Fix hup_background_jobs (née kill_background_jobs) implementation

This was introduced in 1b1bc28c0a but did
not cause any problems until the job control refactor, which caused it
to attempt to signal the calling `exec` builtin's own (invalid) pgrp
with SIGHUP.

Also improved debugging for `j->signal()` failures by printing the
signal we tried sending in case of error, rename the function to
`hup_background_jobs`, and move it from `reader.h`/`reader.cpp` to
`proc.h`/`proc.cpp`.

src/parse_execution.cpp

@@ -786,7 +786,7 @@ parse_execution_result_t parse_execution_context_t::populate_plain_process(
                 return parse_execution_errored;
             }
             else {
-                kill_background_jobs();
+                hup_background_jobs();
             }
         }
     }

src/proc.cpp

@@ -225,7 +225,9 @@ bool job_t::signal(int signal) {
 
     if (pgid != getpgrp()) {
         if (killpg(pgid, signal) == -1) {
-            wperror(L"killpg");
+            char buffer[512];
+            sprintf(buffer, "killpg(%d, %s)", pgid, strsignal(signal));
+            wperror(str2wcstring(buffer).c_str());
             return false;
         }
     } else {
@@ -1199,3 +1201,21 @@ pid_t proc_wait_any() {
     process_clean_after_marking(is_interactive);
     return pid;
 }
+
+void hup_background_jobs() {
+    job_iterator_t jobs;
+
+    while (job_t *j = jobs.next()) {
+        // Make sure we don't try to SIGHUP the calling builtin
+        if (j->pgid == INVALID_PID || !j->get_flag(job_flag_t::JOB_CONTROL)) {
+            continue;
+        }
+
+        if (!j->is_completed()) {
+            if (j->is_stopped()) {
+                j->signal(SIGCONT);
+            }
+            j->signal(SIGHUP);
+        }
+    }
+}

src/proc.h

@@ -404,6 +404,9 @@ int proc_format_status(int status);
 /// Wait for any process finishing.
 pid_t proc_wait_any();
 
+/// Terminate all background jobs
+void hup_background_jobs();
+
 /// Give ownership of the terminal to the specified job.
 ///
 /// \param j The job to give the terminal to.

src/reader.cpp

@@ -1780,7 +1780,7 @@ static void reader_interactive_init() {
                 // Try stopping us.
                 int ret = killpg(shell_pgid, SIGTTIN);
                 if (ret < 0) {
-                    wperror(L"killpg");
+                    wperror(L"killpg(shell_pgid, SIGTTIN)");
                     exit_without_destructors(1);
                 }
             }
@@ -2251,16 +2251,6 @@ void reader_bg_job_warning() {
     fputws(_(L"Use 'disown PID' to remove jobs from the list without terminating them.\n"), stdout);
 }
 
-void kill_background_jobs() {
-    job_iterator_t jobs;
-    while (job_t *j = jobs.next()) {
-        if (!j->is_completed()) {
-            if (j->is_stopped()) j->signal(SIGCONT);
-            j->signal(SIGHUP);
-        }
-    }
-}
-
 /// This function is called when the main loop notices that end_loop has been set while in
 /// interactive mode. It checks if it is ok to exit.
 static void handle_end_loop() {
@@ -2293,7 +2283,7 @@ static void handle_end_loop() {
     }
 
     // Kill remaining jobs before exiting.
-    kill_background_jobs();
+    hup_background_jobs();
 }
 
 static bool selection_is_at_top() {

src/reader.h

@@ -227,9 +227,6 @@ wcstring completion_apply_to_command_line(const wcstring &val_str, complete_flag
                                           const wcstring &command_line, size_t *inout_cursor_pos,
                                           bool append_only);
 
-/// Terminate all background jobs
-void kill_background_jobs();
-
 /// Print warning with list of backgrounded jobs
 void reader_bg_job_warning();