GH-44256: [C++][FS][Azure] Fix edgecase where GetFileInfo incorrectly…

… returns NotFound on flat namespace and Azurite (#44302)

### Rationale for this change
Fix a bug where `GetFileInfo` incorrectly returns `FileType::NotFound` on flat namespace and Azurite.

### What changes are included in this PR?
Fix by detecting the exact edgecase and doing an extra listing operation to disambiguate. 

### Are these changes tested?
Yes, updated automated test

### Are there any user-facing changes?
Only a bug fix.

* GitHub Issue: #44256

Authored-by: Thomas Newton <thomas.w.newton@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/src/arrow/filesystem/azurefs.cc

@@ -1793,6 +1793,8 @@ class AzureFileSystem::Impl {
  // BlobPrefixes. A BlobPrefix always ends with kDelimiter ("/"), so we can
  // distinguish between a directory and a file by checking if we received a
  // prefix or a blob.
+ // This strategy allows us to implement GetFileInfo with just 1 blob storage
+ // operation in almost every case.
  if (!list_response.BlobPrefixes.empty()) {
  // Ensure the returned BlobPrefixes[0] string doesn't contain more characters than
  // the requested Prefix. For instance, if we request with Prefix="dir/abra" and
@@ -1814,6 +1816,25 @@ class AzureFileSystem::Impl {
  info.set_mtime(
  std::chrono::system_clock::time_point{blob.Details.LastModified});
  return info;
+ } else if (blob.Name[options.Prefix.Value().length()] < internal::kSep) {
+ // First list result did not indicate a directory and there is definitely no
+ // exactly matching blob. However, there may still be a directory that we
+ // initially missed because the first list result came before
+ // `options.Prefix + internal::kSep` lexigraphically.
+ // For example the flat namespace storage account has the following blobs:
+ // - container/dir.txt
+ // - container/dir/file.txt
+ // GetFileInfo(container/dir) should return FileType::Directory but in this
+ // edge case `blob = "dir.txt"`, so without further checks we would incorrectly
+ // return FileType::NotFound.
+ // Therefore we make an extra list operation with the trailing slash to confirm
+ // whether the path is a directory.
+ options.Prefix = internal::EnsureTrailingSlash(location.path);
+ auto list_with_trailing_slash_response = container_client.ListBlobs(options);
+ if (!list_with_trailing_slash_response.Blobs.empty()) {
+ info.set_type(FileType::Directory);
+ return info;
+ }
  }
  }
  info.set_type(FileType::NotFound);

cpp/src/arrow/filesystem/azurefs_test.cc

@@ -2065,6 +2065,20 @@ void TestAzureFileSystem::TestGetFileInfoObjectWithNestedStructure() {
  ASSERT_OK(output->Write(lorem_ipsum));
  ASSERT_OK(output->Close());
 
+ // . is immediately before "/" lexicographically, ensure that this doesn't
+ // cause unexpected issues. NOTE: Its seems real Azure blob storage doesn't
+ // allow blob names to end in `.`
+ ASSERT_OK_AND_ASSIGN(output, fs()->OpenOutputStream(
+ data.ContainerPath("test-object-dir/some_other_dir.a"),
+ /*metadata=*/{}));
+ ASSERT_OK(output->Write(lorem_ipsum));
+ ASSERT_OK(output->Close());
+ ASSERT_OK_AND_ASSIGN(output,
+ fs()->OpenOutputStream(data.ContainerPath(kObjectName + ".a"),
+ /*metadata=*/{}));
+ ASSERT_OK(output->Write(lorem_ipsum));
+ ASSERT_OK(output->Close());
+
  AssertFileInfo(fs(), data.ContainerPath(kObjectName), FileType::File);
  AssertFileInfo(fs(), data.ContainerPath(kObjectName) + "/", FileType::NotFound);
  AssertFileInfo(fs(), data.ContainerPath("test-object-dir"), FileType::Directory);