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Contributing to VFS for Git

Thank you for taking the time to contribute!

Guidelines

Code of Conduct
Design Reviews
Platform Specific Code
Tracing and Logging
Error Handling
Background Threads
Coding Conventions
Testing

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Design Reviews

Architectural changes and new features should start with a design review. It's easier and wastes less time to incorporate feedback at this stage.

The design review process is as follows:

Create a pull request that contains a design document in Markdown (.md) format for the proposed change. Assign the design-doc label to the pull request.
Use the pull request for design feedback and for iterating on the design.
Once the design is approved, create a new issue with a description that includes the final design document. Include a link to the pull request that was used for discussion.
Close (without merging!) the pull request used for the design discussion.

Platform Specific Code

Prefer cross-platform code to platform-specific code

Cross-platform code is more easily reused. Reusing code reduces the amount of code that must be written, tested, and maintained.
Platform specific code, and only platform specific code, should go in GVFSPlatform

When platform specific code is required, it should be placed in GVFSPlatform or one of the platforms it contains (e.g. IKernelDriver)

Tracing and Logging

The "Error" logging level is reserved for non-retryable errors that result in I/O failures or the VFS4G process shutting down

The expectation from our customers is that when VFS4G logs an "Error" level message in its log file either:
- VFS4G had to shut down unexpectedly
- VFS4G encountered an issue severe enough that user-initiated I/O would fail.
Log full exception stacks

Full exception stacks (i.e. Exception.ToString) provide more details than the exception message alone (Exception.Message). Full exception stacks make root-causing issues easier.
Do not display full exception stacks to users

Exception call stacks are not usually actionable for the user. Users frequently (sometimes incorrectly) assume that VFS4G has crashed when shown a full stack. The full stack should be included in VFS4G logs, but should not be displayed as part of the error message provided to the user.
Include relevant details when logging exceptions

Sometimes an exception call stack alone is not enough to root cause failures in VFS4G. When catching (or throwing) exceptions, log relevant details that will help diagnose the issue. As a general rule, the closer an exception is caught to where it's thrown, the more relevant details there will be to log.

Example:
```
catch (Exception e)
{
  EventMetadata metadata = new EventMetadata();
  metadata.Add("Area", "Mount");
  metadata.Add(nameof(enlistmentHookPath), enlistmentHookPath);
  metadata.Add(nameof(installedHookPath), installedHookPath);
  metadata.Add("Exception", e.ToString());
  context.Tracer.RelatedError(metadata, $"Failed to compare {hook.Name} version");
}
```

Error Handling

Fail fast: An error or exception that risks data loss or corruption should shut down VFS4G immediately

Preventing data loss and repository corruption is critical. If an error or exception occurs that could lead to data loss, it's better to shut down VFS4G than keep the repository mounted and risk corruption.
Do not catch exceptions that are indicative of a programmer error (e.g. ArgumentNullException)

Any exceptions that result from programmer error (e.g. ArgumentNullException) should be discovered as early in the development process as possible. Avoid catch statements that would hide these errors (e.g. catch(Exception)).

The only exception to this rule is for unhandled exceptions in background threads

Do not use exceptions for normal control flow

Prefer writing code that does not throw exceptions. The TryXXX pattern, for example, avoids the performance costs that come with using exceptions. Additionally, VFS4G typically needs to know exactly where errors occur and handle the errors there. The TryXXX pattern helps ensure errors are handled in that fashion.

Example: Handle errors where they occur (good):

bool TryDoWorkOnDisk(string fileContents, out string error)
{
  if (!TryCreateReadConfig())
  {
    error = "Failed to read config file";
    return false;
  }

  if (!TryCreateTempFile(fileContents))
  {
    error = "Failed to create temp file";
    return false;
  }

  if (!TryRenameTempFile())
  {
    error = "Failed to rename temp file";
    if (!TryDeleteTempFile())
    {
      error += ", and failed to cleanup temp file";
    }

    return false;
  }

  error = null;
  return true;
}

Example: Handle errors in catch without knowing where they came from (bad):

bool TryDoWorkOnDisk(string fileContents, out string error)
{
  try
  {
    CreateReadConfig();
    CreateTempFile(fileContents);
    RenameTempFile();
  }
  catch (Exception ex) when (ex is IOException || ex is UnauthorizedAccessException)
  {
    error = "Something went wrong doing work on disk";

    try
    {
      if (TempFileExists())
      {
        DeleteTempFile();
      }
    }
    catch (Exception e) when (e is IOException || e is UnauthorizedAccessException)
    {
      error += ", and failed to cleanup temp file";
    }

    return false;
  }

  error = null;
  return true;
}

Provide the user with user-actionable messages whenever possible

Don't tell a user what went wrong. Help the user fix the problem.

