fetchTree: Do not re-fetch paths already present

src/libexpr/primops/fetchTree.cc

@@ -185,6 +185,13 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
 // FIXME: document
 static RegisterPrimOp primop_fetchTree("fetchTree", 1, prim_fetchTree);
 
+static void setStorePath(EvalState &state, const StorePath &storePath, Value &v) {
+    state.allowPath(storePath);
+
+    auto path = state.store->printStorePath(storePath);
+    v.mkString(path, PathSet({path}));
+}
+
 static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     const std::string & who, bool unpack, std::string name)
 {
@@ -203,6 +210,8 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
                 url = state.forceStringNoCtx(*attr.value, *attr.pos);
             else if (n == "sha256")
                 expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
+            else if (n == "narHash")
+                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
@@ -230,6 +239,20 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     if (evalSettings.pureEval && !expectedHash)
         throw Error("in pure evaluation mode, '%s' requires a 'sha256' argument", who);
 
+    // early exit if pinned and already in the store
+    if (expectedHash && expectedHash->type == htSHA256) {
+        auto expectedPath =
+            unpack
+            ? state.store->makeFixedOutputPath(FileIngestionMethod::Recursive, *expectedHash, name, {})
+            : state.store->makeFixedOutputPath(FileIngestionMethod::Flat, *expectedHash, name, {});
+
+        auto validPaths = state.store->queryValidPaths({expectedPath}, NoSubstitute);
+        if (!validPaths.empty()) {
+            setStorePath(state, expectedPath, v);
+            return;
+        }
+    }
+
     auto storePath =
         unpack
         ? fetchers::downloadTarball(state.store, *url, name, (bool) expectedHash).first.storePath
@@ -244,10 +267,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
                 *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true));
     }
 
-    state.allowPath(storePath);
+    setStorePath(state, storePath, v);
-
-    auto path = state.store->printStorePath(storePath);
-    v.mkString(path, PathSet({path}));
 }
 
 static void prim_fetchurl(EvalState & state, const Pos & pos, Value * * args, Value & v)

tests/fetchurl.sh

@@ -9,6 +9,10 @@ outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file:
 
 cmp $outPath fetchurl.sh
 
+# Do not re-fetch paths already present.
+outPath2=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file:///does-not-exist/must-remain-unused/fetchurl.sh --argstr sha256 $hash --no-out-link)
+test "$outPath" == "$outPath2"
+
 # Now using a base-64 hash.
 clearStore
 

tests/tarball.sh

@@ -26,10 +26,14 @@ test_tarball() {
     nix-build -o $TEST_ROOT/result '<foo>' -I foo=file://$tarball
 
     nix-build -o $TEST_ROOT/result -E "import (fetchTarball file://$tarball)"
+    # Do not re-fetch paths already present
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTarball { url = file:///does-not-exist/must-remain-unused/$tarball; sha256 = \"$hash\"; })"
 
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree file://$tarball)"
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; })"
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })"
+    # Do not re-fetch paths already present
+    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file:///does-not-exist/must-remain-unused/$tarball; narHash = \"$hash\"; })"
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" 2>&1 | grep 'NAR hash mismatch in input'
 
     nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" >&2