Allow content-addressable paths to have references

This adds a command 'nix make-content-addressable' that rewrites the
specified store paths into content-addressable paths. The advantage of
such paths is that 1) they can be imported without signatures; 2) they
can enable deduplication in cases where derivation changes do not
cause output changes (apart from store path hashes).

For example,

  $ nix make-content-addressable -r nixpkgs.cowsay
  rewrote '/nix/store/g1g31ah55xdia1jdqabv1imf6mcw0nb1-glibc-2.25-49' to '/nix/store/48jfj7bg78a8n4f2nhg269rgw1936vj4-glibc-2.25-49'
  ...
  rewrote '/nix/store/qbi6rzpk0bxjw8lw6azn2mc7ynnn455q-cowsay-3.03+dfsg1-16' to '/nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16'

We can then copy the resulting closure to another store without
signatures:

  $ nix copy --trusted-public-keys '' ---to ~/my-nix /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16

In order to support self-references in content-addressable paths,
these paths are hashed "modulo" self-references, meaning that
self-references are zeroed out during hashing. Somewhat annoyingly,
this means that the NAR hash stored in the Nix database is no longer
necessarily equal to the output of "nix hash-path"; for
content-addressable paths, you need to pass the --modulo flag:

  $ nix path-info --json /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16  | jq -r .[].narHash
  sha256:0ri611gdilz2c9rsibqhsipbfs9vwcqvs811a52i2bnkhv7w9mgw

  $ nix hash-path --type sha256 --base32 /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16
  1ggznh07khq0hz6id09pqws3a8q9pn03ya3c03nwck1kwq8rclzs

  $ nix hash-path --type sha256 --base32 /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16 --modulo iq6g2x4q62xp7y7493bibx0qn5w7xz67
  0ri611gdilz2c9rsibqhsipbfs9vwcqvs811a52i2bnkhv7w9mgw

1 files changed, 21 insertions, 1 deletions

diff --git a/src/nix/hash.cc b/src/nix/hash.cc
index af4105e28..9c06e6116 100644
--- a/src/nix/hash.cc
+++ b/src/nix/hash.cc
@@ -2,6 +2,8 @@
 #include "hash.hh"
 #include "legacy.hh"
 #include "shared.hh"
+#include "references.hh"
+#include "archive.hh"
 
 using namespace nix;
 
@@ -13,6 +15,7 @@ struct CmdHash : Command
     bool truncate = false;
     HashType ht = htSHA256;
     std::vector<std::string> paths;
+    std::experimental::optional<std::string> modulus;
 
     CmdHash(Mode mode) : mode(mode)
     {
@@ -23,6 +26,11 @@ struct CmdHash : Command
         mkFlag()
             .longName("type")
             .mkHashTypeFlag(&ht);
+        mkFlag()
+            .longName("modulo")
+            .description("compute hash modulo specified string")
+            .labels({"modulus"})
+            .dest(&modulus);
         expectArgs("paths", &paths);
     }
 
@@ -41,7 +49,19 @@ struct CmdHash : Command
     void run() override
     {
         for (auto path : paths) {
-            Hash h = mode == mFile ? hashFile(ht, path) : hashPath(ht, path).first;
+
+            std::unique_ptr<AbstractHashSink> hashSink;
+            if (modulus)
+                hashSink = std::make_unique<HashModuloSink>(ht, *modulus);
+            else
+                hashSink = std::make_unique<HashSink>(ht);
+
+            if (mode == mFile)
+                readFile(path, *hashSink);
+            else
+                dumpPath(path, *hashSink);
+
+            Hash h = hashSink->finish().first;
             if (truncate && h.hashSize > 20) h = compressHash(h, 20);
             std::cout << format("%1%\n") %
                 h.to_string(base, base == SRI);