[Support] env vars with empty values on windows

An environment variable can be in one of three states:

1. undefined.
2. defined with a non-empty value.
3. defined but with an empty value.

The windows implementation did not support case 3
(it was not handling errors). The Linux implementation
is already correct.

Differential Revision: https://reviews.llvm.org/D36394

llvm-svn: 311174

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Support/Windows/Process.inc b/llvm/lib/Support/Windows/Process.inc
index 18aef610d54..3fe9f89f1ef 100644
--- a/llvm/lib/Support/Windows/Process.inc
+++ b/llvm/lib/Support/Windows/Process.inc
@@ -129,9 +129,10 @@ Optional<std::string> Process::GetEnv(StringRef Name) {
   size_t Size = MAX_PATH;
   do {
     Buf.reserve(Size);
+    SetLastError(NO_ERROR);
     Size =
-        GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.capacity());
-    if (Size == 0)
+      GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.capacity());
+    if (Size == 0 && GetLastError() == ERROR_ENVVAR_NOT_FOUND)
       return None;
 
     // Try again with larger buffer.