download-from-binary-cache: parallelise fetching of NAR info files

Getting substitute information using the binary cache substituter has
non-trivial latency overhead.  A package or NixOS system configuration
can have hundreds of dependencies, and in the worst case (when the
local info cache is empty) we have to do a separate HTTP request for
each of these.  If the ping time to the server is t, getting N info
files will take tN seconds; e.g., with a ping time of 0.1s to
nixos.org, sequentially downloading 1000 info files (a typical NixOS
config) will take at least 100 seconds.

To fix this problem, the binary cache substituter can now perform
requests in parallel.  This required changing the substituter
interface to support a function querySubstitutablePathInfos() that
queries multiple paths at the same time, and rewriting queryMissing()
to take advantage of parallelism.  (Due to local caching,
parallelising queryMissing() is sufficient for most use cases, since
it's almost always called before building a derivation and thus fills
the local info cache.)

For example, parallelism speeds up querying all 1056 paths in a
particular NixOS system configuration from 116s to 2.6s.  It works so
well because the eccentricity of the top-level derivation in the
dependency graph is only 9.  So we only need 10 round-trips (when
using an unlimited number of parallel connections) to get everything.

Currently we do a maximum of 150 parallel connections to the server.
Thus it's important that the binary cache server (e.g. nixos.org) has
a high connection limit.  Alternatively we could use HTTP pipelining,
but WWW::Curl doesn't support it and libcurl has a hard-coded limit of
5 requests per pipeline.

9 files changed, 282 insertions, 131 deletions

diff --git a/scripts/download-from-binary-cache.pl.in b/scripts/download-from-binary-cache.pl.in
index ea37c818d311..a67818e7f5b4 100644
--- a/scripts/download-from-binary-cache.pl.in
+++ b/scripts/download-from-binary-cache.pl.in
@@ -12,33 +12,40 @@ use strict;
 
 my @binaryCacheUrls = map { s/\/+$//; $_ } split(/ /, ($ENV{"NIX_BINARY_CACHES"} || ""));
 
+my $maxParallelRequests = 150;
+
 my ($dbh, $insertNAR, $queryNAR, $insertNegativeNAR, $queryNegativeNAR);
 my %cacheIds;
 
 my $curlm = WWW::Curl::Multi->new;
 my $activeRequests = 0;
 my $curlIdCount = 1;
-my %curlHandles;
+my %requests;
+my %scheduled;
 my $caBundle = $ENV{"CURL_CA_BUNDLE"} || $ENV{"OPENSSL_X509_CERT_FILE"};
 
 
 sub addRequest {
-    my ($url) = @_;
+    my ($storePath, $url) = @_;
     
     my $curl = WWW::Curl::Easy->new;
     my $curlId = $curlIdCount++;
-    $curlHandles{$curlId} = { handle => $curl, content => "" };
+    $requests{$curlId} = { storePath => $storePath, url => $url, handle => $curl, content => "" };
 
     $curl->setopt(CURLOPT_PRIVATE, $curlId);
     $curl->setopt(CURLOPT_URL, $url);
-    $curl->setopt(CURLOPT_WRITEDATA, \$curlHandles{$curlId}->{content});
+    $curl->setopt(CURLOPT_WRITEDATA, \$requests{$curlId}->{content});
     $curl->setopt(CURLOPT_FOLLOWLOCATION, 1);
     $curl->setopt(CURLOPT_CAINFO, $caBundle) if defined $caBundle;
 
-    $curlm->add_handle($curl);
-    $activeRequests++;
+    if ($activeRequests >= $maxParallelRequests) {
+        $scheduled{$curlId} = 1;
+    } else {
+        $curlm->add_handle($curl);
+        $activeRequests++;
+    }
 
