[PATCH Nix] Flush substituter output upon ‘\r’

[Ludovic Courtès]

The patch below changes the store so that it flushes what it reads from
the substituter’s stderr when it encounters either a newline or a
carriage return.

The intent is to allow substituters to provide some kind of progress
report, where the status is updated by means of a ‘\r’.

As a side effect, this patch removes the “substituter-name: ” prefix on
the lines that are printed.  It can be reinstated if deemed appropriate.

WDYT?

Thanks,
Ludo’.

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 1293a6e..2819843 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1141,8 +1141,10 @@ string 
LocalStore::getLineFromSubstituter(RunningSubstituter & run)
             if (n == 0) throw EndOfFile(format("substituter `%1%' died 
unexpectedly") % run.program);
             err.append(buf, n);
             string::size_type p;
-            while ((p = err.find('\n')) != string::npos) {
-                printMsg(lvlError, run.program + ": " + string(err, 0, p));
+            while (((p = err.find('\n')) != string::npos)
+                  || ((p = err.find('\r')) != string::npos)) {
+               string thing(err, 0, p + 1);
+               writeToStderr(thing);
                 err = string(err, p + 1);
             }
         }