[libcxx] Correct and clean-up filesystem operations error_code paths

[4-rodrigo-salazar]

3 error_code related cleanups/corrections in the std::filesystem operations functions.

1. In __copy, the ec->clear() is unnecessary as ErrorHandler at the start of each function clears the error_code as part of its initialization.

2. In __copy, in the recursive codepath we are not checking the error_code result of it.increment(m_ec2) immediately after use in the for loop condition (and we aren't checking it after the final increment when we don't enter the loop).

3. In __weakly_canonical, it makes calls to __canonical (which internally uses OS APIs implementing POSIX realpath) and we are not checking the error code result from the __canonical call. Both weakly_canonical and canonical are supposed to set the error_code when underlying OS APIs result in an error (https://eel.is/c++draft/fs.err.report#3.1). With this change we propagate up the error_code from __canonical caused by any underlying OS API failure up to the __weakly_canonical. Essentially, if __canonical thinks an error code should be set, then __weakly_canonical must as well. Before this change it would be throwing an exception in the non-error_code form of the function when __canonical fails, while not setting the error code in the error_code form of the function (an inconsistency).

Added a little coverage in weakly_canonical.pass.cpp for the error_code forms of the API that was missing. Though I am lacking utilities in libcxx testing to add granular testing of the failure scenarios (like forcing realpath to fail for a given path, as it could if you had something like a flaky remote filesystem).

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Rodrigo Salazar (4-rodrigo-salazar)

Changes

3 error_code related cleanups/corrections in the std::filesystem operations functions.

1. In __copy, the ec->clear() is unnecessary as ErrorHandler at the start of the each function clears the error_code as part of it's initialization.

2. In __copy, in the recursive codepath we are not checking the error_code result of it.increment(m_ec2) immediately after use in the for loop condition (and we aren't checking it after the final increment when we don't enter the loop).

3. In __weakly_canonical, it makes calls to __canonical (which internally uses OS apis implementing posix realpath) and we are not checking the error code result from the __canonical call. Both weakly_canonical and canonical are supposed to set the error_code when underlying OS APIs result in an error (https://eel.is/c++draft/fs.err.report#3.1). With this change we propagate up the error_code from __canonical caused by any underlying OS api failure up to the __weakly_canonical. Essentially, if __canonical thinks an error code should be set, then __weakly_canonical must as well. Before this change it would be throwing an exception in the non-error_code form of the function when __canonical fails, while not setting the error code in the error_code form of the function (an inconsistency).

Added a little coverage in weakly_canonical.pass.cpp for the error_code forms of the api that was missing. Though I am lacking utilities in libcxx testing to add granular testing of the failure scenarios (like forcing realpath to fail for a given path, as it could if you had something like a flaky remote filesystem).

---

Full diff: https://github.com/llvm/llvm-project/pull/88341.diff

2 Files Affected:

• (modified) libcxx/src/filesystem/operations.cpp (+15-13)
• (modified) libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp (+8)

diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
index 62bb248d5eca9b..6a8e42d2090c40 100644
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -126,9 +126,6 @@ void __copy(const path& from, const path& to, copy_options options, error_code*
     return err.report(errc::function_not_supported);
   }
 
-  if (ec)
-    ec->clear();
-
   if (is_symlink(f)) {
     if (bool(copy_options::skip_symlinks & options)) {
       // do nothing
@@ -166,15 +163,15 @@ void __copy(const path& from, const path& to, copy_options options, error_code*
       return;
     }
     error_code m_ec2;
-    for (; it != directory_iterator(); it.increment(m_ec2)) {
-      if (m_ec2) {
-        return err.report(m_ec2);
-      }
+    for (; !m_ec2 && it != directory_iterator(); it.increment(m_ec2)) {
       __copy(it->path(), to / it->path().filename(), options | copy_options::__in_recursive_copy, ec);
       if (ec && *ec) {
         return;
       }
     }
+    if (m_ec2) {
+      return err.report(m_ec2);
+    }
   }
 }
 
@@ -936,23 +933,28 @@ path __weakly_canonical(const path& p, error_code* ec) {
   --PP;
   vector<string_view_t> DNEParts;
 
+  error_code m_ec;
   while (PP.State != PathParser::PS_BeforeBegin) {
     tmp.assign(createView(p.native().data(), &PP.RawEntry.back()));
-    error_code m_ec;
     file_status st = __status(tmp, &m_ec);
     if (!status_known(st)) {
       return err.report(m_ec);
     } else if (exists(st)) {
-      result = __canonical(tmp, ec);
+      result = __canonical(tmp, &m_ec);
+      if (m_ec) {
+        return err.report(m_ec);
+      }
       break;
     }
     DNEParts.push_back(*PP);
     --PP;
   }
-  if (PP.State == PathParser::PS_BeforeBegin)
-    result = __canonical("", ec);
-  if (ec)
-    ec->clear();
+  if (PP.State == PathParser::PS_BeforeBegin) {
+    result = __canonical("", &m_ec);
+    if (m_ec) {
+      return err.report(m_ec);
+    }
+  }
   if (DNEParts.empty())
     return result;
   for (auto It = DNEParts.rbegin(); It != DNEParts.rend(); ++It)
diff --git a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp
index 053bddcad9b43a..4e4f459f8cf364 100644
--- a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp
@@ -79,6 +79,14 @@ int main(int, char**) {
     TEST_REQUIRE(PathEq(output, expect),
                  TEST_WRITE_CONCATENATED(
                      "Input: ", TC.input.string(), "\nExpected: ", expect.string(), "\nOutput: ", output.string()));
+
+    // Get coverage over the error_code form of the api.
+    std::error_code ec;
+    const fs::path output_c = fs::weakly_canonical(p, ec);
+
+    TEST_REQUIRE(PathEq(output_c, expect),
+                 TEST_WRITE_CONCATENATED(
+                     "Input: ", TC.input.string(), "\nExpected: ", expect.string(), "\nOutput: ", output_c.string()));
   }
   return 0;
 }

[ldionne]

This seems mostly good to me, but I have a question about something I don't quite understand. Thanks for noticing these issues!

[ldionne]

LGTM, thanks for the fix! I applied the nit comment I had.