-    return $curlHandles{$curlId};
+    return $requests{$curlId};
 }
 
 
@@ -55,12 +62,20 @@ sub processRequests {
         if ($curlm->perform() != $activeRequests) {
             while (my ($id, $result) = $curlm->info_read) {
                 if ($id) {
-                    my $handle = $curlHandles{$id}->{handle};
-                    $curlHandles{$id}->{result} = $result;
-                    $curlHandles{$id}->{httpStatus} = $handle->getinfo(CURLINFO_HTTP_CODE);
-                    #print STDERR "\nRequest completed ($id, $result, $curlHandles{$id}->{httpStatus})\n";
+                    my $handle = $requests{$id}->{handle};
+                    $requests{$id}->{result} = $result;
+                    $requests{$id}->{httpStatus} = $handle->getinfo(CURLINFO_HTTP_CODE);
+                    #print STDERR "\nRequest completed ($id, $result, $requests{$id}->{httpStatus})\n";
                     $activeRequests--;
-                    delete $curlHandles{$id}->{handle};
+                    delete $requests{$id}->{handle};
+
+                    if (scalar(keys %scheduled) > 0) {
+                        my $id2 = (keys %scheduled)[0];
+                        $curlm->add_handle($requests{$id2}->{handle});
+                        $activeRequests++;
+                        delete $scheduled{$id2};
+                    }
+                    
                 }
             }
         }
@@ -130,23 +145,21 @@ EOF
 }
 
 
-sub getInfoFrom {
-    my ($storePath, $pathHash, $binaryCacheUrl) = @_;
+sub negativeHit {
+    my ($storePath, $binaryCacheUrl) = @_;
+    $queryNegativeNAR->execute(getCacheId($binaryCacheUrl), basename($storePath));
+    return @{$queryNegativeNAR->fetchall_arrayref()} != 0;
+}
 
-    my $cacheId = getCacheId($binaryCacheUrl);
 
-    # Bail out if there is a negative cache entry.
-    $queryNegativeNAR->execute($cacheId, basename($storePath));
-    return undef if @{$queryNegativeNAR->fetchall_arrayref()} != 0;
+sub processNARInfo {
+    my ($storePath, $binaryCacheUrl, $request) = @_;
     
-    my $infoUrl = "$binaryCacheUrl/$pathHash.narinfo";
-    print STDERR "checking $infoUrl...\n";
-    my $request = addRequest($infoUrl);
-    processRequests;
+    my $cacheId = getCacheId($binaryCacheUrl);
 
     if ($request->{result} != 0 || $request->{httpStatus} != 200) {
         if ($request->{httpStatus} != 404) {
-            print STDERR "could not download ‘$infoUrl’ (" .
+            print STDERR "could not download ‘$request->{url}’ (" .
                 ($request->{result} != 0 ? "Curl error $request->{result}" : "HTTP status $request->{httpStatus}") . ")\n";
         } else {
             $insertNegativeNAR->execute($cacheId, basename($storePath), time());
@@ -172,7 +185,7 @@ sub getInfoFrom {
     }
     return undef if $storePath ne $storePath2;
     if ($storePath ne $storePath2 || !defined $url || !defined $narHash) {
-        print STDERR "bad NAR info file ‘$infoUrl’\n";
+        print STDERR "bad NAR info file ‘$request->{url}’\n";
         return undef;
     }
     
@@ -236,24 +249,65 @@ sub cachedGetInfoFrom {
 }
 
 
-sub getInfo {
-    my ($storePath) = @_;
+sub printInfo {
+    my ($storePath, $info) = @_;
+    print "$storePath\n";
+    print $info->{deriver} ? "$Nix::Config::storeDir/$info->{deriver}" : "", "\n";
+    print scalar @{$info->{refs}}, "\n";
+    print "$Nix::Config::storeDir/$_\n" foreach @{$info->{refs}};
+    print $info->{fileSize} || 0, "\n";
+    print $info->{narSize} || 0, "\n";
+}
 
-    my $pathHash = substr(basename($storePath), 0, 32);
 
-    # First look if we have cached info for one of the URLs.
-    foreach my $binaryCacheUrl (@binaryCacheUrls) {
-        my $info = cachedGetInfoFrom($storePath, $pathHash, $binaryCacheUrl);
-        return $info if defined $info;
+sub printInfoParallel {
+    my @paths = @_;
+
+    # First print all paths for which we have cached info.
+    my @left;
+    foreach my $storePath (@paths) {
+        my $pathHash = substr(basename($storePath), 0, 32);
+        my $found = 0;
+        foreach my $binaryCacheUrl (@binaryCacheUrls) {
+            my $info = cachedGetInfoFrom($storePath, $pathHash, $binaryCacheUrl);
+            if (defined $info) {
+                printInfo($storePath, $info);
+                $found = 1;
+                last;
+            }
+        }
+        push @left, $storePath if !$found;
     }
 
-    # No, so do an HTTP request until we get a hit.
+    return if scalar @left == 0;
+
     foreach my $binaryCacheUrl (@binaryCacheUrls) {
-        my $info = getInfoFrom($storePath, $pathHash, $binaryCacheUrl);
-        return $info if defined $info;
-    }
 
