Never create more checkers than files in a program

internal/compiler/checkerpool.go

@@ -32,7 +32,16 @@ type checkerPool struct {
 
 var _ CheckerPool = (*checkerPool)(nil)
 
-func newCheckerPool(checkerCount int, program *Program) *checkerPool {
+func newCheckerPool(program *Program) *checkerPool {
+	checkerCount := 4
+	if program.SingleThreaded() {
+		checkerCount = 1
+	} else if c := program.Options().Checkers; c != nil {
+		checkerCount = *c
+	}
+
+	checkerCount = min(max(checkerCount, 1), len(program.files), 256)
+
 	pool := &checkerPool{
 		program:      program,
 		checkerCount: checkerCount,

internal/compiler/fileloader.go

@@ -69,6 +69,7 @@ type processedFiles struct {
 	// if file was included using source file and its output is actually part of program
 	// this contains mapping from output to source file
 	outputFileToProjectReferenceSource map[tspath.Path]string
+	finishedProcessing                 bool
 }
 
 type jsxRuntimeImportSpecifier struct {
@@ -236,6 +237,7 @@ func processAllProgramFiles(
 	}
 
 	return processedFiles{
+		finishedProcessing:                   true,
 		resolver:                             loader.resolver,
 		files:                                allFiles,
 		filesByPath:                          filesByPath,

internal/compiler/program.go

@@ -222,8 +222,8 @@ func (p *Program) GetSourceFileFromReference(origin *ast.SourceFile, ref *ast.Fi
 
 func NewProgram(opts ProgramOptions) *Program {
 	p := &Program{opts: opts}
-	p.initCheckerPool()
 	p.processedFiles = processAllProgramFiles(p.opts, p.SingleThreaded())
+	p.initCheckerPool()
 	p.verifyCompilerOptions()
 	return p
 }
@@ -259,16 +259,14 @@ func (p *Program) UpdateProgram(changedFilePath tspath.Path, newHost CompilerHos
 }
 
 func (p *Program) initCheckerPool() {
+	if !p.finishedProcessing {
+		panic("Program must finish processing files before initializing checker pool")
+	}
+
 	if p.opts.CreateCheckerPool != nil {
 		p.checkerPool = p.opts.CreateCheckerPool(p)
 	} else {
-		checkers := 4
-		if p.SingleThreaded() {
-			checkers = 1
-		} else if p.Options().Checkers != nil {
-			checkers = min(max(*p.Options().Checkers, 1), 256)
-		}
-		p.checkerPool = newCheckerPool(checkers, p)
+		p.checkerPool = newCheckerPool(p)
 	}
 }