From 663187219aea93f69df9648fb2e96190e934ed98 Mon Sep 17 00:00:00 2001
From: regnat <rg@regnat.ovh>
Date: Wed, 7 Jul 2021 14:29:58 +0200
Subject: [PATCH] Try and help if a file is missing at eval time

When evaluating a local git flake, if a file a missing at eval-time but
present in the source directory, tell the user that he probably forgot
to check it out

Fix #4507
---
 src/libcmd/installables.cc  | 49 +++++++++++++++++++++++++------------
 tests/flake-missing-file.sh | 39 +++++++++++++++++++++++++++++
 tests/local.mk              |  1 +
 3 files changed, 74 insertions(+), 15 deletions(-)
 create mode 100644 tests/flake-missing-file.sh

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 5758b52ad..2bf61c4df 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -528,26 +528,45 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
     auto cache = openEvalCache(*state, lockedFlake);
     auto root = cache->getRoot();
 
-    for (auto & attrPath : getActualAttrPaths()) {
-        auto attr = root->findAlongAttrPath(
-            parseAttrPath(*state, attrPath),
-            true
-        );
+    try {
+        for (auto & attrPath : getActualAttrPaths()) {
+            auto attr = root->findAlongAttrPath(
+                parseAttrPath(*state, attrPath),
+                true
+            );
 
-        if (!attr) continue;
+            if (!attr) continue;
 
-        if (!attr->isDerivation())
-            throw Error("flake output attribute '%s' is not a derivation", attrPath);
+            if (!attr->isDerivation())
+                throw Error("flake output attribute '%s' is not a derivation", attrPath);
 
-        auto drvPath = attr->forceDerivation();
+            auto drvPath = attr->forceDerivation();
 
-        auto drvInfo = DerivationInfo{
-            std::move(drvPath),
-            state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
-            attr->getAttr(state->sOutputName)->getString()
-        };
+            auto drvInfo = DerivationInfo{
+                std::move(drvPath),
+                    state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
+                    attr->getAttr(state->sOutputName)->getString()
+            };
 
-        return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+            return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+        }
+    } catch (FileError & e) {
+        if (e.errNo != ENOENT) throw;
+        if (!state->store->isInStore(e.path)) throw;
+        auto resolvedInput = lockedFlake->flake.resolvedRef.input;
+        auto [sourceStorePath, relPath] = state->store->toStorePath(e.path);
+        if (sourceStorePath != lockedFlake->flake.lockedRef.input.computeStorePath(*state->store)) throw;
+        if (fetchers::maybeGetStrAttr(resolvedInput.attrs, "type").value_or("") != "git") throw;
+        if (auto repositoryPath = resolvedInput.getSourcePath()) {
+            auto sourcePath = *repositoryPath + "/" + relPath;
+            if (pathExists(sourcePath))
+                e.addTrace(
+                    noPos,
+                    "The file is present in '%s'. "
+                    "Did you forget to track it in Git?",
+                    *repositoryPath);
+        }
+        throw;
     }
 
     throw Error("flake '%s' does not provide attribute %s",
diff --git a/tests/flake-missing-file.sh b/tests/flake-missing-file.sh
new file mode 100644
index 000000000..7f5a21a5d
--- /dev/null
+++ b/tests/flake-missing-file.sh
@@ -0,0 +1,39 @@
+source common.sh
+
+if [[ -z $(type -p git) ]]; then
+    echo "Git not installed; skipping flake tests"
+    exit 99
+fi
+
+clearStore
+rm -rf $TEST_HOME/.cache $TEST_HOME/.config
+
+repo=$TEST_ROOT/flake
+
+rm -rf $repo $repo.tmp
+mkdir $repo
+git -C $repo init
+git -C $repo config user.email "foobar@example.com"
+git -C $repo config user.name "Foobar"
+
+cat > $repo/flake.nix <<EOF
+{
+  description = "Bla bla";
+
+  outputs = inputs: rec {
+    packages.$system.foo = import ./simple.nix;
+    defaultPackage.$system = packages.$system.foo;
+  };
+}
+EOF
+
+git -C $repo add flake.nix
+git -C $repo commit -m 'Initial'
+
+cp ./simple.nix $repo/
+nix build $repo |& grep -q "Did you forget to track it in Git" || \
+    fail "Trying to access a non git-tracked file should suggest that it is probably the issue"
+
+rm $repo/simple.nix
+nix build $repo |& (! grep -q "Did you forget to track it in Git") || \
+    fail "Trying to use an absent file shouldn’t suggest to git add it"
diff --git a/tests/local.mk b/tests/local.mk
index 6f38853bc..f8aff3e87 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -47,6 +47,7 @@ nix_tests = \
   describe-stores.sh \
   flakes.sh \
   flake-local-settings.sh \
+  flake-missing-file.sh \
   build.sh \
   compute-levels.sh \
   repl.sh ca/repl.sh \