* Removed the `id' attribute hack.

* Formalise the notion of fixed-output derivations, i.e., derivations
  for which a cryptographic hash of the output is known in advance.
  Changes to such derivations should not propagate upwards through the
  dependency graph.  Previously this was done by specifying the hash
  component of the output path through the `id' attribute, but this is
  insecure since you can lie about it (i.e., you can specify any hash
  and then produce a completely different output).  Now the
  responsibility for checking the output is moved from the builder to
  Nix itself.

  A fixed-output derivation can be created by specifying the
  `outputHash' and `outputHashAlgo' attributes, the latter taking
  values `md5', `sha1', and `sha256', and the former specifying the
  actual hash in hexadecimal or in base-32 (auto-detected by looking
  at the length of the attribute value).  MD5 is included for
  compatibility but should be considered deprecated.

* Removed the `drvPath' pseudo-attribute in derivation results.  It's
  no longer necessary.

* Cleaned up the support for multiple output paths in derivation store
  expressions.  Each output now has a unique identifier (e.g., `out',
  `devel', `docs').  Previously there was no way to tell output paths
  apart at the store expression level.

* `nix-hash' now has a flag `--base32' to specify that the hash should
  be printed in base-32 notation.

* `fetchurl' accepts parameters `sha256' and `sha1' in addition to
  `md5'.

* `nix-prefetch-url' now prints out a SHA-1 hash in base-32.  (TODO: a
  flag to specify the hash.)

1 files changed, 81 insertions, 36 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index b59232f2c..9208f0b24 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -29,20 +29,60 @@ static PathSet storeExprRootsCached(EvalState & state, const Path & nePath)
 }
 
 
-static Hash hashDerivation(EvalState & state, StoreExpr ne)
+/* Returns the hash of a derivation modulo fixed-output
+   subderivations.  A fixed-output derivation is a derivation with one
+   output (`out') for which an expected hash and hash algorithm are
+   specified (using the `outputHash' and `outputHashAlgo'
+   attributes).  We don't want changes to such derivations to
+   propagate upwards through the dependency graph, changing output
+   paths everywhere.
+
+   For instance, if we change the url in a call to the `fetchurl'
+   function, we do not want to rebuild everything depending on it
+   (after all, (the hash of) the file being downloaded is unchanged).
+   So the *output paths* should not change.  On the other hand, the
+   *derivation store expression paths* should change to reflect the
+   new dependency graph.
+
+   That's what this function does: it returns a hash which is just the
+   of the derivation ATerm, except that any input store expression
+   paths have been replaced by the result of a recursive call to this
+   function, and that for fixed-output derivations we return
+   (basically) its outputHash. */
+static Hash hashDerivationModulo(EvalState & state, StoreExpr ne)
 {
     if (ne.type == StoreExpr::neDerivation) {
+
+        /* Return a fixed hash for fixed-output derivations. */
+        if (ne.derivation.outputs.size() == 1) {
+            DerivationOutputs::iterator i = ne.derivation.outputs.begin();
+            if (i->first == "out" &&
+                i->second.hash != "")
+            {
+                return hashString(htSHA256, "fixed:out:"
+                    + i->second.hashAlgo + ":"
+                    + i->second.hash + ":"
+                    + i->second.path);
+            }
+        }
+
+        /* For other derivations, replace the inputs paths with
+           recursive calls to this function.*/
 	PathSet inputs2;
         for (PathSet::iterator i = ne.derivation.inputs.begin();
-             i != ne.derivation.inputs.end(); i++)
+             i != ne.derivation.inputs.end(); ++i)
         {
-            DrvHashes::iterator j = state.drvHashes.find(*i);
-            if (j == state.drvHashes.end())
-                throw Error(format("don't know expression `%1%'") % (string) *i);
-            inputs2.insert(printHash(j->second));
+            Hash h = state.drvHashes[*i];
+            if (h.type == htUnknown) {
+                StoreExpr ne2 = storeExprFromPath(*i);
+                h = hashDerivationModulo(state, ne2);
+                state.drvHashes[*i] = h;
+            }
+            inputs2.insert(printHash(h));
         }
 	ne.derivation.inputs = inputs2;
     }
+    
     return hashTerm(unparseStoreExpr(ne));
 }
 
@@ -58,9 +98,7 @@ static Path copyAtom(EvalState & state, const Path & srcPath)
     ne.closure.roots.insert(dstPath);
     ne.closure.elems[dstPath] = elem;
 
-    Hash drvHash = hashDerivation(state, ne);
     Path drvPath = writeTerm(unparseStoreExpr(ne), "c");
-    state.drvHashes[drvPath] = drvHash;
 
     state.drvRoots[drvPath] = ne.closure.roots;
 
