Add ability for can_write to indicate why the file is not writable

[rsmith]

Summary

Add an optional output parameter to can_write to allow it to indicate why a file is not considered writable (along with a sample use from Clang where we currently produce stupid diagnostics saying "can't open output file: Success").

I'm not entirely sure this is the right design, and in particular it's a somewhat different interface from the other functions in this file. I also considered:

std::error_code can_write(const Twine &Path, bool &result);

... which seems redundant because result is set to true iff the error code is success, and ...

std::error_code can_write(const Twine &Path);

... which is terrible because bool(can_write(Path)) is true iff the file is *not* writable, and ...

std::error_code cannot_write(const Twine &Path);

... which is inconsistent with other functions near here and ugly.

[• rafael]

• Don't use can_write.
• Change both exists and can_write to "std::error_code access(const

Twine &Path, SomeEnum Mode)".

• Change can_write to "std::error_code write_access(const Twine &Path)".

There is also can_write. OK, I will give it a try at merging these.

Cheers,
Rafael

include/llvm/Support/FileSystem.h

 /// @brief Can we write this file?
 ///
 /// @param Path Input path.
-/// @returns True if we can write to it, false otherwise.
-bool can_write(const Twine &Path);
+/// @param Err If file file is not writable and @p Err is not @c nullptr,
+///            @c *Err is set to a platform-specific @c error_code indicating
+///            the error that would be produced by an attempt to write to the
+///            file.
+/// @returns True if the file exists and we can write to it, false otherwise.
+bool can_write(const Twine &Path, std::error_code *Err = nullptr);
 
 /// @brief Do file_status's represent the same thing?
 ///

lib/Frontend/CompilerInstance.cpp

       llvm::sys::fs::status(OutputPath, Status);
       if (llvm::sys::fs::exists(Status)) {
         // Fail early if we can't write to the final destination.
-        if (!llvm::sys::fs::can_write(OutputPath))
+        if (!llvm::sys::fs::can_write(OutputPath, &Error))
           return nullptr;
 
         // Don't use a temporary if the output is a special file. This handles

lib/Support/Unix/Path.inc

   return std::error_code();
 }
 
-bool can_write(const Twine &Path) {
+bool can_write(const Twine &Path, std::error_code *Err) {
   SmallString<128> PathStorage;
   StringRef P = Path.toNullTerminatedStringRef(PathStorage);
-  return 0 == access(P.begin(), W_OK);
+  if (0 != access(P.begin(), W_OK)) {
+    if (Err)
+      *Err = std::error_code(errno, std::generic_category());
+    return false;
+  }
+  return true;
 }
 
 bool can_execute(const Twine &Path) {

lib/Support/Windows/Path.inc

   return std::error_code();
 }
 
-bool can_write(const Twine &Path) {
+bool can_write(const Twine &Path, std::error_code *Err) {
   // FIXME: take security attributes into account.
   SmallString<128> PathStorage;
   SmallVector<wchar_t, 128> PathUtf16;
 
-  if (UTF8ToUTF16(Path.toStringRef(PathStorage), PathUtf16))
+  if (std::error_code ec =
+          UTF8ToUTF16(Path.toStringRef(PathStorage), PathUtf16)) {
+    if (Err) *Err = ec;
     return false;
+  }
 
   DWORD Attr = ::GetFileAttributesW(PathUtf16.begin());
-  return (Attr != INVALID_FILE_ATTRIBUTES) && !(Attr & FILE_ATTRIBUTE_READONLY);
+  if (Attr == INVALID_FILE_ATTRIBUTES) {
+    if (Err) *Err = windows_error(::GetLastError());
+    return false;
+  }
+  if (Attr & FILE_ATTRIBUTE_READONLY) {
+    if (Err) *Err = windows_error(ERROR_ACCESS_DENIED);
+    return false;
+  }
+  return true;
 }
 
 bool can_execute(const Twine &Path) {

test/Frontend/output-failures.c

-// RUN: not %clang_cc1 -emit-llvm -o %S/doesnotexist/somename %s 2> %t
-// RUN: FileCheck -check-prefix=OUTPUTFAIL -input-file=%t %s
+// RUN: not %clang_cc1 -emit-llvm -o %S/doesnotexist/somename %s 2>&1 >/dev/null \
+// RUN:   | FileCheck -check-prefix=ENOENT %s
+// RUN: touch %t
+// RUN: chmod a-w %t
+// RUN: not %clang_cc1 -emit-llvm -o %t %s 2>&1 >/dev/null \
+// RUN:   | FileCheck -check-prefix=EACCES %s
+// RUN: rm -f %t
 
-// OUTPUTFAIL: error: unable to open output file '{{.*}}{{[/\\]}}test{{[/\\]}}Frontend{{[/\\]}}doesnotexist{{[/\\]}}somename': '{{[nN]}}o such file or directory'
+// REQUIRES: shell
+
+// ENOENT: error: unable to open output file '{{.*}}{{[/\\]}}test{{[/\\]}}Frontend{{[/\\]}}doesnotexist{{[/\\]}}somename': '{{[nN]}}o such file or directory'
+// EACCES: error: unable to open output file '{{.*}}{{[/\\]}}test{{[/\\]}}Frontend{{[/\\]}}doesnotexist{{[/\\]}}somename': '{{[pP]}}ermission denied'