From 5d195096163c6838c2df1327cca21eb5ca3a9045 Mon Sep 17 00:00:00 2001
From: AnotherTest <ali.mpfard@gmail.com>
Date: Tue, 30 Mar 2021 02:19:02 +0430
Subject: [PATCH] Shell: Handle SIGCHLD after sending SIGCONT to job

This fixes `fg` and `bg` causing the shell to go into an infinite loop
of trying to `waitpid` until some current job changes state.

a.k.a. "Fix Shell backgrounding, yet again!" :P
---
 Userland/Shell/Builtin.cpp | 4 ++--
 Userland/Shell/Job.h       | 3 +++
 Userland/Shell/Shell.cpp   | 8 +++++++-
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/Userland/Shell/Builtin.cpp b/Userland/Shell/Builtin.cpp
index 1c79d056e66..8e90a3fc49b 100644
--- a/Userland/Shell/Builtin.cpp
+++ b/Userland/Shell/Builtin.cpp
@@ -111,7 +111,7 @@ int Shell::builtin_bg(int argc, const char** argv)
 
     job->set_running_in_background(true);
     job->set_should_announce_exit(true);
-    job->set_is_suspended(false);
+    job->set_shell_did_continue(true);
 
     dbgln("Resuming {} ({})", job->pid(), job->cmd());
     warnln("Resuming job {} - {}", job->job_id(), job->cmd().characters());
@@ -396,7 +396,7 @@ int Shell::builtin_fg(int argc, const char** argv)
     }
 
     job->set_running_in_background(false);
-    job->set_is_suspended(false);
+    job->set_shell_did_continue(true);
 
     dbgln("Resuming {} ({})", job->pid(), job->cmd());
     warnln("Resuming job {} - {}", job->job_id(), job->cmd().characters());
diff --git a/Userland/Shell/Job.h b/Userland/Shell/Job.h
index f2470b283d7..af05521dddd 100644
--- a/Userland/Shell/Job.h
+++ b/Userland/Shell/Job.h
@@ -86,6 +86,7 @@ public:
     bool should_announce_exit() const { return m_should_announce_exit; }
     bool should_announce_signal() const { return m_should_announce_signal; }
     bool is_suspended() const { return m_is_suspended; }
+    bool shell_did_continue() const { return m_shell_did_continue; }
     void unblock() const;
 
     Core::ElapsedTimer& timer() { return m_command_timer; }
@@ -94,6 +95,7 @@ public:
     void set_signalled(int sig);
 
     void set_is_suspended(bool value) const { m_is_suspended = value; }
+    void set_shell_did_continue(bool value) const { m_shell_did_continue = value; }
 
     void set_running_in_background(bool running_in_background)
     {
@@ -129,6 +131,7 @@ private:
     Core::ElapsedTimer m_command_timer;
     mutable bool m_active { true };
     mutable bool m_is_suspended { false };
+    mutable bool m_shell_did_continue { false };
     bool m_should_be_disowned { false };
     OwnPtr<AST::Command> m_command;
 };
diff --git a/Userland/Shell/Shell.cpp b/Userland/Shell/Shell.cpp
index d1d2fa7bae4..296c24a6509 100644
--- a/Userland/Shell/Shell.cpp
+++ b/Userland/Shell/Shell.cpp
@@ -1057,7 +1057,7 @@ void Shell::block_on_job(RefPtr<Job> job)
     if (!job)
         return;
 
-    if (job->is_suspended())
+    if (job->is_suspended() && !job->shell_did_continue())
         return; // We cannot wait for a suspended job.
 
     ScopeGuard io_restorer { [&]() {
@@ -1655,6 +1655,12 @@ void Shell::notify_child_event()
             }
             if (child_pid == 0) {
                 // If the child existed, but wasn't dead.
+                if (job.is_suspended() && job.shell_did_continue()) {
+                    // The job was suspended, and we sent it a SIGCONT.
+                    job.set_is_suspended(false);
+                    job.set_shell_did_continue(false);
+                    found_child = true;
+                }
                 continue;
             }
             if (child_pid == job.pid()) {