Call commonChildInit() before doing chroot init

This ensures that daemon clients see error messages from the chroot
setup.

nix/libstore/build.cc

@@ -374,8 +374,6 @@ void Goal::trace(const format & f)
 /* Common initialisation performed in child processes. */
 static void commonChildInit(Pipe & logPipe)
 {
-    restoreAffinity();
-
     /* Put the child in a separate session (and thus a separate
        process group) so that it has no controlling terminal (meaning
        that e.g. ssh cannot open /dev/tty) and it doesn't receive
@@ -1974,6 +1972,12 @@ void DerivationGoal::initChild()
 
     try { /* child */
 
+        _writeToStderr = 0;
+
+        restoreAffinity();
+
+        commonChildInit(builderOut);
+
 #if CHROOT_ENABLED
         if (useChroot) {
             /* Initialise the loopback interface. */
@@ -2092,8 +2096,6 @@ void DerivationGoal::initChild()
         }
 #endif
 
-        commonChildInit(builderOut);
-
         if (chdir(tmpDir.c_str()) == -1)
             throw SysError(format("changing into `%1%'") % tmpDir);
 

nix/libutil/util.cc

@@ -466,10 +466,18 @@ void warnOnce(bool & haveWarned, const FormatOrString & fs)
 }
 
 
+static void defaultWriteToStderr(const unsigned char * buf, size_t count)
+{
+    writeFull(STDERR_FILENO, buf, count);
+}
+
+
 void writeToStderr(const string & s)
 {
     try {
-        _writeToStderr((const unsigned char *) s.data(), s.size());
+        auto p = _writeToStderr;
+        if (!p) p = defaultWriteToStderr;
+        p((const unsigned char *) s.data(), s.size());
     } catch (SysError & e) {
         /* Ignore failing writes to stderr if we're in an exception
            handler, otherwise throw an exception.  We need to ignore
@@ -481,12 +489,6 @@ void writeToStderr(const string & s)
 }
 
 
-static void defaultWriteToStderr(const unsigned char * buf, size_t count)
-{
-    writeFull(STDERR_FILENO, buf, count);
-}
-
-
 void (*_writeToStderr) (const unsigned char * buf, size_t count) = defaultWriteToStderr;
 
 
@@ -849,7 +851,7 @@ pid_t startProcess(std::function<void()> fun, const string & errorPrefix)
     if (pid == -1) throw SysError("unable to fork");
 
     if (pid == 0) {
-        _writeToStderr = defaultWriteToStderr;
+        _writeToStderr = 0;
         try {
             restoreAffinity();
             fun();