-    return undef;
+        my @left2;
+        %requests = ();
+        foreach my $storePath (@left) {
+            my $pathHash = substr(basename($storePath), 0, 32);
+            if (negativeHit($storePath, $binaryCacheUrl)) {
+                push @left2, $storePath;
+                next;
+            }
+            my $infoUrl = "$binaryCacheUrl/$pathHash.narinfo";
+            addRequest($storePath, $infoUrl);
+        }
+
+        processRequests;
+
+        foreach my $request (values %requests) {
+            my $info = processNARInfo($request->{storePath}, $binaryCacheUrl, $request);
+            if (defined $info) {
+                printInfo($request->{storePath}, $info);
+            } else {
+                push @left2, $request->{storePath};
+            }
+        }
+
+        @left = @left2;
+    }
 }
 
 
@@ -264,30 +318,37 @@ sub downloadBinary {
 
     cache: foreach my $binaryCacheUrl (@binaryCacheUrls) {
         my $info = cachedGetInfoFrom($storePath, $pathHash, $binaryCacheUrl);
-        $info = getInfoFrom($storePath, $pathHash, $binaryCacheUrl) unless defined $info;
-        if (defined $info) {
-            my $decompressor;
-            if ($info->{compression} eq "bzip2") { $decompressor = "$Nix::Config::bzip2 -d"; }
-            elsif ($info->{compression} eq "xz") { $decompressor = "$Nix::Config::xz -d"; }
-            else {
-                print STDERR "unknown compression method ‘$info->{compression}’\n";
-                next;
-            }
-            print STDERR "\n*** Downloading ‘$info->{url}’ into ‘$storePath’...\n";
-            if (system("$Nix::Config::curl --fail --location $binaryCacheUrl/$info->{url} | $decompressor | $Nix::Config::binDir/nix-store --restore $storePath") != 0) {
-                die "download of `$info->{url}' failed" . ($! ? ": $!" : "") . "\n" unless $? == 0;
-                next;
-            }
-            # The hash in the manifest can be either in base-16 or
-            # base-32.  Handle both.
-            $info->{narHash} =~ /^sha256:(.*)$/ or die "invalid hash";
-            my $hash = $1;
-            my $hash2 = hashPath("sha256", 1, $storePath);
-            die "hash mismatch in downloaded path ‘$storePath’; expected $hash, got $hash2\n"
-                if $hash ne $hash2;
-            print STDERR "\n";
-            return 1;
+
+        unless (defined $info) {
+            next if negativeHit($storePath, $binaryCacheUrl);
+            my $request = addRequest($storePath, "$binaryCacheUrl/$pathHash.narinfo");
+            processRequests;
+            $info = processNARInfo($storePath, $binaryCacheUrl, $request);
+        }
+
+        next unless defined $info;
+        
+        my $decompressor;
+        if ($info->{compression} eq "bzip2") { $decompressor = "$Nix::Config::bzip2 -d"; }
+        elsif ($info->{compression} eq "xz") { $decompressor = "$Nix::Config::xz -d"; }
+        else {
+            print STDERR "unknown compression method ‘$info->{compression}’\n";
+            next;
+        }
+        print STDERR "\n*** Downloading ‘$info->{url}’ into ‘$storePath’...\n";
+        if (system("$Nix::Config::curl --fail --location $binaryCacheUrl/$info->{url} | $decompressor | $Nix::Config::binDir/nix-store --restore $storePath") != 0) {
+            die "download of `$info->{url}' failed" . ($! ? ": $!" : "") . "\n" unless $? == 0;
+            next;
         }
+        # The hash in the manifest can be either in base-16 or
+        # base-32.  Handle both.
+        $info->{narHash} =~ /^sha256:(.*)$/ or die "invalid hash";
+        my $hash = $1;
+        my $hash2 = hashPath("sha256", 1, $storePath);
+        die "hash mismatch in downloaded path ‘$storePath’; expected $hash, got $hash2\n"
+            if $hash ne $hash2;
+        print STDERR "\n";
+        return 1;
     }
 
