WIP: fix flake file not found for untracked files in git

Still WIP, because the installables code is disasteriously duplicated.
So far this implements for:

- nix build
- nix search
- nix flake show

Change-Id: I3196720c2a458b7dab8d969ae1b04b3a7d29869b

src/libcmd/installable-flake.cc

@@ -1,6 +1,7 @@
 #include "globals.hh"
 #include "installable-flake.hh"
 #include "installable-derived-path.hh"
+#include "logging.hh"
 #include "outputs-spec.hh"
 #include "util.hh"
 #include "command.hh"
@@ -25,6 +26,61 @@
 
 namespace nix {
 
+[[nodiscard]] FileError handleGitTrackError(
+    FileError && fileError,
+    FlakeRef const & flakeRef,
+    flake::LockedFlake const & lockedFlake
+)
+{
+    // TODO(Qyriad): do the same thing for other supported VCSes
+    auto const isGit = flakeRef.input.getType() == "git";
+    bool const isNotFound = fileError.errNo == ENOENT;
+    bool const unhelpfulSource =
+        !flakeRef.input.getSourcePath() || !lockedFlake.flake.sourceInfo;
+    if (!isGit || !isNotFound || unhelpfulSource || fileError.referencedPaths.size() != 1) {
+        // If this flake isn't a Git flake or doesn't have a source path,
+        // or if this error is something other than ENOENT, *or* if ENOENT
+        // is about more than one path (or if we have no path information at all),
+        // then we don't have any helpful hints to add. Rethrow.
+        throw;
+    }
+
+    // Otherwise, get the path of the missing file as relative to the flake source tree,
+    // instead of the Nix store.
+
+    // The full path that got ENOENT'd.
+    auto const missingStorePath = fileError.referencedPaths.front();
+
+    // Directory of the original flake.nix (outside the Nix store).
+    auto const flakeSourceDir = *flakeRef.input.getSourcePath();
+
+    // Nix store directory containing the fetched flake.
+    // to_string() returns the path without the /nix/store part.
+    auto const flakeStoreDir = lockedFlake.flake.sourceInfo->storePath.to_string();
+
+    if (missingStorePath.find(flakeStoreDir) == missingStorePath.npos) {
+        // The mising path isn't in the flake source tree. We have no helpful
+        // information to offer.
+        throw;
+    }
+
+    // Strip the root path in the store (e.g. c8kjkvxfq9pfwd4832ishyl2mpw4l58y-source).
+    auto const amountToStrip = missingStorePath.find(flakeStoreDir) + flakeStoreDir.size();
+    auto const origSrcPath = flakeSourceDir + missingStorePath.substr(amountToStrip);
+    if (!pathExists(origSrcPath)) {
+        throw;
+    }
+    ErrorInfo existingInfo(std::move(fileError.info()));
+    existingInfo.msg = HintFmt(
+        "%1%\nnote: '%2%' found in original source; did you run git add?",
+        Uncolored(existingInfo.msg),
+        origSrcPath
+    );
+    auto withTip = FileError(fileError.errNo, std::move(fileError.referencedPaths), "");
+    withTip.err = existingInfo;
+    return withTip;
+}
+
 std::vector<std::string> InstallableFlake::getActualAttrPaths()
 {
     std::vector<std::string> res;
@@ -94,6 +150,15 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
 
     auto attrPath = attr->getAttrPathStr();
 
+    try {
+        // Evaluate here, instead of as a side effect of isDerivation(),
+        // for clarity of error handling.
+        attr->forceValue();
+    } catch (FileError & e) {
+        e.addTrace({}, "in source tree referenced by flake %s", flakeRef);
+        throw handleGitTrackError(std::move(e), flakeRef, *_lockedFlake);
+    }
+
     if (!attr->isDerivation()) {
 
         // FIXME: use eval cache?
@@ -162,7 +227,12 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
 
 std::pair<Value *, PosIdx> InstallableFlake::toValue(EvalState & state)
 {
+    try {
         return {&getCursor(state)->forceValue(), noPos};
+    } catch (FileError & e) {
+        e.addTrace({}, "in source tree referenced by flake %s", flakeRef);
+        throw handleGitTrackError(std::move(e), flakeRef, *_lockedFlake);
+    }
 }
 
 std::vector<ref<eval_cache::AttrCursor>>

src/libcmd/installable-flake.hh

@@ -5,6 +5,12 @@
 
 namespace nix {
 
+[[nodiscard]] FileError handleGitTrackError(
+    FileError && fileError,
+    FlakeRef const & flakeRef,
+    flake::LockedFlake const & lockedFlake
+);
+
 /**
  * Extra info about a \ref DerivedPath "derived path" that ultimately
  * come from a Flake.

src/nix/flake.cc

@@ -1357,10 +1357,15 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
 
         auto cache = openEvalCache(*state, flake);
 
+        try {
             auto j = visit(*cache->getRoot(), {}, fmt(ANSI_BOLD "%s" ANSI_NORMAL, flake->flake.lockedRef), "");
-        if (json)
+            if (json) {
                 logger->cout("%s", j.dump());
             }
+        } catch (FileError & e) {
+            throw handleGitTrackError(std::move(e), flake->flake.originalRef, *flake);
+        }
+    }
 };
 
 struct CmdFlakePrefetch : FlakeCommand, MixJSON

src/nix/search.cc

@@ -3,6 +3,7 @@
 #include "eval.hh"
 #include "eval-inline.hh"
 #include "eval-settings.hh"
+#include "installable-flake.hh"
 #include "names.hh"
 #include "get-drvs.hh"
 #include "common-args.hh"
@@ -108,6 +109,22 @@ struct CmdSearch : InstallableValueCommand, MixJSON
                     }
                 };
 
+                // If this is a flake, let's try to be helpful and add a tip if eval
+                // ends up tripping over an untracked file in git.
+                if (auto instFlake = installable.dynamic_pointer_cast<InstallableFlake>()) {
+                    try {
+                        // nb: isDerivation() below also does this as a side effect.
+                        cursor.forceValue();
+                    } catch (FileError & e) {
+                        e.addTrace({}, "in source tree referenced by flake %s", instFlake->flakeRef);
+                        throw handleGitTrackError(
+                            std::move(e),
+                            instFlake->flakeRef,
+                            *instFlake->_lockedFlake
+                        );
+                    }
+                }
+
                 if (cursor.isDerivation()) {
                     DrvName name(cursor.getAttr(state->sName)->getString());