Serialize exceptions from the sandbox process to the parent

Fixes #4118.

src/libstore/build.cc

@@ -2737,9 +2737,12 @@ void DerivationGoal::startBuilder()
     /* Check if setting up the build environment failed. */
     while (true) {
         string msg = readLine(builderOut.readSide.get());
+        if (string(msg, 0, 1) == "\2") break;
         if (string(msg, 0, 1) == "\1") {
-            if (msg.size() == 1) break;
-            throw Error(string(msg, 1));
+            FdSource source(builderOut.readSide.get());
+            auto ex = readError(source);
+            ex.addTrace({}, "while setting up the build environment");
+            throw ex;
         }
         debug(msg);
     }
@@ -3785,7 +3788,7 @@ void DerivationGoal::runChild()
             args.push_back(rewriteStrings(i, inputRewrites));
 
         /* Indicate that we managed to set up the build environment. */
-        writeFull(STDERR_FILENO, string("\1\n"));
+        writeFull(STDERR_FILENO, string("\2\n"));
 
         /* Execute the program.  This should not return. */
         if (drv->isBuiltin()) {
@@ -3815,8 +3818,11 @@ void DerivationGoal::runChild()
 
         throw SysError("executing '%1%'", drv->builder);
 
-    } catch (std::exception & e) {
-        writeFull(STDERR_FILENO, "\1while setting up the build environment: " + string(e.what()) + "\n");
+    } catch (Error & e) {
+        writeFull(STDERR_FILENO, "\1\n");
+        FdSink sink(STDERR_FILENO);
+        sink << e;
+        sink.flush();
         _exit(1);
     }
 }

src/libutil/serialise.cc

@@ -266,6 +266,24 @@ Sink & operator << (Sink & sink, const StringSet & s)
     return sink;
 }
 
+Sink & operator << (Sink & sink, const Error & ex)
+{
+    auto info = ex.info();
+    sink
+        << "Error"
+        << info.level
+        << info.name
+        << info.description
+        << (info.hint ? info.hint->str() : "")
+        << 0 // FIXME: info.errPos
+        << info.traces.size();
+    for (auto & trace : info.traces) {
+        sink << 0; // FIXME: trace.pos
+        sink << trace.hint.str();
+    }
+    return sink;
+}
+
 
 void readPadding(size_t len, Source & source)
 {
@@ -319,6 +337,30 @@ template Paths readStrings(Source & source);
 template PathSet readStrings(Source & source);
 
 
+Error readError(Source & source)
+{
+    auto type = readString(source);
+    assert(type == "Error");
+    ErrorInfo info;
+    info.level = (Verbosity) readInt(source);
+    info.name = readString(source);
+    info.description = readString(source);
+    auto hint = readString(source);
+    if (hint != "") info.hint = hintformat(std::move(format("%s") % hint));
+    auto havePos = readNum<size_t>(source);
+    assert(havePos == 0);
+    auto nrTraces = readNum<size_t>(source);
+    for (size_t i = 0; i < nrTraces; ++i) {
+        havePos = readNum<size_t>(source);
+        assert(havePos == 0);
+        info.traces.push_back(Trace {
+            .hint = hintformat(std::move(format("%s") % readString(source)))
+        });
+    }
+    return Error(std::move(info));
+}
+
+
 void StringSink::operator () (const unsigned char * data, size_t len)
 {
     static bool warned = false;

src/libutil/serialise.hh

@@ -321,6 +321,7 @@ inline Sink & operator << (Sink & sink, uint64_t n)
 Sink & operator << (Sink & sink, const string & s);
 Sink & operator << (Sink & sink, const Strings & s);
 Sink & operator << (Sink & sink, const StringSet & s);
+Sink & operator << (Sink & in, const Error & ex);
 
 
 MakeError(SerialisationError, Error);
@@ -382,6 +383,8 @@ Source & operator >> (Source & in, bool & b)
     return in;
 }
 
+Error readError(Source & source);
+
 
 /* An adapter that converts a std::basic_istream into a source. */
 struct StreamToSourceAdapter : Source