to blocking even if a job tweaks it.
+from pexpect_helper import SpawnedProc
+import sys
+import signal
+import time
+import os
+
+sp = SpawnedProc()
+send, sendline, expect_prompt, expect_str, sleep = (
+    sp.send,
+    sp.sendline,
+    sp.expect_prompt,
+    sp.expect_str,
+    sp.sleep,
+)
+
+# Launch fish_test_helper.
+expect_prompt()
+exe_path = os.environ.get("fish_test_helper")
+sendline(exe_path + " stdin_make_nonblocking")
+
+expect_str("stdin was blocking")
+sleep(0.1)
+
+send("\x1A")  # ctrl-Z
+expect_prompt("has stopped")
+
+# We don't "restore" non-blocking state when continuing a stopped job.
+sleep(0.1)
+sendline("fg")
+expect_str("stdin was blocking")
+
+# Kill the job and do it again.
+send("\x03")  # ctrl-c
+expect_prompt("")
+sendline(exe_path + " stdin_make_nonblocking")
+expect_str("stdin was blocking")
+send("\x03")  # ctrl-c