     return 0;
@@ -300,29 +361,20 @@ initCache();
 if ($ARGV[0] eq "--query") {
 
     while (<STDIN>) {
-        my $cmd = $_; chomp $cmd;
-
+        chomp;
+        my ($cmd, @args) = split " ", $_;
+        
         if ($cmd eq "have") {
             my $storePath = <STDIN>; chomp $storePath;
             # FIXME: want to give correct info here, but it's too slow.
-            #print "0\n";
-            my $info = getInfo($storePath);
-            if (defined $info) { print "1\n"; } else { print "0\n"; }
+            print "0\n";
+            #my $info = getInfo($storePath);
+            #if (defined $info) { print "1\n"; } else { print "0\n"; }
         }
 
         elsif ($cmd eq "info") {
-            my $storePath = <STDIN>; chomp $storePath;
-            my $info = getInfo($storePath);
-            if (defined $info) {
-                print "1\n";
-                print $info->{deriver} ? "$Nix::Config::storeDir/$info->{deriver}" : "", "\n";
-                print scalar @{$info->{refs}}, "\n";
-                print "$Nix::Config::storeDir/$_\n" foreach @{$info->{refs}};
-                print $info->{fileSize} || 0, "\n";
-                print $info->{narSize} || 0, "\n";
-            } else {
-                print "0\n";
-            }
+            printInfoParallel(@args);
+            print "\n";
         }
 
         else { die "unknown command `$cmd'"; }
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index d5bbd540b34d..1c84e5b9f92d 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2352,10 +2352,12 @@ void SubstitutionGoal::tryNext()
     sub = subs.front();
     subs.pop_front();
 
-    if (!worker.store.querySubstitutablePathInfo(sub, storePath, info)) {
-        tryNext();
-        return;
-    }
+    SubstitutablePathInfos infos;
+    PathSet dummy(singleton<PathSet>(storePath));
+    worker.store.querySubstitutablePathInfos(sub, dummy, infos);
+    SubstitutablePathInfos::iterator k = infos.find(storePath);
+    if (k == infos.end()) { tryNext(); return; }
+    info = k->second;
 
     /* To maintain the closure invariant, we first have to realise the
        paths referenced by this one. */
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 5c22f1406649..f660ed68df0d 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -155,8 +155,9 @@ void setDefaultsFromEnvironment()
 
     string subs = getEnv("NIX_SUBSTITUTERS", "default");
     if (subs == "default") {
-        substituters.push_back(nixLibexecDir + "/nix/substituters/copy-from-other-stores.pl");
-        substituters.push_back(nixLibexecDir + "/nix/substituters/download-using-manifests.pl");
+        //substituters.push_back(nixLibexecDir + "/nix/substituters/copy-from-other-stores.pl");
+        //substituters.push_back(nixLibexecDir + "/nix/substituters/download-using-manifests.pl");
+        substituters.push_back(nixLibexecDir + "/nix/substituters/download-from-binary-cache.pl");
     } else
         substituters = tokenizeString(subs, ":");
 
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 1ce62aeafcef..b4ea4b748178 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -936,37 +936,57 @@ bool LocalStore::hasSubstitutes(const Path & path)
 }
 
 
-bool LocalStore::querySubstitutablePathInfo(const Path & substituter,
-    const Path & path, SubstitutablePathInfo & info)
+void LocalStore::querySubstitutablePathInfos(const Path & substituter,
+    PathSet & paths, SubstitutablePathInfos & infos)
 {
     RunningSubstituter & run(runningSubstituters[substituter]);
     startSubstituter(substituter, run);
 
-    writeLine(run.to, "info\n" + path);
+    string s = "info ";
+    foreach (PathSet::const_iterator, i, paths)
+        if (infos.find(*i) == infos.end()) { s += *i; s += " "; }
+    writeLine(run.to, s);
 
-    if (!getIntLine<int>(run.from)) return false;
-    
-    info.deriver = readLine(run.from);
-    if (info.deriver != "") assertStorePath(info.deriver);
-    int nrRefs = getIntLine<int>(run.from);
-    while (nrRefs--) {
-        Path p = readLine(run.from);
-        assertStorePath(p);
-        info.references.insert(p);
+    while (true) {
+        Path path = readLine(run.from);
+        if (path == "") break;
+        assert(paths.find(path) != paths.end());
+        paths.erase(path);
+        SubstitutablePathInfo & info(infos[path]);
+        info.deriver = readLine(run.from);
+        if (info.deriver != "") assertStorePath(info.deriver);
+        int nrRefs = getIntLine<int>(run.from);
+        while (nrRefs--) {
+            Path p = readLine(run.from);
+            assertStorePath(p);
+            info.references.insert(p);
+        }
+        info.downloadSize = getIntLine<long long>(run.from);
+        info.narSize = getIntLine<long long>(run.from);
     }
-    info.downloadSize = getIntLine<long long>(run.from);
-    info.narSize = getIntLine<long long>(run.from);
-    
-    return true;
 }
 
