hnnsgstfssn opened a new pull request, #38288:
URL: https://github.com/apache/beam/pull/38288
Implement custom WindowFn registration for the Go SDK, reaching capability
parity with Java/Python AssignContext.
Custom WindowFns are plain structs registered via RegisterWindowFn and
validated structurally at registration time, matching the DoFn pattern used
elsewhere in the SDK. Two AssignWindows shapes are accepted:
AssignWindows(typex.EventTime) []typex.Window
AssignWindows(typex.EventTime, T) []typex.Window
The second form is element-aware: the element value drives which window(s)
it lands in. Validation happens via reflection at registration, so misuse fails
at pipeline construction rather than at runtime. A package-level registry
records the struct type and optional element type for cross-package lookup via
LookupWindowFnMeta.
An interface-based shape (WindowAssigner) was explored first. Structural
typing was chosen instead because it keeps custom WindowFns consistent with
DoFns, avoids forcing users to satisfy a Go-specific interface, and lets the
same registry carry the element-type metadata that the dispatch and translation
paths need.
WindowFnInvoker dispatches in three tiers: typed interface (zero alloc),
element-aware typed interface (zero alloc), and reflect.Value.Method.Call as a
fallback (2 allocs/element).
Serialization piggybacks on the Beam model proto (FunctionSpec with URN +
JSON payload), so the internal v1 proto needs no new fields. The side-input
mapping path panics for element-aware WindowFns, matching the existing sessions
guard, since side-input windows have no element to feed AssignWindows.
Integration coverage exercises an element-aware WindowFn end-to-end through
the direct runner, verifying that elements with different SizeMs values land in
the windows dictated by their own data.
Fixes #20627
------------------------
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