nix-channel: Use binary caches advertised by channels

Channels can now advertise a binary cache by creating a file
<channel-url>/binary-cache-url.  The channel unpacker puts these in
its "binary-caches" subdirectory.  Thus, the URLS of the binary caches
for the channels added by root appear in
/nix/var/nix/profiles/per-user/eelco/channels/binary-caches/*.  The
binary cache substituter reads these and adds them to the list of
binary caches.

corepkgs/unpack-channel.nix

@@ -8,17 +8,21 @@ let
       cd $out
       ${bzip2} -d < $src | ${tar} xf - --warning=no-timestamp
       mv * $out/$channelName
+      if [ -n "$binaryCacheURL" ]; then
+        mkdir $out/binary-caches
+        echo -n "$binaryCacheURL" > $out/binary-caches/$channelName
+      fi
     '';
 
 in
 
-{ name, channelName, src }:
+{ name, channelName, src, binaryCacheURL ? "" }:
 
 derivation {
   system = builtins.currentSystem;
   builder = shell;
   args = [ "-e" builder ];
-  inherit name channelName src;
+  inherit name channelName src binaryCacheURL;
 
   PATH = "${nixBinDir}:${coreutils}";
 

doc/manual/conf-file.xml

@@ -329,6 +329,18 @@ build-use-chroot = /dev /proc /bin</programlisting>
   </varlistentry>
 
 
+  <varlistentry><term><literal>binary-caches-files</literal></term>
+
+    <listitem><para>A list of names of files that will be read to
+    obtain additional binary cache URLs.  The default is
+    <literal>/nix/var/nix/profiles/per-user/root/channels/binary-caches/*</literal>,
+    which ensures that Nix will use the binary caches corresponding to
+    the channels installed by root.  Do not set this option to read
+    files created by untrusted users!</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><literal>trusted-binary-caches</literal></term>
 
     <listitem><para>A list of URLs of binary caches, separated by

scripts/download-from-binary-cache.pl.in

@@ -176,6 +176,16 @@ sub getAvailableCaches {
         ($Nix::Config::config{"binary-caches"}
          // ($Nix::Config::storeDir eq "/nix/store" ? "http://nixos.org/binary-cache" : ""));
 
+    my $urlsFiles = $Nix::Config::config{"binary-cache-files"}
+        // "/nix/var/nix/profiles/per-user/root/channels/binary-caches/*";
+    foreach my $urlFile (glob $urlsFiles) {
+        next unless -f $urlFile;
+        open FILE, "<$urlFile" or die "cannot open ‘$urlFile’\n";
+        my $url = <FILE>; chomp $url;
+        close FILE;
+        push @urls, strToList($url);
+    }
+
     # Allow Nix daemon users to override the binary caches to a subset
     # of those listed in the config file.  Note that ‘untrusted-*’
     # denotes options passed by the client.

scripts/nix-channel.in

@@ -80,12 +80,6 @@ sub update {
 
     readChannels;
 
-    # Create the manifests directory if it doesn't exist.
-    mkdir $manifestDir, 0755 unless -e $manifestDir;
-
-    # Do we have write permission to the manifests directory?
-    die "$0: you do not have write permission to `$manifestDir'!\n" unless -W $manifestDir;
-
     # Download each channel.
     my $exprs = "";
     foreach my $name (keys %channels) {
@@ -102,10 +96,19 @@ sub update {
         $headers =~ s/\r//g;
         $url = $1 if $headers =~ /^Location:\s*(.*)\s*$/m;
 
-        # Pull the channel manifest.
+        # Check if the channel advertises a binary cache.
+        my $binaryCacheURL = `$Nix::Config::curl --silent '$url'/binary-cache-url`;
+        my $extraAttrs = "";
+        if ($? == 0 && $binaryCacheURL ne "") {
+            $extraAttrs .= "binaryCacheURL = \"$binaryCacheURL\"; ";
+        } else {
+            # No binary cache, so pull the channel manifest.
+            mkdir $manifestDir, 0755 unless -e $manifestDir;
+            die "$0: you do not have write permission to `$manifestDir'!\n" unless -W $manifestDir;
             $ENV{'NIX_ORIG_URL'} = $origUrl;
             system("$Nix::Config::binDir/nix-pull", "--skip-wrong-store", "$url/MANIFEST") == 0
                 or die "cannot pull manifest from `$url'\n";
+        }
 
         # Download the channel tarball.
         my $fullURL = "$url/nixexprs.tar.bz2";
@@ -120,7 +123,7 @@ sub update {
         my $cname = $name;
         $cname .= $1 if basename($url) =~ /(-\d.*)$/;
 
-        $exprs .= "'f: f { name = \"$cname\"; channelName = \"$name\"; src = builtins.storePath \"$path\"; }' ";
+        $exprs .= "'f: f { name = \"$cname\"; channelName = \"$name\"; src = builtins.storePath \"$path\"; $extraAttrs }' ";
     }
 
     # Unpack the channel tarballs into the Nix store and install them