 
 bool LocalStore::querySubstitutablePathInfo(const Path & path,
     SubstitutablePathInfo & info)
 {
-    foreach (Paths::iterator, i, substituters)
-        if (querySubstitutablePathInfo(*i, path, info)) return true;
-    return false;
+    SubstitutablePathInfos infos;
+    querySubstitutablePathInfos(singleton<PathSet>(path), infos);
+    SubstitutablePathInfos::iterator i = infos.find(path);
+    if (i == infos.end()) return false;
+    info = i->second;
+    return true;
+}
+
+
+void LocalStore::querySubstitutablePathInfos(const PathSet & paths,
+    SubstitutablePathInfos & infos)
+{
+    PathSet todo = paths;
+    foreach (Paths::iterator, i, substituters) {
+        if (todo.empty()) break;
+        querySubstitutablePathInfos(*i, todo, infos);
+    }
 }
 
 
@@ -1110,7 +1130,7 @@ Path LocalStore::addToStore(const Path & _srcPath,
        method for very large paths, but `copyPath' is mainly used for
        small files. */
     StringSink sink;
-    if (recursive) 
+    if (recursive)
         dumpPath(srcPath, sink, filter);
     else
         sink.s = readFile(srcPath);
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index aa8e8582fb0d..c4d8be692a36 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -128,8 +128,11 @@ public:
     bool querySubstitutablePathInfo(const Path & path,
         SubstitutablePathInfo & info);
     
-    bool querySubstitutablePathInfo(const Path & substituter,
-        const Path & path, SubstitutablePathInfo & info);
+    void querySubstitutablePathInfos(const Path & substituter,
+        PathSet & paths, SubstitutablePathInfos & infos);
+    
+    void querySubstitutablePathInfos(const PathSet & paths,
+        SubstitutablePathInfos & infos);
     
     Path addToStore(const Path & srcPath,
         bool recursive = true, HashType hashAlgo = htSHA256,
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index 093499936349..aa5f6ff727c9 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -55,45 +55,97 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
     
     PathSet todo(targets.begin(), targets.end()), done;
 
+    bool useSubstitutes = queryBoolSetting("build-use-substitutes", true);
+
+    /* Getting substitute info has high latency when using the binary
+       cache substituter.  Thus it's essential to do substitute
+       queries in parallel as much as possible.  To accomplish this
+       we do the following:
+
+       - For all paths still to be processed (‘todo’), we add all
+         paths for which we need info to the set ‘query’.  For an
+         unbuilt derivation this is the output paths; otherwise, it's
+         the path itself.
+
+       - We get info about all paths in ‘query’ in parallel.
+
+       - We process the results and add new items to ‘todo’ if
+         necessary.  E.g. if a path is substitutable, then we need to
+         get info on its references.
+
+       - Repeat until ‘todo’ is empty.
+    */
+
     while (!todo.empty()) {
-        Path p = *(todo.begin());
-        todo.erase(p);
-        if (done.find(p) != done.end()) continue;
-        done.insert(p);
-
-        if (isDerivation(p)) {
-            if (!store.isValidPath(p)) {
-                unknown.insert(p);
-                continue;
+              
+        PathSet query, todoDrv, todoNonDrv;
+
+        foreach (PathSet::iterator, i, todo) {
+            if (done.find(*i) != done.end()) continue;
+            done.insert(*i);
+
+            if (isDerivation(*i)) {
+                if (!store.isValidPath(*i)) {
+                    // FIXME: we could try to substitute p.
+                    unknown.insert(*i);
+                    continue;
+                }
+                Derivation drv = derivationFromPath(store, *i);
+
+                PathSet invalid;
+                foreach (DerivationOutputs::iterator, j, drv.outputs)
+                    if (!store.isValidPath(j->second.path)) invalid.insert(j->second.path);
+                if (invalid.empty()) continue;
+                
+                todoDrv.insert(*i);
+                if (useSubstitutes) query.insert(invalid.begin(), invalid.end());
+            }
+
+            else {
+                if (store.isValidPath(*i)) continue;
+                query.insert(*i);
+                todoNonDrv.insert(*i);
             }
-            Derivation drv = derivationFromPath(store, p);
+        }
+
+        todo.clear();
+        
+        SubstitutablePathInfos infos;
+        store.querySubstitutablePathInfos(query, infos);
+
+        foreach (PathSet::iterator, i, todoDrv) {
+            // FIXME: cache this
+            Derivation drv = derivationFromPath(store, *i);
 
