SL-10153: Introduce ll_convert, windows_message() templates.

Add ll_convert<TO, FROM> template, used as (e.g.):
ll_convert<std::string>(value_of_some_other_string_type);
There is no generic template implementation -- the template exists solely to
provide generic aliases for a bewildering family of llstring.h string-
conversion functions with highly-specific names. There's a generic
implementation, though, for the degenerate case where FROM and TO are
identical.

Add ll_convert<> specialization aliases for most of the string-conversion
functions declared in llstring.h, including the Windows-specific ones
involving llutf16string and std::wstring.

Add a mini-lecture in llstring.h about appropriate use of string types on
Windows.

Add LL_WCHAR_T_NATIVE llpreprocessor.h macro so we can detect whether to
provide separate conversions for llutf16string and std::wstring, or whether
those would collide because the types are identical.

Add inline ll_convert_wide_to_string(const std::wstring&) overloads so caller
isn't required to call arg.c_str(), which naturally permits an ll_convert
alias.

Add ll_convert_wide_to_wstring(), ll_convert_wstring_to_wide() as placeholders
for converting between Windows std::wstring and Linden LLWString, with
corresponding ll_convert aliases. We don't yet have library code to perform
such conversions officially; for now, just copy characters.

Add LLStringUtil::getenv(key) and getoptenv(key) functions. The latter returns
boost::optional<string_type> in case the caller needs to detect absence of a
given environment variable rather than simply accepting a default value.
Naturally getenv(), which accepts a default, is implemented using getoptenv().
getoptenv(), in turn, is implemented using an underlying llstring_getoptenv().

On Windows, llstring_getoptenv() returns boost::optional<std::wstring> (based
on GetEnvironmentVariableW()), whereas elsewhere, llstring_getoptenv() returns
boost::optional<std::string> (based on classic Posix getenv()).

The beauty of generic ll_convert is that the portable LLStringUtilBase<T>::
getoptenv() template can call the platform-specific llstring_getoptenv() and
transparently perform whatever conversion is necessary to return the desired
string_type.

Add windows_message<T>(error) template, with an overload that implicitly calls
GetLastError(). We provide a single concrete windows_message<std::wstring>()
implementation because that's what we get from Windows FormatMessageW() --
everything else is a generic conversion to the desired target string type.

This obviates llprocess.cpp's previous WindowsErrorString() implementation --
reimplement using windows_message<std::string>().

1 files changed, 3 insertions, 24 deletions

diff --git a/indra/llcommon/llprocess.cpp b/indra/llcommon/llprocess.cpp
index 5753efdc59..1fa53f322b 100644
--- a/indra/llcommon/llprocess.cpp
+++ b/indra/llcommon/llprocess.cpp
@@ -1205,30 +1205,9 @@ static LLProcess::Status interpret_status(int status)
 /// GetLastError()/FormatMessage() boilerplate
 static std::string WindowsErrorString(const std::string& operation)
 {
-	int result = GetLastError();
-
-	LPTSTR error_str = 0;
-	if (FormatMessage( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
-					   NULL,
-					   result,
-					   0,
-					   (LPTSTR)&error_str,
-					   0,
-					   NULL)
-		!= 0) 
-	{
-		// convert from wide-char string to multi-byte string
-		char message[256];
-		wcstombs(message, error_str, sizeof(message));
-		message[sizeof(message)-1] = 0;
-		LocalFree(error_str);
-		// convert to std::string to trim trailing whitespace
-		std::string mbsstr(message);
-		mbsstr.erase(mbsstr.find_last_not_of(" \t\r\n"));
-		return STRINGIZE(operation << " failed (" << result << "): " << mbsstr);
-	}
-	return STRINGIZE(operation << " failed (" << result
-					 << "), but FormatMessage() did not explain");
+	auto result = GetLastError();
+	return STRINGIZE(operation << " failed (" << result << "): "
+					 << windows_message<std::string>(result));
 }
 
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