+------------------------------------------------------------------------------+
| |
| Git-Credential-Manager |
| |
+-+-------------+--------------+-----+---------------------+-----------------+-+
| | | | | |
| | | | Windows | Windows |
| | | | | |
| +-----------v-----------+ | | +----------------v---------------+ |
| | | | | | | |
| | GitHub <-------------+ GitHub.UI.Windows | |
| | | | | | | |
| +-+---------------------+ | | +-+------------------------------+ |
| | | | | |
| | +---------------------v-+ | | +------------------------------v-+
| | | | | | | |
| | | Atlassian.Bitbucket <------------+ Atlassian.Bitbucket.UI.Windows |
| | | | | | | |
| | +-+---------------------+ | | +---------------+----------------+
| | | | | |
| | | +----------------------v-+ | |
| | | | | | |
| | | | Microsoft.AzureRepos | | |
| | | | | | |
| | | +-----------+------------+ | |
| | | | | |
+-v---v----v--------------v------------+ +-v-----------------v----------------+
| | | |
| Core <--+ Core.UI |
| | | |
+--------------------------------------+ +------------------------------------+
Git Credential Manager Core (GCM Core) is built to be Git host and platform/OS
agonstic. Most of the shared logic (command execution, the abstract platform
subsystems, etc) can be found in the Core class
library (C#). The library targets .NET Standard as well as .NET Framework.
Note
The reason for also targeting .NET Framework directly is that the
Microsoft.Identity.Client(MSAL.NET) library requires a .NET Framework target to be able to show the embedded web browser auth pop-up on Windows platforms.There are extension points that now exist in MSAL.NET meaning we can plug-in our own browser pop-up handling code on .NET meaning both Windows and Mac. We haven't yet gotten around to exploring this.
See this issue for more information.
The entry-point for GCM Core can be found in the Git-Credential-Manager
project, a console application that targets both .NET and .NET Framework.
This project emits the git-credential-manager-core(.exe) executable, and
contains very little code - registration of all supported host providers and
running the Application object found in Core.
Providers have their own projects/assemblies that take dependencies on the
Core core assembly, and are dependents of the main
entry point application Git-Credential-Manager. Code in these binaries is
expected to run on all supported platforms and typically (see MSAL.NET note
above) does not include any graphical user interface; they use terminal prompts
only.
Where a provider needs some platform-specific interaction or graphical user interface, the recommended model is to have a separate 'helper' executable that the shared, core binaries shell out to. Currently the Bitbucket and GitHub providers each have a WPF (Windows only) helper executable that shows authentication prompts and messages.
The Microsoft.Git.CredentialHelper.UI project is a WPF (Windows only) assembly
that contains common WPF components and styles that are shared between provider
helpers on Windows.
We hope to be able to migrate the WPF/Windows only helpers to Avalonia in order to gain cross-platform graphical user interface support. See this issue for up-to-date progress on this effort.
For authentication using Microsoft Accounts or Azure Active Directory, things
are a little different. The MicrosoftAuthentication component is present in
the core Core assembly, rather than bundled with a
specific host provider. This was done to allow any service that may wish to in
the future integrate with Microsoft Accounts or Azure Active Directory can make
use of this reusable authentication component.
GCM Core makes use of the async/await model of .NET and C# in almost all
parts of the codebase where appropriate as usually requests end up going to the
network at some point.
+---------------+
| |
| Git |
| |
+---+-------^---+
| |
+---v---+---+---+
| stdin | stdout|
+---+---+---^---+
| |
(2) | | (7)
Select | | Serialize
Command | | Result
| |
(3) | |
Select | |
+---------------+ Provider +---v-------+---+
| Host Provider | | |
| Registry <------------+ Command |
| | | |
+-------^-------+ +----+------^---+
| | |
| (4) | | (6)
| Execute | | Return
| Operation | | Result
| (1) | |
| Register +----v------+---+
| | |
+--------------------+ Host Provider |
| |
+-------^-------+
|
(5) Use services |
|
+-------v-------+
| Command |
| Context |
+---------------+
Git Credential Manager Core maintains a set of known commands including
Get|Store|EraseCommand, as well as commands for install and help/usage.
GCM Core also maintains a set of known, registered host providers that implement
the IHostProvider interface. Providers register themselves by adding an
instance of the provider to the Application object via the RegisterProvider
method in Core.Program.
The GenericHostProvider is registered last so that it can handle all other
HTTP-based remotes as a catch-all, and provide basic username/password auth and
detect the presence of Windows Integrated Authentication (Kerberos, NTLM,
Negotiate) support (1).
For each invocation of GCM Core, the first argument on the command-line is matched against the known commands and if there is a successful match, the input from Git (over standard input) is deserialized and the command is executed (2).
