* Ad hoc per-package logging. When Nix performs a derivation, it now

writes stdout/stderr of the builder to ${prefix}/var/log/nix/x,
  where x is the file name of the derivation expression, e.g.,

    /nix/var/log/nix/54256391624be04fcb426048ae3ea0a4-d-pan-0.14.2.nix

  Note that consecutive builds of the same expression overwrite,
  rather than append to, existing log files.

src/libnix/exec.cc

@@ -16,14 +16,14 @@ static string pathNullDevice = "/dev/null";
 
 /* Run a program. */
 void runProgram(const string & program, 
-    const Strings & args, const Environment & env)
+    const Strings & args, const Environment & env,
+    const string & logFileName)
 {
     /* Create a log file. */
-    string logFileName = nixLogDir + "/run.log";
     string logCommand = 
 	verbosity >= lvlDebug 
-	? "tee -a "  + logFileName + " >&2"
+	? "tee "  + logFileName + " >&2"
-	: "cat >> " + logFileName;
+	: "cat > " + logFileName;
     /* !!! auto-pclose on exit */
     FILE * logFile = popen(logCommand.c_str(), "w"); /* !!! escaping */
     if (!logFile)

src/libnix/exec.hh

@@ -15,7 +15,8 @@ typedef map<string, string> Environment;
 
 /* Run a program. */
 void runProgram(const string & program, 
-    const Strings & args, const Environment & env);
+    const Strings & args, const Environment & env,
+    const string & logFileName);
 
 
 #endif /* !__EXEC_H */

src/libnix/normalise.cc

@@ -164,7 +164,8 @@ Path normaliseNixExpr(const Path & _nePath, PathSet pending)
 
         /* Run the builder. */
         msg(lvlChatty, format("building..."));
-        runProgram(ne.derivation.builder, ne.derivation.args, env);
+        runProgram(ne.derivation.builder, ne.derivation.args, env,
+            nixLogDir + "/" + baseNameOf(nePath));
         msg(lvlChatty, format("build completed"));
         
     } else