internal/lsp: limit diagnostics concurrency

Diagnostics runs cannot be canceled until they finish a package. If a
user has a very expensive package, we may stack up diagnostics runs to
the point where the machine runs out of memory. We see hints of this in
various issues.

To avoid that, only allow one diagnostics run at a time. We can change
the limit later if we want.

Updates golang/go#37223.

Change-Id: Icd0eec4da936153306cf0a1f7175ae2b4b265272
Reviewed-on: https://go-review.googlesource.com/c/tools/+/219958
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/diagnostics.go

@@ -44,6 +44,14 @@ func (s *Server) diagnose(ctx context.Context, snapshot source.Snapshot, alwaysA
 	ctx, done := trace.StartSpan(ctx, "lsp:background-worker")
 	defer done()
 
+	// Wait for a free diagnostics slot.
+	select {
+	case <-ctx.Done():
+		return nil
+	case s.diagnosticsSema <- struct{}{}:
+	}
+	defer func() { <-s.diagnosticsSema }()
+
 	allReports := make(map[diagnosticKey][]source.Diagnostic)
 	var reportsMu sync.Mutex
 	var wg sync.WaitGroup

internal/lsp/lsp_test.go

@@ -85,10 +85,7 @@ func testLSP(t *testing.T, exporter packagestest.Exporter) {
 			t.Fatal(err)
 		}
 		r := &runner{
-			server: &Server{
-				session:   session,
-				delivered: map[span.URI]sentDiagnostics{},
-			},
+			server: NewServer(session, nil),
 			data:   datum,
 			ctx:    ctx,
 		}

internal/lsp/server.go

@@ -16,6 +16,8 @@ import (
 	"golang.org/x/tools/internal/span"
 )
 
+const concurrentAnalyses = 1
+
 // NewServer creates an LSP server and binds it to handle incoming client
 // messages on on the supplied stream.
 func NewServer(session source.Session, client protocol.Client) *Server {
@@ -23,6 +25,7 @@ func NewServer(session source.Session, client protocol.Client) *Server {
 		delivered:       make(map[span.URI]sentDiagnostics),
 		session:         session,
 		client:          client,
+		diagnosticsSema: make(chan struct{}, concurrentAnalyses),
 	}
 }
 
@@ -54,6 +57,9 @@ type Server struct {
 	// delivered is a cache of the diagnostics that the server has sent.
 	deliveredMu sync.Mutex
 	delivered   map[span.URI]sentDiagnostics
+
+	// diagnosticsSema limits the concurrency of diagnostics runs, which can be expensive.
+	diagnosticsSema chan struct{}
 }
 
 // sentDiagnostics is used to cache diagnostics that have been sent for a given file.