Some effort to minimize flake dependencies

For example, if the top-level flake depends on
"nixpkgs/release-19.03", and one of its dependencies depends on
"nixpkgs", then the latter will be mapped to "nixpkgs/release-19.03",
rather than whatever the default branch of "nixpkgs" is. Thus you get
only one "nixpkgs" dependency rather than two.

This currently only works in a breadth-first way, so the other way
around (i.e. if the top-level flake depends on "nixpkgs", and a
dependency depends on "nixpkgs/release-19.03") still results in two
"nixpkgs" dependencies.

src/libexpr/flake/flake.cc

@@ -112,7 +112,9 @@ static FlakeRef lookupFlake(EvalState & state, const FlakeRef & flakeRef, const
     return flakeRef;
 }
 
-FlakeRef maybeLookupFlake(
+/* If 'allowLookup' is true, then resolve 'flakeRef' using the
+   registries. */
+static FlakeRef maybeLookupFlake(
     EvalState & state,
     const FlakeRef & flakeRef,
     bool allowLookup)
@@ -126,6 +128,23 @@ FlakeRef maybeLookupFlake(
         return flakeRef;
 }
 
+typedef std::vector<std::pair<FlakeRef, FlakeRef>> RefMap;
+
+static FlakeRef lookupInRefMap(
+    const RefMap & refMap,
+    const FlakeRef & flakeRef)
+{
+    // FIXME: inefficient.
+    for (auto & i : refMap) {
+        if (flakeRef.contains(i.first)) {
+            debug("mapping '%s' to previously seen input '%s' -> '%s",
+                flakeRef, i.first, i.second);
+            return i.second;
+        }
+    }
+
+    return flakeRef;
+}
 
 static SourceInfo fetchFlake(EvalState & state, const FlakeRef & resolvedRef)
 {
@@ -205,15 +224,21 @@ static void expectType(EvalState & state, ValueType type,
             showType(type), showType(value.type), pos);
 }
 
-Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup)
+static Flake getFlake(EvalState & state, const FlakeRef & originalRef,
+    bool allowLookup, RefMap & refMap)
 {
-    auto flakeRef = maybeLookupFlake(state, originalRef, allowLookup);
+    auto flakeRef = lookupInRefMap(refMap,
+        maybeLookupFlake(state,
+            lookupInRefMap(refMap, originalRef), allowLookup));
 
     SourceInfo sourceInfo = fetchFlake(state, flakeRef);
     debug("got flake source '%s' with flakeref %s", sourceInfo.storePath, sourceInfo.resolvedRef.to_string());
 
     FlakeRef resolvedRef = sourceInfo.resolvedRef;
 
+    refMap.push_back({originalRef, resolvedRef});
+    refMap.push_back({flakeRef, resolvedRef});
+
     state.store->assertStorePath(sourceInfo.storePath);
 
     if (state.allowedPaths)
@@ -314,13 +339,27 @@ Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup
     return flake;
 }
 
-static SourceInfo getNonFlake(EvalState & state, const FlakeRef & flakeRef)
+Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup)
 {
+    RefMap refMap;
+    return getFlake(state, originalRef, allowLookup, refMap);
+}
+
+static SourceInfo getNonFlake(EvalState & state, const FlakeRef & originalRef,
+    bool allowLookup, RefMap & refMap)
+{
+    auto flakeRef = lookupInRefMap(refMap,
+        maybeLookupFlake(state,
+            lookupInRefMap(refMap, originalRef), allowLookup));
+
     auto sourceInfo = fetchFlake(state, flakeRef);
     debug("got non-flake source '%s' with flakeref %s", sourceInfo.storePath, sourceInfo.resolvedRef.to_string());
 
     FlakeRef resolvedRef = sourceInfo.resolvedRef;
 
+    refMap.push_back({originalRef, resolvedRef});
+    refMap.push_back({flakeRef, resolvedRef});
+
     state.store->assertStorePath(sourceInfo.storePath);
 
     if (state.allowedPaths)
@@ -360,6 +399,7 @@ bool allowedToUseRegistries(HandleLockFile handle, bool isTopRef)
    Note that this is lazy: we only recursively fetch inputs that are
    not in the lockfile yet. */
 static std::pair<Flake, LockedInput> updateLocks(
+    RefMap & refMap,
     const std::string & inputPath,
     EvalState & state,
     const Flake & flake,
@@ -372,6 +412,8 @@ static std::pair<Flake, LockedInput> updateLocks(
         flake.originalRef,
         flake.sourceInfo.narHash);
 
