Improve interactivity in face of concurrent diagnostics

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
+* Fix `textDocument/completion` (and other interactive requests) stalling when analyzers are enabled on a multi-project solution: `workspace/diagnostic` now analyzes one project at a time instead of fanning out across every project concurrently
+  - By @razzmatazz in https://github.com/razzmatazz/csharp-language-server/pull/421
 * Fix every position-based request answering null on multi-targeted projects: a file of a `<TargetFrameworks>` project has one Roslyn document per target framework, which the document lookup treated as ambiguous and resolved to `None`; the lookup now picks the flavor with the best TFM (same rule as workspace-global TFM selection, mirroring how Visual Studio auto-selects a default project context), leaving genuinely linked files (one path in different project files) ambiguous as before
   - By @pbednarcik
 * Report per-project progress while loading a solution: `$/progress` reports now carry a message like `Project (34/88)` and a monotonically increasing percentage, so clients can display load progress and estimate readiness

docs/codebase-architecture.md

@@ -223,16 +223,23 @@ When `config.analyzersEnabled = true`, calls
 back to `semanticModel.GetDiagnostics(ct)`.
 
 **Workspace pull (`getWorkspaceDiagnosticReports`):** Takes `config`, a `knownResultIds`
-map (for unchanged-report optimization), and the list of workspace folders. For each project:
+map (for unchanged-report optimization), and the list of workspace folders. Processes
+projects **strictly sequentially** — one project at a time, across all workspace
+folders — via a plain nested `for` loop over `asyncSeq { yield! ... }`; there is no
+`Async.Start` fan-out. This is deliberate: both compilation construction
+(`GetCompilationAsync`/`GetSourceGeneratedDocumentsAsync`) and the analyzer pass are
+CPU-bound, and running them concurrently across many projects previously starved
+interactive requests (completion, hover, …) sharing the same `ThreadPool` — see
+`plans/interactive-request-latency-vs-analyzers.md` for the full history. A single
+project's failure is caught with `try/with` around its `AsyncSeq` so it doesn't abort
+the rest of the sweep. For each project (`generateProjectDiagnosticReports`):
 - Builds `resultId = "{project.Version}/{analyzersEnabled}"` so toggling `analyzersEnabled`
   invalidates any cached "unchanged" result the client holds
 - Checks if the client already holds results for this `resultId` → emits `Unchanged`
 - Otherwise calls `project.GetCompilationAsync(ct)`; when `config.analyzersEnabled = true`
   uses `Analyzers.getCompilationDiagnosticsWithAnalyzers project compilation`, otherwise
-  `compilation.GetDiagnostics()`; groups diagnostics by document URI and emits `Full` reports
-- Results flow through a bounded `Channel<WorkspaceDiagnosticsReportsChannelItem>(256)` with
-  one `Async.Start` per project writing to the channel and the main consumer yielding from
-  it as `AsyncSeq`
+  `compilation.GetDiagnostics()`; groups diagnostics by document URI and yields `Full`
+  reports directly from the `asyncSeq`
 
 The `handleWorkspaceDiagnostic` function supports two modes based on `PartialResultToken`:
 - `None` → collects all reports into a single `WorkspaceDiagnosticReport`
@@ -252,9 +259,17 @@ Two functions used by all three diagnostic code paths:
 
 **`getCompilationDiagnosticsWithAnalyzers project compilation ct`**
 - Falls back to `compilation.GetDiagnostics(ct)` when no analyzers are present.
-- Otherwise calls `compilation.WithAnalyzers(analyzers, project.AnalyzerOptions)` →
+- Otherwise calls `compilation.WithAnalyzers(analyzers, analysisOptions)` →
   `GetAllDiagnosticsAsync(ct)`.
 
+**`concurrentAnalysis: false`:** Both functions pass a `CompilationWithAnalyzersOptions`
+with `concurrentAnalysis = false`, matching the precedent set by Roslyn's own IDE layer
+(`DiagnosticIncrementalAnalyzer.CompilationManager`) and OmniSharp. This prevents a
+*single* project's analyzer pass from internally fanning out across every core via
+Roslyn's `AnalyzerDriver`, bounding all analyzer-driven diagnostic computation (push,
+pull-per-document, pull-workspace) to one core system-wide by construction, on top of
+the sequential per-project processing in `getWorkspaceDiagnosticReports` above.
+
 **Key design point:** `project.AnalyzerOptions` is the `AnalyzerOptions` instance that
 `MSBuildWorkspace` builds from the project's `<Analyzer>` items and `.editorconfig` files.
 It already carries the `AnalyzerConfigOptionsProvider` that makes `.editorconfig`