[clang] [clang][analyzer] Support `fflush` in the StreamChecker (PR #74296)

[Balázs Kéri via cfe-commits]

================
@@ -299,6 +299,74 @@ void error_fseek_0(void) {
   fclose(F);
 }
 
+void error_fflush_after_fclose(void) {
+  FILE *F = tmpfile();
+  int Ret;
+  fflush(NULL);                      // no-warning
+  if (!F)
+    return;
+  if ((Ret = fflush(F)) != 0)
+    clang_analyzer_eval(Ret == EOF); // expected-warning {{TRUE}}
+  fclose(F);
+  fflush(F);                         // expected-warning {{Stream might be already closed}}
+}
+
+void error_fflush_on_open_failed_stream(void) {
+  FILE *F = tmpfile();
+  if (!F) {
+    fflush(F); // no-warning
+    return;
+  }
+  fclose(F);
+}
+
+void error_fflush_on_unknown_stream(FILE *F) {
+  fflush(F);   // no-warning
+}
+
+void error_fflush_on_non_null_stream_clear_error_states(void) {
+  FILE *F0 = tmpfile(), *F1 = tmpfile();
+  // `fflush` clears a non-EOF stream's error state.
+  if (F0) {
+    StreamTesterChecker_make_ferror_stream(F0);
+    if (fflush(F0) == 0) {             // no-warning
+      clang_analyzer_eval(ferror(F0)); // expected-warning {{FALSE}}
+      clang_analyzer_eval(feof(F0));   // expected-warning {{FALSE}}
+    }
+    fclose(F0);
+  }
+  // `fflush` clears an EOF stream's error state.
+  if (F1) {
+    StreamTesterChecker_make_ferror_stream(F1);
+    StreamTesterChecker_make_feof_stream(F1);
+    if (fflush(F1) == 0) {             // no-warning
+      clang_analyzer_eval(ferror(F1)); // expected-warning {{FALSE}}
+      clang_analyzer_eval(feof(F1));   // expected-warning {{TRUE}}
+    }
+    fclose(F1);
+  }
+}
+
+void error_fflush_on_null_stream_clear_error_states(void) {
+  FILE *F0 = tmpfile(), *F1 = tmpfile();
+  // `fflush` clears all stream's error states, while retains their EOF states.
+  if (F0 && F1) {
+    StreamTesterChecker_make_ferror_stream(F0);
+    StreamTesterChecker_make_ferror_stream(F1);
----------------
balazske wrote:

Because `StreamTesterChecker_make_feof_stream` removes the error flag, this line is not useful. The test can check if the EOF flag remains active after `fflush` and the error flag is not set.

https://github.com/llvm/llvm-project/pull/74296