+    std::vector<std::function<void()>> postponed;
+
     for (auto & [id, input] : flake.inputs) {
         auto inputPath2 = (inputPath.empty() ? "" : inputPath + "/") + id;
         auto i = oldEntry.inputs.find(id);
@@ -380,29 +422,36 @@ static std::pair<Flake, LockedInput> updateLocks(
         } else {
             if (handleLockFile == AllPure || handleLockFile == TopRefUsesRegistries)
                 throw Error("cannot update flake input '%s' in pure mode", id);
-            if (input.isFlake) {
-                auto actualInput = getFlake(state, input.ref,
-                        allowedToUseRegistries(handleLockFile, false));
+
+            auto warn = [&](const SourceInfo & sourceInfo) {
                 if (i == oldEntry.inputs.end())
-                    printMsg(lvlWarn, "mapped flake input '%s' to '%s'",
-                        inputPath2, actualInput.sourceInfo.resolvedRef);
+                    printInfo("mapped flake input '%s' to '%s'",
+                        inputPath2, sourceInfo.resolvedRef);
                 else
                     printMsg(lvlWarn, "updated flake input '%s' from '%s' to '%s'",
-                        inputPath2, i->second.originalRef, actualInput.sourceInfo.resolvedRef);
-                newEntry.inputs.insert_or_assign(id,
-                    updateLocks(inputPath2, state, actualInput, handleLockFile, {}, false).second);
+                        inputPath2, i->second.originalRef, sourceInfo.resolvedRef);
+            };
+
+            if (input.isFlake) {
+                auto actualInput = getFlake(state, input.ref,
+                    allowedToUseRegistries(handleLockFile, false), refMap);
+                warn(actualInput.sourceInfo);
+                postponed.push_back([&, id{id}, inputPath2, actualInput]() {
+                    newEntry.inputs.insert_or_assign(id,
+                        updateLocks(refMap, inputPath2, state, actualInput, handleLockFile, {}, false).second);
+                });
             } else {
-                auto sourceInfo = getNonFlake(state,
-                    maybeLookupFlake(state, input.ref,
-                        allowedToUseRegistries(handleLockFile, false)));
-                printMsg(lvlWarn, "mapped flake input '%s' to '%s'",
-                    inputPath2, sourceInfo.resolvedRef);
+                auto sourceInfo = getNonFlake(state, input.ref,
+                    allowedToUseRegistries(handleLockFile, false), refMap);
+                warn(sourceInfo);
                 newEntry.inputs.insert_or_assign(id,
                     LockedInput(sourceInfo.resolvedRef, input.ref, sourceInfo.narHash));
             }
         }
     }
 
+    for (auto & f : postponed) f();
+
     return {flake, newEntry};
 }
 
@@ -423,8 +472,10 @@ ResolvedFlake resolveFlake(EvalState & state, const FlakeRef & topRef, HandleLoc
             + "/" + flake.sourceInfo.resolvedRef.subdir + "/flake.lock");
     }
 
+    RefMap refMap;
+
     LockFile lockFile(updateLocks(
-            "", state, flake, handleLockFile, oldLockFile, true).second);
+            refMap, "", state, flake, handleLockFile, oldLockFile, true).second);
 
     if (!(lockFile == oldLockFile)) {
         if (allowedToWrite(handleLockFile)) {
@@ -500,7 +551,8 @@ static void prim_callFlake(EvalState & state, const Pos & pos, Value * * args, V
 
         callFlake(state, flake, lazyInput->lockedInput, v);
     } else {
-        auto sourceInfo = getNonFlake(state, lazyInput->lockedInput.ref);
+        RefMap refMap;
+        auto sourceInfo = getNonFlake(state, lazyInput->lockedInput.ref, false, refMap);
 
         if (sourceInfo.narHash != lazyInput->lockedInput.narHash)
             throw Error("the content hash of repository '%s' doesn't match the hash recorded in the referring lockfile", sourceInfo.resolvedRef);

src/libexpr/flake/flake.hh

@@ -80,13 +80,6 @@ struct Flake
 
 Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool allowLookup);
 
-/* If 'allowLookup' is true, then resolve 'flakeRef' using the
-   registries. */
-FlakeRef maybeLookupFlake(
-    EvalState & state,
-    const FlakeRef & flakeRef,
-    bool allowLookup);
-
 /* Fingerprint of a locked flake; used as a cache key. */
 typedef Hash Fingerprint;
 

src/libexpr/flake/flakeref.cc

@@ -255,6 +255,23 @@ FlakeRef FlakeRef::baseRef() const // Removes the ref and rev from a FlakeRef.
     return result;
 }
 
+bool FlakeRef::contains(const FlakeRef & other) const
+{
+    if (!(data == other.data))
+        return false;
+
+    if (ref && ref != other.ref)
+        return false;
+
+    if (rev && rev != other.rev)
+        return false;
+
+    if (subdir != other.subdir)
+        return false;
+
+    return true;
+}
+
 std::optional<FlakeRef> parseFlakeRef(
     const std::string & uri, bool allowRelative)
 {

src/libexpr/flake/flakeref.hh

@@ -182,6 +182,12 @@ struct FlakeRef
         return std::get_if<FlakeRef::IsPath>(&data)
             && rev == Hash(rev->type);
     }
+
+    /* Return true if 'other' is not less specific than 'this'. For
+       example, 'nixpkgs' contains 'nixpkgs/release-19.03', and both
+       'nixpkgs' and 'nixpkgs/release-19.03' contain
+       'nixpkgs/release-19.03/<hash>'. */
+    bool contains(const FlakeRef & other) const;
 };
 
 std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);