Add a fish_test_helper command to print blocked signals

src/fish_test_helper.cpp

@@ -69,6 +69,23 @@ static void print_fds() {
     fputc('\n', stdout);
 }
 
+static void print_blocked_signals() {
+    sigset_t sigs;
+    sigemptyset(&sigs);
+    if (sigprocmask(SIG_SETMASK, nullptr, &sigs)) {
+        perror("sigprocmask");
+        exit(EXIT_FAILURE);
+    }
+    // There is no obviously portable way to get the maximum number of signals; here we limit it to 128.
+    for (int sig=1; sig < 128; sig++) {
+        if (sigismember(&sigs, sig)) {
+            if (const char *s = strsignal(sig)) {
+                fprintf(stderr, "%s\n", s);
+            }
+        }
+    }
+}
+
 static void show_help();
 
 /// A thing that fish_test_helper can do.
@@ -93,6 +110,7 @@ static fth_command_t s_commands[] = {
     {"print_pid_then_sleep", print_pid_then_sleep, "Print our pid, then sleep for .5 seconds"},
     {"print_pgrp", print_pgrp, "Print our pgroup to stdout"},
     {"print_fds", print_fds, "Print the list of active FDs to stdout"},
+    {"print_blocked_signals", print_blocked_signals, "Print to stdout the name(s) of blocked signals"},
     {"help", show_help, "Print list of fish_test_helper commands"},
 };