diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 8e7999157..e3232ce1d 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -24,12 +24,8 @@ static void fixupInput(Input & input)
     // Check common attributes.
     input.getType();
     input.getRef();
-    if (input.getRev())
-        input.locked = true;
     input.getRevCount();
     input.getLastModified();
-    if (input.getNarHash())
-        input.locked = true;
 }
 
 Input Input::fromURL(const ParsedURL & url)
@@ -94,6 +90,11 @@ bool Input::isDirect() const
     return !scheme || scheme->isDirect(*this);
 }
 
+bool Input::isLocked() const
+{
+    return scheme && scheme->isLocked(*this);
+}
+
 std::optional<CanonPath> Input::isRelative() const
 {
     assert(scheme);
@@ -172,8 +173,6 @@ void Input::checkLocks(Input & input) const
 
     // FIXME
     #if 0
-    input.locked = true;
-
     assert(input.hasAllInfo());
     #endif
 }
diff --git a/src/libfetchers/fetchers.hh b/src/libfetchers/fetchers.hh
index bb7fa04e6..85214af12 100644
--- a/src/libfetchers/fetchers.hh
+++ b/src/libfetchers/fetchers.hh
@@ -27,7 +27,6 @@ struct Input
 
     std::shared_ptr<InputScheme> scheme; // note: can be null
     Attrs attrs;
-    bool locked = false;
 
 public:
     static Input fromURL(const std::string & url);
@@ -50,7 +49,7 @@ public:
 
     /* Check whether this is a "locked" input, that is,
        one that contains a commit hash or content hash. */
-    bool isLocked() const { return locked; }
+    bool isLocked() const;
 
     /* Only for relative path flakes, i.e. 'path:./foo', returns the
        relative path, i.e. './foo'. */
@@ -140,6 +139,9 @@ struct InputScheme
     virtual bool isDirect(const Input & input) const
     { return true; }
 
+    virtual bool isLocked(const Input & input) const
+    { return false; }
+
     virtual std::optional<CanonPath> isRelative(const Input & input) const
     { return std::nullopt; }
 
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index a072a6cd0..697e87f28 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -482,7 +482,6 @@ struct GitInputScheme : InputScheme
             //input.attrs.erase("narHash");
             auto narHash = store->queryPathInfo(storePath)->narHash;
             input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
-            input.locked = true;
 
             return storePath;
         };
@@ -732,8 +731,6 @@ struct GitInputScheme : InputScheme
             input.attrs.insert_or_assign(
                 "revCount",
                 getRevCount(repoInfo, repoInfo.url, *input.getRev()));
-
-            input.locked = true;
         }
 
         // FIXME: maybe we should use the timestamp of the last
@@ -744,6 +741,11 @@ struct GitInputScheme : InputScheme
 
         return {makeFSInputAccessor(CanonPath(repoInfo.url), listFiles(repoInfo), std::move(makeNotAllowedError)), input};
     }
+
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
+    }
 };
 
 static auto rGitInputScheme = OnStartup([] { registerInputScheme(std::make_unique<GitInputScheme>()); });
diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index d101de433..9f61c0331 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -194,8 +194,6 @@ struct GitArchiveInputScheme : InputScheme
         auto rev = input.getRev();
         if (!rev) rev = getRevFromRef(store, input);
 
-        input.locked = true;
-
         input.attrs.erase("ref");
         input.attrs.insert_or_assign("rev", rev->gitRev());
 
@@ -240,6 +238,11 @@ struct GitArchiveInputScheme : InputScheme
 
         return {accessor, input2};
     }
+
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
+    }
 };
 
 struct GitHubInputScheme : GitArchiveInputScheme
diff --git a/src/libfetchers/mercurial.cc b/src/libfetchers/mercurial.cc
index 416be7412..7139abdcb 100644
--- a/src/libfetchers/mercurial.cc
+++ b/src/libfetchers/mercurial.cc
@@ -229,7 +229,6 @@ struct MercurialInputScheme : InputScheme
             assert(input.getRev());
             assert(!origRev || origRev == input.getRev());
             input.attrs.insert_or_assign("revCount", getIntAttr(infoAttrs, "revCount"));
-            input.locked = true;
             return storePath;
         };
 
@@ -336,6 +335,11 @@ struct MercurialInputScheme : InputScheme
 
         return {makeStorePathAccessor(store, storePath), input};
     }
+
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
+    }
 };
 
 static auto rMercurialInputScheme = OnStartup([] { registerInputScheme(std::make_unique<MercurialInputScheme>()); });
diff --git a/src/libfetchers/path.cc b/src/libfetchers/path.cc
index 1acac82a6..831e15a77 100644
--- a/src/libfetchers/path.cc
+++ b/src/libfetchers/path.cc
@@ -43,7 +43,10 @@ struct PathInputScheme : InputScheme
             /* Allow the user to pass in "fake" tree info
                attributes. This is useful for making a pinned tree
                work the same as the repository from which is exported
-               (e.g. path:/nix/store/...-source?lastModified=1585388205&rev=b0c285...). */
+               (e.g. path:/nix/store/...-source?lastModified=1585388205&rev=b0c285...).
+               FIXME: remove this hack once we have a prepopulated
+               flake input cache mechanism.
+            */
             if (name == "type" || name == "rev" || name == "revCount" || name == "lastModified" || name == "narHash" || name == "path")
                 // checked in Input::fromAttrs
                 ;
@@ -76,6 +79,11 @@ struct PathInputScheme : InputScheme
             return CanonPath(path);
     }
 
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getNarHash();
+    }
+
     bool hasAllInfo(const Input & input) const override
     {
         return true;
diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index cb4772547..010d71dba 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -237,6 +237,10 @@ struct CurlInputScheme : InputScheme
         return true;
     }
 
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getNarHash();
+    }
 };
 
 struct FileInputScheme : CurlInputScheme
@@ -261,7 +265,6 @@ struct FileInputScheme : CurlInputScheme
         // FIXME: remove?
         auto narHash = store->queryPathInfo(file.storePath)->narHash;
         input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
-        input.locked = true;
 
         return {makeStorePathAccessor(store, file.storePath), input};
     }
@@ -290,7 +293,6 @@ struct TarballInputScheme : CurlInputScheme
         // FIXME: remove?
         auto narHash = store->queryPathInfo(storePath)->narHash;
         input.attrs.insert_or_assign("narHash", narHash.to_string(SRI, true));
-        input.locked = true;
 
         return {makeStorePathAccessor(store, storePath), input};
     }