Thread pgroups into builtin_eval

Ensure that if eval is invoked as part of a pipeline, any jobs spawned
by eval will have the same pgroup as the parent job.

cherry-pick of 82f2d86718

Partially fixes #6806

src/builtin_eval.cpp

@@ -27,7 +27,8 @@ int builtin_eval(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
     const auto cached_exec_count = parser.libdata().exec_count;
     int status = STATUS_CMD_OK;
     if (argc > 1) {
-        if (parser.eval(new_cmd, *streams.io_chain) != eval_result_t::ok) {
+        if (parser.eval(new_cmd, *streams.io_chain, block_type_t::top, streams.parent_pgid) !=
+            eval_result_t::ok) {
             status = STATUS_CMD_ERROR;
         } else if (cached_exec_count == parser.libdata().exec_count) {
             // Issue #5692, in particular, to catch `eval ""`, `eval "begin; end;"`, etc.

src/exec.cpp

@@ -911,6 +911,7 @@ static bool exec_process_in_job(parser_t &parser, process_t *p, const std::share
 
         case process_type_t::builtin: {
             io_streams_t builtin_io_streams{stdout_read_limit};
+            if (j->pgid != INVALID_PID) builtin_io_streams.parent_pgid = j->pgid;
             if (!exec_internal_builtin_proc(parser, j, p, pipe_read.get(), process_net_io_chain,
                                             builtin_io_streams)) {
                 return false;

src/io.h

@@ -454,6 +454,11 @@ struct io_streams_t {
     // Actual IO redirections. This is only used by the source builtin. Unowned.
     const io_chain_t *io_chain{nullptr};
 
+    // The pgid of the job, if any. This enables builtins which run more code like eval() to share
+    // pgid.
+    // TODO: this is awkwardly placed, consider just embedding a lineage here.
+    maybe_t<pid_t> parent_pgid{};
+
     // io_streams_t cannot be copied.
     io_streams_t(const io_streams_t &) = delete;
     void operator=(const io_streams_t &) = delete;

tests/checks/pipeline-pgroup.fish

@@ -37,3 +37,8 @@ end
 and echo "All pgroups agreed"
 or echo "Pgroups disagreed. Found $a0 $a1 $a2, and $b0 $b1 $b2"
 # CHECK: All pgroups agreed
+
+# Ensure that eval retains pgroups - #6806.
+# Our regex will capture the first pgroup and use a positive lookahead on the second.
+$fth print_pgrp | tr \n ' ' 1>&2 | eval '$fth print_pgrp' 1>&2
+# CHECKERR: {{(\d+) (?=\1)\d+}}