Support building a derivation if some outputs are already valid

This handles the chroot and build hook cases, which are easy.
Supporting the non-chroot-build case will require more work (hash
rewriting!).

Issue #21.

scripts/build-remote.pl.in

@@ -6,6 +6,7 @@ use IO::Handle;
 use Nix::Config;
 use Nix::SSH qw/sshOpts openSSHConnection/;
 use Nix::CopyClosure;
+use Nix::Store;
 no warnings('once');
 
 
@@ -18,7 +19,7 @@ no warnings('once');
 #   less than the maximum load for that machine, we try to get an
 #   exclusive lock on $currentLoad/$machine-$slot (without blocking).
 #   If we get such a lock, we send "accept" to the caller.  Otherwise,
-#   we send "postpone" and exit. 
+#   we send "postpone" and exit.
 # - We release the exclusive lock on $currentLoad/main-lock.
 # - We perform the build on $neededSystem.
 # - We release the exclusive lock on $currentLoad/$machine-$slot.
@@ -102,14 +103,14 @@ REQ: while (1) {
         sendReply "decline";
         next;
     }
-    
+
     # Acquire the exclusive lock on $currentLoad/main-lock.
     mkdir $currentLoad, 0777 or die unless -d $currentLoad;
     my $mainLock = "$currentLoad/main-lock";
     sysopen MAINLOCK, "$mainLock", O_RDWR|O_CREAT, 0600 or die;
     flock(MAINLOCK, LOCK_EX) or die;
-    
-    
+
+
     while (1) {
         # Find all machine that can execute this build, i.e., that
         # support builds for the given platform and features, and are
@@ -141,7 +142,7 @@ REQ: while (1) {
                     close $slotLock;
                     $slot++;
                 }
-                
+
                 push @available, { machine => $cur, load => $load, free => $free }
                 if $load < $cur->{maxJobs};
             }
@@ -160,7 +161,7 @@ REQ: while (1) {
             if ($rightType && !$canBuildLocally) {
                 sendReply "postpone";
             } else {
-                sendReply "decline";                
+                sendReply "decline";
             }
             close MAINLOCK;
             next REQ;
@@ -182,9 +183,9 @@ REQ: while (1) {
 
 
         # Select the best available machine and lock a free slot.
-        my $selected = $available[0]; 
+        my $selected = $available[0];
         my $machine = $selected->{machine};
-        
+
         $slotLock = openSlotLock($machine, $selected->{free});
         flock($slotLock, LOCK_EX | LOCK_NB) or die;
         utime undef, undef, $slotLock;
@@ -196,7 +197,7 @@ REQ: while (1) {
         @sshOpts = ("-i", $machine->{sshKeys}, "-x");
         $hostName = $machine->{hostName};
         last REQ if openSSHConnection $hostName;
-    
+
         warn "unable to open SSH connection to $hostName, trying other available machines...\n";
         $machine->{enabled} = 0;
     }
@@ -286,10 +287,10 @@ if (system("exec ssh $hostName @sshOpts '(read; kill -INT -\$\$) <&0 & nix-store
 foreach my $output (@outputs) {
     my $maybeSignRemote = "";
     $maybeSignRemote = "--sign" if $UID != 0;
-
+    next if isValidPath($output);
     system("exec ssh $hostName @sshOpts 'nix-store --export $maybeSignRemote $output'" .
            "| NIX_HELD_LOCKS=$output @bindir@/nix-store --import > /dev/null") == 0
-	or die "cannot copy $output from $hostName: $?";
+        or die "cannot copy $output from $hostName: $?";
 }
 
 

src/libstore/build.cc

@@ -756,6 +756,9 @@ class DerivationGoal : public Goal
     /* Referenceable paths (i.e., input and output paths). */
     PathSet allPaths;
 
+    /* Outputs that are already valid. */
+    PathSet validPaths;
+
     /* User selected for running the builder. */
     UserLock buildUser;
 
@@ -1141,7 +1144,7 @@ void DerivationGoal::tryToBuild()
        omitted, but that would be less efficient.)  Note that since we
        now hold the locks on the output paths, no other process can
        build this derivation, so no further checks are necessary. */
-    PathSet validPaths = checkPathValidity(true);
+    validPaths = checkPathValidity(true);
     if (validPaths.size() == drv.outputs.size()) {
         debug(format("skipping build of derivation `%1%', someone beat us to it")
             % drvPath);
@@ -1150,22 +1153,16 @@ void DerivationGoal::tryToBuild()
         return;
     }
 
-    if (validPaths.size() > 0)
-        /* !!! fix this; try to delete valid paths */
-        throw Error(
-            format("derivation `%1%' is blocked by its output paths")
-            % drvPath);
+    printMsg(lvlError, format("BLOCKERS: %1%") % showPaths(validPaths));
 
     /* If any of the outputs already exist but are not valid, delete
        them. */
     foreach (DerivationOutputs::iterator, i, drv.outputs) {
         Path path = i->second.path;
-        if (worker.store.isValidPath(path))
-            throw Error(format("obstructed build: path `%1%' exists") % path);
-        if (pathExists(path)) {
-            debug(format("removing unregistered path `%1%'") % path);
-            deletePathWrapped(path);
-        }
+        if (worker.store.isValidPath(path)) continue;
+        if (!pathExists(path)) continue;
+        debug(format("removing unregistered path `%1%'") % path);
+        deletePathWrapped(path);
     }
 
     /* Check again whether any output previously failed to build,
@@ -1283,6 +1280,10 @@ void DerivationGoal::buildDone()
             Path path = i->second.path;
 
             if (useChroot && pathExists(chrootRootDir + path)) {
+                /* Move output paths from the chroot to the Nix store.
+                   If the output was already valid, just skip
+                   (discard) it. */
+                if (validPaths.find(path) != validPaths.end()) continue;
                 if (rename((chrootRootDir + path).c_str(), path.c_str()) == -1)
                     throw SysError(format("moving build output `%1%' from the chroot to the Nix store") % path);
             }
@@ -1747,6 +1748,9 @@ void DerivationGoal::startBuilder()
 #else
         throw Error("chroot builds are not supported on this platform");
 #endif
+    } else { // !useChroot
+        if (validPaths.size() > 0)
+            throw Error(format("derivation `%1%' is blocked by its output paths %2%") % drvPath % showPaths(validPaths));
     }