From 22ba4dc78d956020e06e0618f020e11700749823 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
Date: Mon, 26 Aug 2024 21:14:20 +0200
Subject: [PATCH] builtins.readDir: fix nix error trace on filesystem errors
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Before:

nix-env % ./src/nix/nix eval --impure --expr 'let f = builtins.readDir "/nix/store/hs3yxdq9knimwdm51gvbs4dvncz46f9d-hello-2.12.1/foo"; in f' --show-trace
error: filesystem error: directory iterator cannot open directory: No such file or directory [/nix/store/hs3yxdq9knimwdm51gvbs4dvncz46f9d-hello-2.12.1/foo]

After:

error:
       … while calling the 'readDir' builtin
         at «string»:1:9:
            1| let f = builtins.readDir "/nix/store/hs3yxdq9knimwdm51gvbs4dvncz46f9d-hello-2.12.1/foo"; in f
             |         ^

       error: reading directory '/nix/store/hs3yxdq9knimwdm51gvbs4dvncz46f9d-hello-2.12.1/foo': No such file or directory
---
 src/libutil/posix-source-accessor.cc | 40 +++++++++++++++-------------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/src/libutil/posix-source-accessor.cc b/src/libutil/posix-source-accessor.cc
index 8cec3388d..f26f74d58 100644
--- a/src/libutil/posix-source-accessor.cc
+++ b/src/libutil/posix-source-accessor.cc
@@ -132,23 +132,24 @@ SourceAccessor::DirEntries PosixSourceAccessor::readDirectory(const CanonPath &
 {
     assertNoSymlinks(path);
     DirEntries res;
-    for (auto & entry : std::filesystem::directory_iterator{makeAbsPath(path)}) {
-        checkInterrupt();
-        auto type = [&]() -> std::optional<Type> {
-            std::filesystem::file_type nativeType;
-            try {
-                nativeType = entry.symlink_status().type();
-            } catch (std::filesystem::filesystem_error & e) {
-                // We cannot always stat the child. (Ideally there is no
-                // stat because the native directory entry has the type
-                // already, but this isn't always the case.)
-                if (e.code() == std::errc::permission_denied || e.code() == std::errc::operation_not_permitted)
-                    return std::nullopt;
-                else throw;
-            }
+    try {
+        for (auto & entry : std::filesystem::directory_iterator{makeAbsPath(path)}) {
+            checkInterrupt();
+            auto type = [&]() -> std::optional<Type> {
+                std::filesystem::file_type nativeType;
+                try {
+                    nativeType = entry.symlink_status().type();
+                } catch (std::filesystem::filesystem_error & e) {
+                    // We cannot always stat the child. (Ideally there is no
+                    // stat because the native directory entry has the type
+                    // already, but this isn't always the case.)
+                    if (e.code() == std::errc::permission_denied || e.code() == std::errc::operation_not_permitted)
+                        return std::nullopt;
+                    else throw;
+                }
 
-            // cannot exhaustively enumerate because implementation-specific
-            // additional file types are allowed.
+                // cannot exhaustively enumerate because implementation-specific
+                // additional file types are allowed.
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wswitch-enum"
             switch (nativeType) {
@@ -158,8 +159,11 @@ SourceAccessor::DirEntries PosixSourceAccessor::readDirectory(const CanonPath &
             default: return tMisc;
             }
 #pragma GCC diagnostic pop
-        }();
-        res.emplace(entry.path().filename().string(), type);
+            }();
+            res.emplace(entry.path().filename().string(), type);
+        }
+    } catch (std::filesystem::filesystem_error & e) {
+        throw SysError("reading directory %1%", showPath(path));
     }
     return res;
 }