Be less aggressive about reclaiming the foreground pgrp

Prior to this fix, in every call to job_continue, fish would reclaim the
foreground pgrp. This would cause other jobs in the pipeline (which may
have another pgrp) to receive SIGTTIN / SIGTTOU.

Only reclaim the foreground pgrp if it was held at the point of job_continue.

This partially addresses #5765

src/builtin_bg.cpp

@@ -30,7 +30,7 @@ static int send_to_bg(parser_t &parser, io_streams_t &streams, job_t *j) {
                               j->command_wcstr());
     j->promote();
     j->set_flag(job_flag_t::FOREGROUND, false);
-    j->continue_job(j->is_stopped());
+    j->continue_job(true, j->is_stopped());
     return STATUS_CMD_OK;
 }
 

src/builtin_fg.cpp

@@ -106,6 +106,6 @@ int builtin_fg(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
     job->promote();
     job->set_flag(job_flag_t::FOREGROUND, true);
 
-    job->continue_job(job->is_stopped());
+    job->continue_job(true, job->is_stopped());
     return STATUS_CMD_OK;
 }

src/exec.cpp

@@ -1021,6 +1021,9 @@ bool exec_job(parser_t &parser, shared_ptr<job_t> j) {
         return true;
     }
 
+    // Check to see if we should reclaim the foreground pgrp after the job finishes or stops.
+    const bool reclaim_foreground_pgrp = (tcgetpgrp(STDIN_FILENO) == getpgrp());
+
     const std::shared_ptr<job_t> parent_job = j->get_parent();
 
     // Perhaps inherit our parent's pgid and job control flag.
@@ -1120,7 +1123,7 @@ bool exec_job(parser_t &parser, shared_ptr<job_t> j) {
         return false;
     }
 
-    j->continue_job(false);
+    j->continue_job(reclaim_foreground_pgrp, false);
     return true;
 }
 

src/proc.cpp

@@ -827,7 +827,7 @@ return false;
     return true;
 }
 
-void job_t::continue_job(bool send_sigcont) {
+void job_t::continue_job(bool reclaim_foreground_pgrp, bool send_sigcont) {
     // Put job first in the job list.
     promote();
     set_flag(job_flag_t::NOTIFIED, false);
@@ -839,7 +839,7 @@ void job_t::continue_job(bool send_sigcont) {
     // Make sure we retake control of the terminal before leaving this function.
     bool term_transferred = false;
     cleanup_t take_term_back([&]() {
-        if (term_transferred) {
+        if (term_transferred && reclaim_foreground_pgrp) {
             terminal_return_from_job(this);
         }
     });

src/proc.h

@@ -408,8 +408,10 @@ class job_t {
     /// saved terminal modes and send the process group a SIGCONT signal to wake it up before we
     /// block.
     ///
-    /// \param send_sigcont Whether SIGCONT should be sent to the job if it is in the foreground.
-    void continue_job(bool send_sigcont);
+    /// \param reclaim_foreground_pgrp whether, when the job finishes or stops, to reclaim the
+    /// foreground pgrp (via tcsetpgrp). \param send_sigcont Whether SIGCONT should be sent to the
+    /// job if it is in the foreground.
+    void continue_job(bool reclaim_foreground_pgrp, bool send_sigcont);
 
     /// Promotes the job to the front of the job list.
     void promote();