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, 13 insertions, 0 deletions

diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 6f3bf7ae8..2e416edef 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -1260,6 +1260,19 @@ string replaceStrings(const std::string & s,
 }
 
 
+std::string rewriteStrings(const std::string & _s, const StringMap & rewrites)
+{
+    auto s = _s;
+    for (auto & i : rewrites) {
+        if (i.first == i.second) continue;
+        size_t j = 0;
+        while ((j = s.find(i.first, j)) != string::npos)
+            s.replace(j, i.first.size(), i.second);
+    }
+    return s;
+}
+
+
 string statusToString(int status)
 {
     if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {