Add hashed-mirrors back

Some users have their own hashed-mirrors setup, that is used to mirror
things in addition to what’s available on tarballs.nixos.org. Although
this should be feasable to do with a Binary Cache, it’s not always
easy, since you have to remember what "name" each of the tarballs has.
Continuing to support hashed-mirrors is cheap, so it’s best to leave
support in Nix. Note that NIX_HASHED_MIRRORS is also supported in
Nixpkgs through fetchurl.nix.

Note that this excludes tarballs.nixos.org from the default, as in
\#3689. All of these are available on cache.nixos.org.

doc/manual/command-ref/conf-file.xml

@@ -370,6 +370,33 @@ false</literal>.</para>
 
   </varlistentry>
 
+  <varlistentry xml:id="conf-hashed-mirrors"><term><literal>hashed-mirrors</literal></term>
+
+    <listitem><para>A list of web servers used by
+    <function>builtins.fetchurl</function> to obtain files by hash.
+    Given a hash type <replaceable>ht</replaceable> and a base-16 hash
+    <replaceable>h</replaceable>, Nix will try to download the file
+    from
+    <literal>hashed-mirror/<replaceable>ht</replaceable>/<replaceable>h</replaceable></literal>.
+    This allows files to be downloaded even if they have disappeared
+    from their original URI. For example, given the hashed mirror
+    <literal>http://tarballs.example.com/</literal>, when building the
+    derivation
+
+<programlisting>
+builtins.fetchurl {
+  url = "https://example.org/foo-1.2.3.tar.xz";
+  sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";
+}
+</programlisting>
+
+    Nix will attempt to download this file from
+    <literal>http://tarballs.example.com/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae</literal>
+    first. If it is not available there, if will try the original URI.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry xml:id="conf-http-connections"><term><literal>http-connections</literal></term>
 
     <listitem><para>The maximum number of parallel TCP connections

src/libstore/builtins/fetchurl.cc

@@ -58,6 +58,20 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         }
     };
 
+    /* Try the hashed mirrors first. */
+    if (getAttr("outputHashMode") == "flat")
+        for (auto hashedMirror : settings.hashedMirrors.get())
+            try {
+                if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
+                auto ht = parseHashType(getAttr("outputHashAlgo"));
+                auto h = Hash(getAttr("outputHash"), ht);
+                fetch(hashedMirror + printHashType(h.type) + "/" + h.to_string(Base16, false));
+                return;
+            } catch (Error & e) {
+                debug(e.what());
+            }
+
+    /* Otherwise try the specified URL. */
     fetch(mainUrl);
 }
 

src/libstore/globals.hh

@@ -335,6 +335,9 @@ public:
         "setuid/setgid bits or with file capabilities."};
 #endif
 
+    Setting<Strings> hashedMirrors{this, {}, "hashed-mirrors",
+        "A list of servers used by builtins.fetchurl to fetch files by hash."};
+
     Setting<uint64_t> minFree{this, 0, "min-free",
         "Automatically run the garbage collector when free disk space drops below the specified amount."};