Make <nix/unpack-channel.nix> a builtin builder

This was the last function using a shell script, so this allows us to
get rid of tar, coreutils, bash etc.

src/libstore/builtins/unpack-channel.cc

@@ -0,0 +1,39 @@
+#include "rust.hh"
+#include "builtins.hh"
+#include "compression.hh"
+
+namespace nix {
+
+void builtinUnpackChannel(const BasicDerivation & drv)
+{
+    auto getAttr = [&](const string & name) {
+        auto i = drv.env.find(name);
+        if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
+        return i->second;
+    };
+
+    Path out = getAttr("out");
+    auto channelName = getAttr("channelName");
+    auto src = getAttr("src");
+
+    createDirs(out);
+
+    auto source = sinkToSource([&](Sink & sink) {
+        auto decompressor =
+            hasSuffix(src, ".bz2") ? makeDecompressionSink("bzip2", sink) :
+            hasSuffix(src, ".xz") ? makeDecompressionSink("xz", sink) :
+            makeDecompressionSink("none", sink);
+        readFile(src, *decompressor);
+        decompressor->finish();
+    });
+
+    unpack_tarfile(*source, out);
+
+    auto entries = readDirectory(out);
+    if (entries.size() != 1)
+        throw Error("channel tarball '%s' contains more than one file", src);
+    if (rename((out + "/" + entries[0].name).c_str(), (out + "/" + channelName).c_str()) == -1)
+        throw SysError("renaming channel directory");
+}
+
+}