4 files changed, 56 insertions, 1 deletions
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 5f221944594b..5df03da74e00 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -26,6 +26,7 @@ typedef enum {
     actVerifyPaths = 107,
     actSubstitute = 108,
     actQueryPathInfo = 109,
+    actPostBuildHook = 110,
 } ActivityType;
 
 typedef enum {
@@ -36,6 +37,7 @@ typedef enum {
     resSetPhase = 104,
     resProgress = 105,
     resSetExpected = 106,
+    resPostBuildLogLine = 107,
 } ResultType;
 
 typedef uint64_t ActivityId;
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 969e4dff383d..a344a5ac7520 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -179,6 +179,36 @@ struct TeeSource : Source
     }
 };
 
+/* A reader that consumes the original Source until 'size'. */
+struct SizedSource : Source
+{
+    Source & orig;
+    size_t remain;
+    SizedSource(Source & orig, size_t size)
+        : orig(orig), remain(size) { }
+    size_t read(unsigned char * data, size_t len)
+    {
+        if (this->remain <= 0) {
+            throw EndOfFile("sized: unexpected end-of-file");
+        }
+        len = std::min(len, this->remain);
+        size_t n = this->orig.read(data, len);
+        this->remain -= n;
+        return n;
+    }
+
+    /* Consume the original source until no remain data is left to consume. */
+    size_t drainAll()
+    {
+        std::vector<unsigned char> buf(8192);
+        size_t sum = 0;
+        while (this->remain > 0) {
+            size_t n = read(buf.data(), buf.size());
+            sum += n;
+        }
+        return sum;
+    }
+};
 
 /* Convert a function into a sink. */
 struct LambdaSink : Sink
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 17aee2d5c3d0..44fa72482552 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -84,6 +84,15 @@ void clearEnv()
         unsetenv(name.first.c_str());
 }
 
+void replaceEnv(std::map<std::string, std::string> newEnv)
+{
+    clearEnv();
+    for (auto newEnvVar : newEnv)
+    {
+        setenv(newEnvVar.first.c_str(), newEnvVar.second.c_str(), 1);
+    }
+}
+
 
 Path absPath(Path path, Path dir)
 {
@@ -1019,10 +1028,22 @@ void runProgram2(const RunOptions & options)
     if (options.standardOut) out.create();
     if (source) in.create();
 
+    ProcessOptions processOptions;
+    // vfork implies that the environment of the main process and the fork will
+    // be shared (technically this is undefined, but in practice that's the
+    // case), so we can't use it if we alter the environment
+    if (options.environment)
+        processOptions.allowVfork = false;
+
     /* Fork. */
     Pid pid = startProcess([&]() {
+        if (options.environment)
+            replaceEnv(*options.environment);
         if (options.standardOut && dup2(out.writeSide.get(), STDOUT_FILENO) == -1)
             throw SysError("dupping stdout");
+        if (options.mergeStderrToStdout)
+            if (dup2(STDOUT_FILENO, STDERR_FILENO) == -1)
+                throw SysError("cannot dup stdout into stderr");
         if (source && dup2(in.readSide.get(), STDIN_FILENO) == -1)
             throw SysError("dupping stdin");
 
@@ -1047,7 +1068,7 @@ void runProgram2(const RunOptions & options)
             execv(options.program.c_str(), stringsToCharPtrs(args_).data());
 
         throw SysError("executing '%1%'", options.program);
-    });
+    }, processOptions);
 
     out.writeSide = -1;
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index fce3cab8def5..b538a0b41ce8 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -270,12 +270,14 @@ struct RunOptions
     std::optional<uid_t> uid;
     std::optional<uid_t> gid;
     std::optional<Path> chdir;
+    std::optional<std::map<std::string, std::string>> environment;
     Path program;
     bool searchPath = true;
     Strings args;
     std::optional<std::string> input;
     Source * standardIn = nullptr;
     Sink * standardOut = nullptr;
+    bool mergeStderrToStdout = false;
     bool _killStderr = false;
 
     RunOptions(const Path & program, const Strings & args)