             bool mustBuild = false;
-            foreach (DerivationOutputs::iterator, i, drv.outputs)
-                if (!store.isValidPath(i->second.path) &&
-                    !(queryBoolSetting("build-use-substitutes", true) && store.hasSubstitutes(i->second.path)))
-                    mustBuild = true;
+            if (useSubstitutes) {
+                foreach (DerivationOutputs::iterator, j, drv.outputs)
+                    if (!store.isValidPath(j->second.path) &&
+                        infos.find(j->second.path) == infos.end())
+                        mustBuild = true;
+            } else
+                mustBuild = true;
 
             if (mustBuild) {
-                willBuild.insert(p);
+                willBuild.insert(*i);
                 todo.insert(drv.inputSrcs.begin(), drv.inputSrcs.end());
                 foreach (DerivationInputs::iterator, i, drv.inputDrvs)
                     todo.insert(i->first);
-            } else 
+            } else
                 foreach (DerivationOutputs::iterator, i, drv.outputs)
-                    todo.insert(i->second.path);
+                    todoNonDrv.insert(i->second.path);
         }
-
-        else {
-            if (store.isValidPath(p)) continue;
-            SubstitutablePathInfo info;
-            if (store.querySubstitutablePathInfo(p, info)) {
-                willSubstitute.insert(p);
-                downloadSize += info.downloadSize;
-                narSize += info.narSize;
-                todo.insert(info.references.begin(), info.references.end());
+        
+        foreach (PathSet::iterator, i, todoNonDrv) {
+            done.insert(*i);
+            SubstitutablePathInfos::iterator info = infos.find(*i);
+            if (info != infos.end()) {
+                willSubstitute.insert(*i);
+                downloadSize += info->second.downloadSize;
+                narSize += info->second.narSize;
+                todo.insert(info->second.references.begin(), info->second.references.end());
             } else
-                unknown.insert(p);
+                unknown.insert(*i);
         }
     }
 }
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 5e5561a6aecf..1cf67d3731d1 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -256,6 +256,19 @@ bool RemoteStore::querySubstitutablePathInfo(const Path & path,
 }
 
 
+void RemoteStore::querySubstitutablePathInfos(const PathSet & paths,
+    SubstitutablePathInfos & infos)
+{
+    if (paths.empty()) return;
+    printMsg(lvlError, format("QUERYING %1% (REMOTE)") % showPaths(paths));
+    foreach (PathSet::const_iterator, i, paths) {
+        SubstitutablePathInfo info;
+        if (querySubstitutablePathInfo(*i, info))
+            infos[*i] = info;
+    }
+}
+
+
 ValidPathInfo RemoteStore::queryPathInfo(const Path & path)
 {
     openConnection();
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index e9f40da6dbf4..1056a6115849 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -48,6 +48,9 @@ public:
     bool querySubstitutablePathInfo(const Path & path,
         SubstitutablePathInfo & info);
     
+    void querySubstitutablePathInfos(const PathSet & paths,
+        SubstitutablePathInfos & infos);
+    
     Path addToStore(const Path & srcPath,
         bool recursive = true, HashType hashAlgo = htSHA256,
         PathFilter & filter = defaultPathFilter);
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index bf3269f57818..92b2ddb1e730 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -86,6 +86,8 @@ struct SubstitutablePathInfo
     unsigned long long narSize; /* 0 = unknown */
 };
 
+typedef std::map<Path, SubstitutablePathInfo> SubstitutablePathInfos;
+
 
 struct ValidPathInfo 
 {
@@ -147,6 +149,9 @@ public:
        substitutable path. */
     virtual bool querySubstitutablePathInfo(const Path & path,
         SubstitutablePathInfo & info) = 0;
+
+    virtual void querySubstitutablePathInfos(const PathSet & paths,
+        SubstitutablePathInfos & infos) = 0;
     
     /* Copy the contents of a path to the store and register the
        validity the resulting path.  The resulting path is returned.