Upgrade downstreamPlaceholder to a type with methods

This gets us ready for dynamic derivation dependencies (part of RFC 92).

src/libexpr/eval.cc

@@ -4,6 +4,7 @@
 #include "util.hh"
 #include "store-api.hh"
 #include "derivations.hh"
+#include "downstream-placeholder.hh"
 #include "globals.hh"
 #include "eval-inline.hh"
 #include "filetransfer.hh"
@@ -1058,7 +1059,7 @@ void EvalState::mkOutputString(
             ? store->printStorePath(*std::move(optOutputPath))
             /* Downstream we would substitute this for an actual path once
                we build the floating CA derivation */
-            : downstreamPlaceholder(*store, drvPath, outputName),
+            : DownstreamPlaceholder { drvPath, outputName }.render(),
         NixStringContext {
             NixStringContextElem::Built {
                 .drvPath = drvPath,
@@ -2380,7 +2381,7 @@ DerivedPath EvalState::coerceToDerivedPath(const PosIdx pos, Value & v, std::str
             // This is testing for the case of CA derivations
             auto sExpected = optOutputPath
                 ? store->printStorePath(*optOutputPath)
-                : downstreamPlaceholder(*store, b.drvPath, output);
+                : DownstreamPlaceholder { b.drvPath, output }.render();
             if (s != sExpected)
                 error(
                     "string '%s' has context with the output '%s' from derivation '%s', but the string is not the right placeholder for this derivation output. It should be '%s'",

src/libexpr/eval.hh

@@ -483,7 +483,7 @@ public:
      * Coerce to `DerivedPath`.
      *
      * Must be a string which is either a literal store path or a
-     * "placeholder (see `downstreamPlaceholder()`).
+     * "placeholder (see `DownstreamPlaceholder`).
      *
      * Even more importantly, the string context must be exactly one
      * element, which is either a `NixStringContextElem::Opaque` or
@@ -622,7 +622,7 @@ public:
      * @param optOutputPath Optional output path for that string. Must
      * be passed if and only if output store object is input-addressed.
      * Will be printed to form string if passed, otherwise a placeholder
-     * will be used (see `downstreamPlaceholder()`).
+     * will be used (see `DownstreamPlaceholder`).
      */
     void mkOutputString(
         Value & value,

src/libexpr/primops.cc

@@ -1,5 +1,6 @@
 #include "archive.hh"
 #include "derivations.hh"
+#include "downstream-placeholder.hh"
 #include "eval-inline.hh"
 #include "eval.hh"
 #include "globals.hh"
@@ -87,7 +88,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
         auto outputs = resolveDerivedPath(*store, drv);
         for (auto & [outputName, outputPath] : outputs) {
             res.insert_or_assign(
-                downstreamPlaceholder(*store, drv.drvPath, outputName),
+                DownstreamPlaceholder { drv.drvPath, outputName }.render(),
                 store->printStorePath(outputPath)
             );
         }

src/libstore/derivations.cc

@@ -1,4 +1,5 @@
 #include "derivations.hh"
+#include "downstream-placeholder.hh"
 #include "store-api.hh"
 #include "globals.hh"
 #include "util.hh"
@@ -810,13 +811,7 @@ std::string hashPlaceholder(const std::string_view outputName)
     return "/" + hashString(htSHA256, concatStrings("nix-output:", outputName)).to_string(Base32, false);
 }
 
-std::string downstreamPlaceholder(const Store & store, const StorePath & drvPath, std::string_view outputName)
-{
-    auto drvNameWithExtension = drvPath.name();
-    auto drvName = drvNameWithExtension.substr(0, drvNameWithExtension.size() - 4);
-    auto clearText = "nix-upstream-output:" + std::string { drvPath.hashPart() } + ":" + outputPathName(drvName, outputName);
-    return "/" + hashString(htSHA256, clearText).to_string(Base32, false);
-}
+
 
 
 static void rewriteDerivation(Store & store, BasicDerivation & drv, const StringMap & rewrites)
@@ -880,7 +875,7 @@ std::optional<BasicDerivation> Derivation::tryResolve(
         for (auto & outputName : inputOutputs) {
             if (auto actualPath = get(inputDrvOutputs, { inputDrv, outputName })) {
                 inputRewrites.emplace(
-                    downstreamPlaceholder(store, inputDrv, outputName),
+                    DownstreamPlaceholder { inputDrv, outputName }.render(),
                     store.printStorePath(*actualPath));
                 resolved.inputSrcs.insert(*actualPath);
             } else {

src/libstore/derivations.hh

@@ -6,6 +6,7 @@
 #include "hash.hh"
 #include "content-address.hh"
 #include "repair-flag.hh"
+#include "derived-path.hh"
 #include "sync.hh"
 #include "comparator.hh"
 
@@ -495,17 +496,6 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
  */
 std::string hashPlaceholder(const std::string_view outputName);
 
-/**
- * This creates an opaque and almost certainly unique string
- * deterministically from a derivation path and output name.
- *
- * It is used as a placeholder to allow derivations to refer to
- * content-addressed paths whose content --- and thus the path
- * themselves --- isn't yet known. This occurs when a derivation has a
- * dependency which is a CA derivation.
- */
-std::string downstreamPlaceholder(const Store & store, const StorePath & drvPath, std::string_view outputName);
-
 extern const Hash impureOutputHash;
 