@@ -109,16 +147,11 @@ static void processBinding(EvalState & state, Expr e, StoreExpr & ne,
             if (!a) throw Error("derivation name missing");
             Path drvPath = evalPath(state, a);
 
-            a = queryAttr(e, "drvHash");
-            if (!a) throw Error("derivation hash missing");
-            Hash drvHash = parseHash(htMD5, evalString(state, a));
-
             a = queryAttr(e, "outPath");
             if (!a) throw Error("output path missing");
             PathSet drvRoots;
             drvRoots.insert(evalPath(state, a));
             
-            state.drvHashes[drvPath] = drvHash;
             state.drvRoots[drvPath] = drvRoots;
 
             ss.push_back(addInput(state, drvPath, ne));
@@ -188,8 +221,9 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
     ne.type = StoreExpr::neDerivation;
 
     string drvName;
-    Hash outHash;
-    bool outHashGiven = false;
+    
+    string outputHash;
+    string outputHashAlgo;
 
     for (ATermIterator i(attrs.keys()); i; ++i) {
         string key = aterm2String(*i);
@@ -222,10 +256,8 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
             if (key == "builder") ne.derivation.builder = s;
             else if (key == "system") ne.derivation.platform = s;
             else if (key == "name") drvName = s;
-            else if (key == "id") { 
-                outHash = parseHash(htMD5, s);
-                outHashGiven = true;
-            }
+            else if (key == "outputHash") outputHash = s;
+            else if (key == "outputHashAlgo") outputHashAlgo = s;
         }
     }
     
@@ -237,6 +269,24 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
     if (drvName == "")
         throw Error("required attribute `name' missing");
 
+    /* If an output hash was given, check it. */
+    if (outputHash == "")
+        outputHashAlgo = "";
+    else {
+        HashType ht = parseHashType(outputHashAlgo);
+        if (ht == htUnknown)
+            throw Error(format("unknown hash algorithm `%1%'") % outputHashAlgo);
+        Hash h;
+        if (outputHash.size() == Hash(ht).hashSize * 2)
+            /* hexadecimal representation */
+            h = parseHash(ht, outputHash);
+        else
+            /* base-32 representation */
+            h = parseHash32(ht, outputHash);
+        string s = outputHash;
+        outputHash = printHash(h);
+    }
+
     /* Check the derivation name.  It shouldn't contain whitespace,
        but we are conservative here: we check whether only
        alphanumerics and some other characters appear. */
@@ -252,38 +302,33 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
         }
 
     /* Construct the "masked" derivation store expression, which is
-       the final one except that the list of output paths is set to
-       the set of output names, and the corresponding environment
-       variables have an empty value.  This ensures that changes in
-       the set of output names do get reflected in the hash. */
+       the final one except that in the list of outputs, the output
+       paths are empty, and the corresponding environment variables
+       have an empty value.  This ensures that changes in the set of
+       output names do get reflected in the hash. */
     ne.derivation.env["out"] = "";
-    ne.derivation.outputs.insert("out");
+    ne.derivation.outputs["out"] =
+        DerivationOutput("", outputHashAlgo, outputHash);
         
-    /* Determine the output path by hashing the Nix expression with no
-       outputs to produce a unique but deterministic path name for
-       this derivation. */
-    if (!outHashGiven) outHash = hashDerivation(state, ne);
+    /* Use the masked derivation expression to compute the output
+       path. */
     Path outPath = makeStorePath("output:out",
-        outHash, drvName);
+        hashDerivationModulo(state, ne), drvName);
 
     /* Construct the final derivation store expression. */
     ne.derivation.env["out"] = outPath;
-    ne.derivation.outputs.clear();
-    ne.derivation.outputs.insert(outPath);
+    ne.derivation.outputs["out"] =
+        DerivationOutput(outPath, outputHashAlgo, outputHash);
 
     /* Write the resulting term into the Nix store directory. */
-    Hash drvHash = outHashGiven
-        ? hashString(printHash(outHash) + outPath, htMD5)
-        : hashDerivation(state, ne);
     Path drvPath = writeTerm(unparseStoreExpr(ne), "d-" + drvName);
 
     printMsg(lvlChatty, format("instantiated `%1%' -> `%2%'")
         % drvName % drvPath);
 
+    /* !!! assumes a single output */
     attrs.set("outPath", makeAttrRHS(makePath(toATerm(outPath)), makeNoPos()));
     attrs.set("drvPath", makeAttrRHS(makePath(toATerm(drvPath)), makeNoPos()));
-    attrs.set("drvHash",
-        makeAttrRHS(makeStr(toATerm(printHash(drvHash))), makeNoPos()));
     attrs.set("type", makeAttrRHS(makeStr(toATerm("derivation")), makeNoPos()));
 
     return makeAttrs(attrs);