Example:

"You can only specify --hydrate if the repository is mounted. Run 'gvfs mount' and try again."

Background Threads

Avoid using the thread pool (and avoid using async)

HttpRequestor.SendRequest makes a blocking call to HttpClient.SendAsync. That blocking call consumes a thread from the managed thread pool. Until that design changes, the rest of VFS4G must avoid using the thread pool unless absolutely necessary. If the thread pool is required, any long running tasks should be moved to a separate thread managed by VFS4G itself (see GitMaintenanceQueue for an example).

Long-running or blocking work scheduled on the managed thread pool can prevent the normal operation of VFS4G. For example, it could prevent downloading file sizes, loose objects, or file contents in a timely fashion.
Catch all exceptions on long-running tasks and background threads

Wrap all code that runs in the background thread in a top-level try/catch(Exception). Any exceptions caught by this handler should be logged, and then VFS4G should be forced to terminate with Environment.Exit. It's not safe to allow VFS4G to continue to run after an unhandled exception stops a background thread or long-running task. Testing has shown that Environment.Exit consistently terminates the VFS4G mount process regardless of how background threads are started (e.g. native thread, new Thread(), Task.Factory.StartNew()).

An example of this pattern can be seen in BackgroundFileSystemTaskRunner.ProcessBackgroundTasks.

Coding Conventions

Most C# coding style guidelines are covered by StyleCop

Fix any StyleCop issues reported in changed code. When adding new projects to VFS4G, be sure that StyleCop is analyzing them as part of the build.
Prefer explicit types to interfaces and implicitly typed variables

Avoid the use of var (C#), dynamic (C#), and auto (C++). Prefer concrete/explicit types to interfaces (e.g. prefer List to IList).

The VFS4G codebase uses this approach because:
- Interfaces can hide the performance characteristics of their underlying type. For example, an IDictionary could be a SortedList or a Dictionary (or several other data types).
- Interfaces can hide the thread safety (or lack thereof) of their underlying type. For example, an IDictionary could be a Dictionary or a ConcurrentDictionary.
- Explicit types make these performance and thread safety characteristics explicit when reviewing code.
- VFS4G is not a public API and its components are always shipped together. Develoepers are free to make API changes to VFS4G's public methods.
Method names start with a verb (e.g. "GetProjectedFolderEntryData" rather than "ProjectedFolderEntryData")

Starting with a verb in the name improves readability and helps ensure consistency with the rest of the VFS4G codebase.

Aim to write self-commenting code. When necessary, comments should give background needed to understand the code.

Helpful (good) comment:

// Order the folders in descending order so that we walk the tree from bottom up.
// Traversing the folders in this order:
//  1. Ensures child folders are deleted before their parents
//  2. Ensures that folders that have been deleted by git 
//     (but are still in the projection) are found before their
//     parent folder is re-expanded (only applies on platforms 
//     where EnumerationExpandsDirectories is true)
foreach (PlaceholderListDatabase.PlaceholderData folderPlaceholder in   placeholderFoldersListCopy.OrderByDescending(x => x.Path))

Obvious (bad) comment:

// Check if enumeration expands directories on the current platform
if (GVFSPlatform.Instance.KernelDriver.EnumerationExpandsDirectories)

Add new interfaces when it makes sense for the product, not simply for unit testing

When a class needs to be mocked (or have a subset of its behavior mocked), prefer using virtual methods instead of adding a new interface. VFS4G uses interfaces when multiple implementations of the interface exist in the product code.
Check for null using the equality (==) and inequality (!=) operators rather than is

A corollary to this guideline is that equality/inequality operators that break null checks should not be added (see this post for an example).
Use nameof(...) rather than hardcoded strings

Using nameof ensures that when methods/variables are renamed the logging of those method/variable names will also be updated. However, hard coded strings are still appropriate when they are used for generating reports and changing the strings would break the reports.

C/C++

Do not use C-style casts. Use C++-style casts.

C++ style casts (e.g. static_cast<T>) more clearly express the intent of the programmer, allow for better validation by the compiler, and are easier to search for in the codebase.
Declare static functions at the top of source files

This ensures that the functions can be called from anywhere inside the file.
Do not use namespace using statements in header files

using statements inside header files are picked up by all source files that include the headers and can cause unexpected errors if there are name collisions.

Prefer using to full namespaces in source files

Example:

// Inside MyFavSourceFile.cpp

using std::string;

// Do not use `std::` namespace with `string`
static string s_myString;

Use a meaningful prefix for "public" free functions, and use the same prefix for all functions in a given header file