The Get|Store|EraseCommands consult the host provider registry for the most
appropriate host provider. The default registry implementation select the a host
provider by asking each registered provider in turn if they understand the
request. The provider selection can be overridden by the user via the
credential.provider or GCM_PROVIDER
configuration and environment variable respectively (3)).
The Get|Store|EraseCommands call the corresponding
Get|Store|EraseCredentialAsync methods on the IHostProvider, passing the
request from Git together with an instance of the ICommandContext (4). The
host provider can then make use of various services available on the command
context to complete the requested operation (5).
Once a credential has been created, retrieved, stored or erased, the host
provider returns the credential (for get operations only) to the calling
command (6). The credential is then serialized and returned to Git over standard
output (7) and GCM Core terminates with a successful exit code.
Host providers implement the IHostProvider interface. They can choose to
directly implement the interface they can also derive from the HostProvider
abstract class (which itself implements the IHostProvider interface).
The HostProvider abstract class implements the
Get|Store|EraseCredentialAsync methods and instead has the
GenerateCredentialAsync abstract method, and the GetServiceName virtual
method. Calls to get, store, or erase result in first a call to
GetServiceName which should return a stable and unique value for the provider
and request. This value forms part of the attributes associated with any stored
credential in the credential store. During a get operation the
credential store is queried for an existing credential with such service name.
If a credential is found it is returned immediately. Similarly, calls to store
and erase are handles automatically to store credentials against, and erase
credentials matching the service name. Methods are implemented as virtual
meaning you can always override this behaviour, for example to clear other
custom caches on an erase request, without having to reimplement the
lookup/store credential logic.
The default implementation of GetServiceName is usually sufficient for most
providers. It returns the computed remote URL (without a trailing slash) from
the input arguments from Git - <protocol>://<host>[/<path>] - no username is
included even if present.
Host providers are queried in turn, by priority (then registration order) via
the IHostProvider.IsSupported(InputArguments) method and passed the input
received from Git. If the provider recognises the request, for example by a
matching known host name, they can return true. If the provider wants to
cancel and abort an authentication request, for example if this is a HTTP (not
HTTPS) request for a known host, they should still return true and later
cancel the request.
Host providers can also be queried via the IHostProvider.IsSupported(HttpResponseMessage)
method and passed the response message from a HEAD call made to the remote URI.
This is useful for detecting on-premises instances based on header values. GCM
will only query a provider via this method overload if no other provider at the
same registration priority has returned true to the InputArguments overload.
Depending on the request from Git, one of GetCredentialAsync (for get
requests), StoreCredentialAsync (for store requests) or
EraseCredentialAsync (for erase requests) will be called. The argument
InputArguments contains the request information passed over standard input
from Git/the caller; the same as was passed to IsSupported.
The return value for the get operation must be an ICredential that Git can
use to complete authentication.
Note:
The credential can also be an instance where both username and password are the empty string, to signal to Git it should let cURL use "any auth" detection - typically to use Windows Integrated Authentication.
There are no return values for the store and erase operations as Git ignores
any output or exit codes for these commands. Failures for these operations are
best communicated via writing to the Standard Error stream via
ICommandContext.Streams.Error.
The ICommandContext which contains numerous services which are useful for
interacting with various platform subsystems, such as the file system or
environment variables. All services on the command context are exposed as
interfaces for ease of testing and portability between different operating
systems and platforms.
| Component | Description |
|---|---|
| CredentialStore | A secure operating system controlled location for storing and retrieving ICredential objects. |
| Settings | Abstraction over all GCM Core settings. |
| Streams | Abstraction over standard input, output and error streams connected to the parent process (typically Git). |
| Terminal | Provides interactions with an attached terminal, if it exists. |
| SessionManager | Provides information about the current user session. |
| Trace | Provides tracing information that may be useful for debugging issues in the wild. Secret information MUST be filtered out completely or via the Write___Secret method(s). |
| FileSystem | Abstraction over file system operations. |
| HttpClientFactory | Factory for creating HttpClient instances that are configured with the correct user agent, headers, and proxy settings. |
| Git | Provides interactions with Git and Git configuration. |
| Environment | Abstraction over the current system/user environment variables. |
| SystemPrompts | Provides services for showing system/OS native credential prompts. |
GCM Core operates a 'fail fast' approach to unrecoverable errors. This usually
means throwing an Exception which will propagate up to the entry-point and be
caught, a non-zero exit code returned, and the error message printed with the
"fatal:" prefix. For errors originating from interop/native code, you should
throw an exception of the InteropException type. Error messages in exceptions
should be human readable. When there is a known or user-fixable issue,
instructions on how to self-remedy the issue, or links to relevant
documentation should be given.
Warnings can be emitted over the standard error stream
(ICommandContext.Streams.Error) when you want to alert the user to a potential
issue with their configuration that does not necessarily stop the
operation/authentication.
The ITrace component can be found on the ICommandContext object or passed in
directly to some constructors. Verbose and diagnostic information is be written
to the trace object in most places of GCM Core.