Add an option for specifying remote builders

This is useful for one-off situations where you want to specify a
builder on the command line instead of having to mess with
nix.machines. E.g.

  $ nix-build -A hello --argstr system x86_64-darwin \
    --option builders 'root@macstadium1 x86_64-darwin'

will perform the specified build on "macstadium1".

It also removes the need for a separate nix.machines file since you
can specify builders in nix.conf directly. (In fact nix.machines is
yet another hack that predates the general nix.conf configuration
file, IIRC.)

Note: this option is supported by the daemon for trusted users. The
fact that this allows trusted users to specify paths to SSH keys to
which they don't normally have access is maybe a bit too much trust...

src/build-remote/build-remote.cc

@@ -45,7 +45,7 @@ int main (int argc, char * * argv)
         unsetenv("DISPLAY");
         unsetenv("SSH_ASKPASS");
 
-        if (argc != 5)
+        if (argc != 6)
             throw UsageError("called without required arguments");
 
         auto store = openStore();
@@ -54,6 +54,7 @@ int main (int argc, char * * argv)
         settings.maxSilentTime = std::stoll(argv[2]);
         settings.buildTimeout = std::stoll(argv[3]);
         verbosity = (Verbosity) std::stoll(argv[4]);
+        settings.builders = argv[5];
 
         /* It would be more appropriate to use $XDG_RUNTIME_DIR, since
            that gets cleared on reboot, but it wouldn't work on OS X. */
@@ -62,13 +63,7 @@ int main (int argc, char * * argv)
         std::shared_ptr<Store> sshStore;
         AutoCloseFD bestSlotLock;
 
-        Machines machines;
+        auto machines = getMachines();
-        try {
-            parseMachines(readFile(getEnv("NIX_REMOTE_SYSTEMS", SYSCONFDIR "/nix/machines")), machines);
-        } catch (const SysError & e) {
-            if (e.errNo != ENOENT)
-                throw;
-        }
         debug("got %d remote builders", machines.size());
 
         if (machines.empty()) {

src/build-remote/local.mk

@@ -7,5 +7,3 @@ build-remote_INSTALL_DIR := $(libexecdir)/nix
 build-remote_LIBS = libmain libutil libformat libstore
 
 build-remote_SOURCES := $(d)/build-remote.cc
-
-build-remote_CXXFLAGS = -DSYSCONFDIR="\"$(sysconfdir)\""

src/libstore/build.cc

@@ -614,7 +614,8 @@ HookInstance::HookInstance()
             settings.thisSystem,
             std::to_string(settings.maxSilentTime),
             std::to_string(settings.buildTimeout),
-            std::to_string(verbosity)
+            std::to_string(verbosity),
+            settings.builders
         };
 
         execv(settings.buildHook.get().c_str(), stringsToCharPtrs(args).data());

src/libstore/globals.hh

@@ -130,6 +130,9 @@ public:
     PathSetting buildHook{this, true, nixLibexecDir + "/nix/build-remote", "build-hook",
         "The path of the helper program that executes builds to remote machines."};
 
+    Setting<std::string> builders{this, "", "builders",
+        "A semicolon-separated list of build machines, in the format of nix.machines."};
+
     Setting<off_t> reservedSize{this, 8 * 1024 * 1024, "gc-reserved-space",
         "Amount of reserved disk space for the garbage collector."};
 

src/libstore/machines.cc

@@ -44,7 +44,7 @@ bool Machine::mandatoryMet(const std::set<string> & features) const {
 
 void parseMachines(const std::string & s, Machines & machines)
 {
-    for (auto line : tokenizeString<std::vector<string>>(s, "\n")) {
+    for (auto line : tokenizeString<std::vector<string>>(s, "\n;")) {
         chomp(line);
         line.erase(std::find(line.begin(), line.end(), '#'), line.end());
         if (line.empty()) continue;
@@ -62,4 +62,20 @@ void parseMachines(const std::string & s, Machines & machines)
     }
 }
 
+Machines getMachines()
+{
+    Machines machines;
+
+    try {
+        parseMachines(readFile(getEnv("NIX_REMOTE_SYSTEMS", settings.nixConfDir + "/machines")), machines);
+    } catch (const SysError & e) {
+        if (e.errNo != ENOENT)
+            throw;
+    }
+
+    parseMachines(settings.builders, machines);
+
+    return machines;
+}
+
 }

src/libstore/machines.hh

@@ -32,4 +32,6 @@ typedef std::vector<Machine> Machines;
 
 void parseMachines(const std::string & s, Machines & machines);
 
+Machines getMachines();
+
 }