tests/tcg: Add two follow-fork-mode tests

Add follow-fork-mode child and and follow-fork-mode parent tests.
Check for the obvious pitfalls, such as lingering breakpoints,
catchpoints, and single-step mode.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240219141628.246823-13-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240305121005.3528075-14-alex.bennee@linaro.org>

tests/tcg/multiarch/follow-fork-mode.c

@@ -0,0 +1,56 @@
+/*
+ * Test GDB's follow-fork-mode.
+ *
+ * fork() a chain of processes.
+ * Parents sends one byte to their children, and children return their
+ * position in the chain, in order to prove that they survived GDB's fork()
+ * handling.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+#include <assert.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+void break_after_fork(void)
+{
+}
+
+int main(void)
+{
+    int depth = 42, err, i, fd[2], status;
+    pid_t child, pid;
+    ssize_t n;
+    char b;
+
+    for (i = 0; i < depth; i++) {
+        err = pipe(fd);
+        assert(err == 0);
+        child = fork();
+        break_after_fork();
+        assert(child != -1);
+        if (child == 0) {
+            close(fd[1]);
+
+            n = read(fd[0], &b, 1);
+            close(fd[0]);
+            assert(n == 1);
+            assert(b == (char)i);
+        } else {
+            close(fd[0]);
+
+            b = (char)i;
+            n = write(fd[1], &b, 1);
+            close(fd[1]);
+            assert(n == 1);
+
+            pid = waitpid(child, &status, 0);
+            assert(pid == child);
+            assert(WIFEXITED(status));
+            return WEXITSTATUS(status) - 1;
+        }
+    }
+
+    return depth;
+}