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?'.

2 files changed, 8 insertions, 8 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()) {
diff --git a/nix/libstore/local-store.cc b/nix/libstore/local-store.cc
index c304e2ddd1..675d1ba66f 100644
--- a/nix/libstore/local-store.cc
+++ b/nix/libstore/local-store.cc
@@ -780,8 +780,8 @@ Path LocalStore::queryPathFromHashPart(const string & hashPart)
     });
 }
 
-/* Read a line from the substituter's stdout, while also processing
-   its stderr. */
+/* Read a line from the substituter's reply file descriptor, while also
+   processing its stderr. */
 string LocalStore::getLineFromSubstituter(Agent & run)
 {
     string res, err;
@@ -802,9 +802,9 @@ string LocalStore::getLineFromSubstituter(Agent & run)
         }
 
         /* Completely drain stderr before dealing with stdout. */
-        if (FD_ISSET(run.builderOut.readSide, &fds)) {
+        if (FD_ISSET(run.fromAgent.readSide, &fds)) {
             char buf[4096];
-            ssize_t n = read(run.builderOut.readSide, (unsigned char *) buf, sizeof(buf));
+            ssize_t n = read(run.fromAgent.readSide, (unsigned char *) buf, sizeof(buf));
             if (n == -1) {
                 if (errno == EINTR) continue;
                 throw SysError("reading from substituter's stderr");
@@ -822,9 +822,9 @@ string LocalStore::getLineFromSubstituter(Agent & run)
         }
 
         /* Read from stdout until we get a newline or the buffer is empty. */
-        else if (FD_ISSET(run.fromAgent.readSide, &fds)) {
+        else if (FD_ISSET(run.builderOut.readSide, &fds)) {
 	    unsigned char c;
-	    readFull(run.fromAgent.readSide, (unsigned char *) &c, 1);
+	    readFull(run.builderOut.readSide, (unsigned char *) &c, 1);
 	    if (c == '\n') {
 		if (!err.empty()) printMsg(lvlError, "substitute: " + err);
 		return res;