createProgram calls host.getSourceFile multiple times

TypeScript Version: 3.1.0-dev.20180807

Search Terms:

Repro

declare let oldProgram: ts.Program;
const originalHost = ts.createCompilerHost(true);
const host = {
  ... originalHost,
  getSourceFile(fileName, ...args) {
    console.log(fileName);
    return originalHost.getSoruceFile(fileName, ...args);
  }
const newProgram = ts.createProgram(oldProgram.getRootFileNames(), oldProgram.getCompilerOptions(), host, oldProgram);

Expected behavior:

Each fileName is logged once.

Actual behavior:

If structureIsReused !== Completely fileNames are logged twice: one time from tryReuseStructureFromOldProgram and another time from findSourceFile. That means these SourceFiles are requested multiple times. This is especially a problem with the default CompilerHost which has no caching. It therefore reads and parses each file multiple times, but only the last one is actually used.

Start with the createProgram repro and trace the calls through tryReuseStructureFromOldProgram and findSourceFile. Verify the behavior with a logging CompilerHost, then ensure each fileName is requested once when reusing a non-complete old program without losing the expected SourceFile behavior.