daemon: 'guix substitute' replies on FD 4.

This avoids the situation where error messages would unintentionally go
to stderr and be wrongfully interpreted as a reply by the daemon.

Fixes <https://bugs.gnu.org/46362>.
This is a followup to ee3226e9d54891c7e696912245e4904435be191c.

* guix/scripts/substitute.scm (display-narinfo-data): Add 'port'
parameter and honor it.
(process-query): Likewise.
(process-substitution): Likewise.
(%error-to-file-descriptor-4?, with-redirected-error-port): Remove.
(%reply-file-descriptor): New variable.
(guix-substitute): Remove use of 'with-redirected-error-port'.  Define
'reply-port' and pass it to 'process-query' and 'process-substitution'.
* nix/libstore/build.cc (SubstitutionGoal::handleChildOutput): Swap
'builderOut' and 'fromAgent'.
* nix/libstore/local-store.cc (LocalStore::getLineFromSubstituter):
Likewise.
* tests/substitute.scm <top level>: Set '%reply-file-descriptor'
rather than '%error-to-file-descriptor-4?'.

1 files changed, 2 insertions, 2 deletions

diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 4f486f0822..5697ae5a43 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -3158,13 +3158,13 @@ void SubstitutionGoal::finished()
 void SubstitutionGoal::handleChildOutput(int fd, const string & data)
 {
     if (verbosity >= settings.buildVerbosity
-	&& fd == substituter->builderOut.readSide) {
+	&& fd == substituter->fromAgent.readSide) {
 	writeToStderr(data);
 	/* Don't write substitution output to a log file for now.  We
 	   probably should, though. */
     }
 
-    if (fd == substituter->fromAgent.readSide) {
+    if (fd == substituter->builderOut.readSide) {
 	/* DATA may consist of several lines.  Process them one by one.  */
 	string input = data;
 	while (!input.empty()) {