[flang][driver] Disallow non-existent input files in the frontend driver

This patch adds a check that verifies that the input file used when
calling the frontend driver (i.e. `flang-new -fc1`) actually exists.
This was not required for the compiler driver, `flang-new`, as that's
already handled in libclangDriver.

Once all input/output file management is moved to the driver, we should
also check that for input from `stdin` the corresponding file descriptor
was successfully acquired.

This patch also makes sure that the default action in the frontend is
`ParseSyntaxOnly`. This is consistent with Clang. Before this change
`flang-new -fc1` would do nothing, which makes testing changes like the
one introduced here a bit tricky.

Reviewed By: SouraVX

Differential Revision: https://reviews.llvm.org/D95127

flang/lib/Frontend/CompilerInvocation.cpp

@@ -84,6 +84,10 @@ bool Fortran::frontend::ParseDiagnosticArgs(clang::DiagnosticOptions &opts,
 
 static InputKind ParseFrontendArgs(FrontendOptions &opts,
     llvm::opt::ArgList &args, clang::DiagnosticsEngine &diags) {
+
+  // By default the frontend driver creates a ParseSyntaxOnly action.
+  opts.programAction_ = ParseSyntaxOnly;
+
   // Identify the action (i.e. opts.ProgramAction)
   if (const llvm::opt::Arg *a =
           args.getLastArg(clang::driver::options::OPT_Action_Group)) {

flang/lib/Frontend/FrontendAction.cpp

@@ -11,7 +11,9 @@
 #include "flang/Frontend/FrontendActions.h"
 #include "flang/Frontend/FrontendOptions.h"
 #include "flang/FrontendTool/Utils.h"
+#include "clang/Basic/DiagnosticFrontend.h"
 #include "llvm/Support/Errc.h"
+#include "llvm/Support/VirtualFileSystem.h"
 
 using namespace Fortran::frontend;
 
@@ -31,6 +33,30 @@ bool FrontendAction::BeginSourceFile(
     CompilerInstance &ci, const FrontendInputFile &realInput) {
 
   FrontendInputFile input(realInput);
+
+  // Return immediately if the input file does not exist or is not a file. Note
+  // that we cannot check this for input from stdin.
+  if (input.file() != "-") {
+    if (!llvm::sys::fs::is_regular_file(input.file())) {
+      // Create an diagnostic ID to report
+      unsigned diagID;
+      if (llvm::vfs::getRealFileSystem()->exists(input.file())) {
+        ci.diagnostics().Report(clang::diag::err_fe_error_reading)
+            << input.file();
+        diagID = ci.diagnostics().getCustomDiagID(
+            clang::DiagnosticsEngine::Error, "%0 is not a regular file");
+      } else {
+        diagID = ci.diagnostics().getCustomDiagID(
+            clang::DiagnosticsEngine::Error, "%0 does not exist");
+      }
+
+      // Report the diagnostic and return
+      ci.diagnostics().Report(diagID) << input.file();
+      BeginSourceFileCleanUp(*this, ci);
+      return false;
+    }
+  }
+
   assert(!instance_ && "Already processing a source file!");
   assert(!realInput.IsEmpty() && "Unexpected empty filename!");
   set_currentInput(realInput);

flang/test/Flang-Driver/missing-input.f90

@@ -1,5 +1,31 @@
-! RUN: not %flang-new  2>&1 | FileCheck %s
-
 ! REQUIRES: new-flang-driver
 
-! CHECK: flang-new: error: no input files
+! Test the behaviour of the driver when input is missing or is invalid. Note
+! that with the compiler driver (flang-new), the input _has_ to be specified.
+! Indeed, the driver decides what "job/command" to create based on the input
+! file's extension. No input file means that it doesn't know what to do
+! (compile?  preprocess? link?). The frontend driver (flang-new -fc1) simply
+! assumes that "no explicit input == read from stdin"
+
+!--------------------------
+! FLANG DRIVER (flang-new)
+!--------------------------
+! RUN: not %flang-new  2>&1 | FileCheck %s --check-prefix=FLANG-NO-FILE
+! RUN: not %flang-new %t 2>&1 | FileCheck %s --check-prefix=FLANG-NONEXISTENT-FILE
+
+!-----------------------------------------
+! FLANG FRONTEND DRIVER (flang-new -fc1)
+!-----------------------------------------
+! RUN: not %flang-new -fc1 %t 2>&1  | FileCheck %s --check-prefix=FLANG-FC1-NONEXISTENT-FILE
+! RUN: not %flang-new -fc1 %S 2>&1  | FileCheck %s --check-prefix=FLANG-FC1-DIR
+
+!-----------------------
+! EXPECTED OUTPUT
+!-----------------------
+! FLANG-NO-FILE: flang-new: error: no input files
+
+! FLANG-NONEXISTENT-FILE: flang-new: error: no such file or directory: {{.*}}
+! FLANG-NONEXISTENT-FILE: flang-new: error: no input files
+
+! FLANG-FC1-NONEXISTENT-FILE: error: {{.*}} does not exist
+! FLANG-FC1-DIR: error: {{.*}} is not a regular file