 }

src/libstore/downstream-placeholder.cc

@@ -0,0 +1,38 @@
+#include "downstream-placeholder.hh"
+#include "derivations.hh"
+
+namespace nix {
+
+std::string DownstreamPlaceholder::render() const
+{
+    return "/" + hash.to_string(Base32, false);
+}
+
+
+inline Hash DownstreamPlaceholder::worker1(const StorePath & drvPath, std::string_view outputName)
+{
+    auto drvNameWithExtension = drvPath.name();
+    auto drvName = drvNameWithExtension.substr(0, drvNameWithExtension.size() - 4);
+    auto clearText = "nix-upstream-output:" + std::string { drvPath.hashPart() } + ":" + outputPathName(drvName, outputName);
+    return hashString(htSHA256, clearText);
+}
+
+DownstreamPlaceholder::DownstreamPlaceholder(const StorePath & drvPath, std::string_view outputName)
+    : hash(worker1(drvPath, outputName))
+{}
+
+inline Hash DownstreamPlaceholder::worker2(const DownstreamPlaceholder & placeholder, std::string_view outputName)
+{
+    experimentalFeatureSettings.require(Xp::DynamicDerivations);
+    auto compressed = compressHash(placeholder.hash, 20);
+    auto clearText = "nix-computed-output:"
+        + compressed.to_string(Base32, false)
+        + ":" + std::string { outputName };
+    return hashString(htSHA256, clearText);
+}
+
+DownstreamPlaceholder::DownstreamPlaceholder(const DownstreamPlaceholder & placeholder, std::string_view outputName)
+    : hash(worker2(placeholder, outputName))
+{}
+
+}

src/libstore/downstream-placeholder.hh

@@ -0,0 +1,71 @@
+#pragma once
+///@file
+
+#include "hash.hh"
+#include "path.hh"
+
+namespace nix {
+
+/**
+ * Downstream Placeholders are opaque and almost certainly unique values
+ * used to allow derivations to refer to store objects which are yet to
+ * be built and for we do not yet have store paths for.
+ *
+ * They correspond to `DerivedPaths` that are not `DerivedPath::Opaque`,
+ * except for the cases involving input addressing or fixed outputs
+ * where we do know a store path for the derivation output in advance.
+ *
+ * Unlike `DerivationPath`, however, `DownstreamPlaceholder` is
+ * purposefully opaque and obfuscated. This is so they are hard to
+ * create by accident, and so substituting them (once we know what the
+ * path to store object is) is unlikely to capture other stuff it
+ * shouldn't.
+ *
+ * We use them with `Derivation`: the `render()` method is called to
+ * render an opaque string which can be used in the derivation, and the
+ * resolving logic can substitute those strings for store paths when
+ * resolving `Derivation.inputDrvs` to `BasicDerivation.inputSrcs`.
+ */
+class DownstreamPlaceholder
+{
+    /**
+     * `DownstreamPlaceholder` is just a newtype of `Hash`.
+     * This its only field.
+     */
+    Hash hash;
+
+public:
+    /**
+     * This creates an opaque and almost certainly unique string
+     * deterministically from the placeholder.
+     */
+    std::string render() const;
+
+    /**
+     * For content-addressed derivations
+     *
+     * The derivation is known (we have a store path for it), but the
+     * output doesn't yet have a known store path.
+     */
+    DownstreamPlaceholder(const StorePath & drvPath, std::string_view outputName);
+
+    /**
+     * For dynamic derivations
+     *
+     * The derivation is not yet known, and we just have (another)
+     * placeholder for it.
+     */
+    DownstreamPlaceholder(const DownstreamPlaceholder & drvPlaceholder, std::string_view outputName);
+
+private:
+    /**
+     * Private worker function for constructor
+     */
+    static inline Hash worker1(const StorePath & drvPath, std::string_view outputName);
+    /**
+     * Private worker function for constructor
+     */
+    static inline Hash worker2(const DownstreamPlaceholder & placeholder, std::string_view outputName);
+};
+
+}

src/libutil/experimental-features.cc

@@ -207,6 +207,9 @@ constexpr std::array<ExperimentalFeatureDetails, 13> xpFeatureDetails = {{
 
               - "text hashing" derivation outputs, so we can build .drv
                 files.
+
+              - dependencies in derivations on the outputs of
+                derivations that are themselves derivations outputs.
         )",
     },
 }};

src/nix/app.cc

@@ -7,6 +7,7 @@
 #include "names.hh"
 #include "command.hh"
 #include "derivations.hh"
+#include "downstream-placeholder.hh"
 
 namespace nix {
 
@@ -23,7 +24,7 @@ StringPairs resolveRewrites(
         if (auto drvDep = std::get_if<BuiltPathBuilt>(&dep.path))
             for (auto & [ outputName, outputPath ] : drvDep->outputs)
                 res.emplace(
-                    downstreamPlaceholder(store, drvDep->drvPath, outputName),
+                    DownstreamPlaceholder { drvDep->drvPath, outputName }.render(),
                     store.printStorePath(outputPath)
                 );
     return res;