docs: initial version of varlink guideline

docs/VARLINK.md

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+---
+title: Varlink API Style
+category: Contributing
+layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
+---
+
+# General guideline
+
+- Varlink field names should use camelCase. This guideline does not apply to
+  well-known and documented configuration options, such as those defined in
+  [systemd.unit](https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html),
+  where existing naming conventions should be preserved for
+  compatibility and clarity.
+
+- Every field and method should include meaningful documentation. It's
+  acceptable to reference existing documentation where appropriate.
+  Documentation may be omitted only when the meaning is self-evident, even to
+  someone not already familiar with varlink interface/method.
+
+- Varlink fields should optimize toward clarity:
+  * avoid abbreviations: `cacheDir` -> `cacheDirectory`
+  * prefer string values over numeric codes when possible,
+    to make interfaces more self-descriptive and easier to understand.
+
+# Interface structure
+
+- Varlink methods should consider splitting their output into 'context' and
+  'runtime' sections. The guiding principle is simple: if a property makes
+  sense to include in a configuration (e.g. unit file), it belongs to 'context';
+  otherwise, it goes under 'runtime'. This split ensures a consistent and
+  reusable structure. Functions that describe an object can produce context
+  data that other functions can later consume to create a similar object.
+
+  Example: `io.systemd.Unit.List` outputs unit configuration, which can later
+  be reused to create another unit via `io.systemd.Unit.StartTransient` (not
+  implemented yet). The `io.systemd.Unit.StartTransient` call should accept
+  only the 'context' portion of the output, without requiring any runtime data
+  such as state (e.g. pid) or statistics.
+
+- Following the guideline above, any field within 'context' should be nullable
+  by default. This ensures that when a context structure is used as input, the
+  caller is not required to provide every field explicitly. Omitted fields are
+  automatically assigned their default values, allowing partial context
+  definitions to be valid and simplifying reuse across different operations.
+  Fields that cannot logically be omitted in input (e.g. a unit type) may remain
+  non-nullable.
+
+# Enums
+
+- Enum fields in the codebase must be exposed as string values in Varlink, not
+  as their underlying integer representations. Use `SD_VARLINK_DEFINE_ENUM_TYPE`
+  to declare an enum type in the Varlink specification.
+
+- The Varlink IDL validator does not permit enum values that contain dashes.
+  Therefore, when defining an enum for Varlink, replace dashes with underscores.
+
+- Varlink interface should output enum values using the underscore form. For
+  input, it should accept both the original dash-containing form and the
+  underscore form. The following helpers simplify this:
+  * `JSON_BUILD_STRING_UNDERSCORIFY` - outputs a stringified enum value
+    with dashes converted to underscores.
+  * `JSON_DISPATCH_ENUM_DEFINE` - creates a `json_dispatch_*` function that
+    accepts both the original and the underscorified enum value as valid input.
+
+- An internal enum may be exposed as a simple string field instead of a Varlink
+  enum type when the field is output-only and never provided or controlled by
+  the user. However, such fields should avoid using dashes to prevent breaking
+  changes if they are later converted into enums (see below).
+
+- A varlink string field that has a finite set of possible values may later be
+  converted into an enum without introducing a breaking change. This allows the
+  interface to evolve from loosely defined string values to a more explicit and
+  type-safe enumeration once the valid options are well established.