Make the hashes mirrors used by builtins.fetchurl configurable

In particular, this allows it to be disabled in our tests.

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

@@ -660,6 +660,34 @@ password <replaceable>my-password</replaceable>
   </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. The default is
+    <literal>http://tarballs.nixos.org/</literal>. 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 default mirror
+    <literal>http://tarballs.nixos.org/</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.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae</literal>
+    first. If it is not available there, if will try the original URI.</para></listitem>
+
+  </varlistentry>
+
+
 </variablelist>
 
 </para>

src/libstore/builtins.cc

@@ -38,9 +38,12 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
 
     std::shared_ptr<std::string> data;
 
-    try {
     if (getAttr("outputHashMode") == "flat")
-            data = fetch("http://tarballs.nixos.org/" + getAttr("outputHashAlgo") + "/" + getAttr("outputHash"));
+        for (auto hashedMirror : settings.hashedMirrors.get())
+            try {
+                if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
+                data = fetch(hashedMirror + getAttr("outputHashAlgo") + "/" + getAttr("outputHash"));
+                break;
             } catch (Error & e) {
                 debug(e.what());
             }

src/libstore/globals.hh

@@ -327,6 +327,9 @@ public:
         "Whether builders can acquire new privileges by calling programs with "
         "setuid/setgid bits or with file capabilities."};
 #endif
+
+    Setting<Strings> hashedMirrors{this, {"http://tarballs.nixos.org/"}, "hashed-mirrors",
+        "A list of servers used by builtins.fetchurl to fetch files by hash."};
 };
 
 

tests/fetchurl.sh

@@ -5,7 +5,7 @@ clearStore
 # Test fetching a flat file.
 hash=$(nix-hash --flat --type sha256 ./fetchurl.sh)
 
-outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha256 $hash --no-out-link)
+outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha256 $hash --no-out-link --option hashed-mirrors '')
 
 cmp $outPath fetchurl.sh
 
@@ -14,7 +14,7 @@ clearStore
 
 hash=$(nix hash-file --type sha512 --base64 ./fetchurl.sh)
 
-outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha512 $hash --no-out-link)
+outPath=$(nix-build '<nix/fetchurl.nix>' --argstr url file://$(pwd)/fetchurl.sh --argstr sha512 $hash --no-out-link --option hashed-mirrors '')
 
 cmp $outPath fetchurl.sh