[clangd] Add a testcase for empty preamble.

[hokein]

Summary

This is a patch of add a testcase for https://reviews.llvm.org/D50628.

[ilya-biryukov]

Maybe also add a test for find-definition that was broken before? (non-empty preamble -> empty preamble -> bad gotodef that goes to included file instead of the local variable)
To have a regression test against similar failures.

unittests/clangd/TUSchedulerTests.cpp

312

Maybe call PrecompiledPreamble directly? it requires some setup, but tests the modified code without extra layers that TUScheduler adds.

[hokein]

In D50627#1196988, @ilya-biryukov wrote:

Maybe also add a test for find-definition that was broken before? (non-empty preamble -> empty preamble -> bad gotodef that goes to included file instead of the local variable)
To have a regression test against similar failures.

I think this patch is in a good scope (for empty preamble). I'd leave it as it is. The failure of GoTodefinition test is caused by an inconsistent behavior of using lastBuiltPreamble/NewPreamble in TUScheduler, I will send out a separate patch fixing it (based on our discussion).

[ilya-biryukov]

In D50627#1198595, @hokein wrote:

I think this patch is in a good scope (for empty preamble). I'd leave it as it is. The failure of GoTodefinition test is caused by an inconsistent behavior of using lastBuiltPreamble/NewPreamble in TUScheduler, I will send out a separate patch fixing it (based on our discussion).

LG, thanks!

+ a few more nits to the tests

unittests/clangd/TUSchedulerTests.cpp

333

NIT: just use cantFail(std::move(Preamble)) to get the underlying result, no need for an extra ASSERT_TRUE here.

348

NIT: also use cantFail?

[hokein]

Address review comments.

[ilya-biryukov]

LGTM, but note the comment: we don't need ASSERT_TRUE(bool(Preamble)).

unittests/clangd/TUSchedulerTests.cpp

333

Remove ASSERT_TRUE(bool(Preamble))?
We assume everyone runs the tests run with assertions enable on every commit (and builtbots will catch the ones that don't), so cantFail will handle the error case for us.

[hokein]

Remove ASSERT_TRUE(bool(Preamble)).

unittests/clangd/TUSchedulerTests.cpp

TEST_F(TUSchedulerTests, EvictedAST) {
   ASSERT_EQ(BuiltASTCounter.load(), 4);
 
   // Check the AST for foo.cpp is retained now and one of the others got
   // evicted.
   EXPECT_THAT(S.getFilesWithCachedAST(),
               UnorderedElementsAre(Foo, AnyOf(Bar, Baz)));
 }
 
+TEST_F(TUSchedulerTests, EmptyPreamble) {
+  TUScheduler S(
+      /*AsyncThreadsCount=*/4, /*StorePreambleInMemory=*/true,
+      PreambleParsedCallback(),
+      /*UpdateDebounce=*/std::chrono::steady_clock::duration::zero(),
+      ASTRetentionPolicy());
+
+  auto Foo = testPath("foo.cpp");
+  auto Header = testPath("foo.h");
+
+  Files[Header] = "void foo()";
+  Timestamps[Header] = time_t(0);
+  auto WithPreamble = R"cpp(
+    #include "foo.h"
+    int main() {}
+  )cpp";
+  auto WithEmptyPreamble = R"cpp(int main() {})cpp";
+  S.update(Foo, getInputs(Foo, WithPreamble), WantDiagnostics::Auto,
+           [](std::vector<Diag>) {});
+  S.runWithPreamble("getNonEmptyPreamble", Foo,
+                    [&](llvm::Expected<InputsAndPreamble> Preamble) {
+                      // We expect to get a non-empty preamble.
+                      EXPECT_GT(cantFail(std::move(Preamble))
+                                    .Preamble->Preamble.getBounds()
+                                    .Size,
+                                0u);
+                    });
+  // Wait for the preamble is being built.
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(1)));
+
+  // Update the file which results in an empty preamble.
+  S.update(Foo, getInputs(Foo, WithEmptyPreamble), WantDiagnostics::Auto,
+           [](std::vector<Diag>) {});
+  // Wait for the preamble is being built.
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(1)));
+  S.runWithPreamble("getEmptyPreamble", Foo,
+                    [&](llvm::Expected<InputsAndPreamble> Preamble) {
+                      // We expect to get an empty preamble.
+                      EXPECT_EQ(cantFail(std::move(Preamble))
+                                    .Preamble->Preamble.getBounds()
+                                    .Size,
+                                0u);
+                    });
+}
+
 TEST_F(TUSchedulerTests, RunWaitsForPreamble) {
   // Testing strategy: we update the file and schedule a few preamble reads at
   // the same time. All reads should get the same non-null preamble.
   TUScheduler S(
       /*AsyncThreadsCount=*/4, /*StorePreambleInMemory=*/true,
       PreambleParsedCallback(),
       /*UpdateDebounce=*/std::chrono::steady_clock::duration::zero(),
       ASTRetentionPolicy());