[MCContext] Don't use getenv inside class constructor

[igor-laevsky]

Summary

In our llvm use case we are observing rare crashes inside MCContext constructor. Apparently they were caused by the non thread safe getenv call. This change handles this problem by moving getenv call into initialisation of a static command line option. It's slightly better then explicit locks and less complicated than adding new constructor argument.

This issue was already discussed sometime ago (see http://lists.llvm.org/pipermail/llvm-dev/2013-May/062367.html) but I haven't found any evidence that it was fixed back then.

[rnk]

lgtm

[grosbach]

How is calling getenv() in a global static initializer any better? Seems a step backwards.

[rnk]

Hopefully programs don't start threads during static initialization, so the
getenv call isn't racy with setenv. OTOH, programs that use setenv and
threads are already busted, and we can't really help them.

llvm/trunk/lib/MC/MCContext.cpp

 #include "llvm/MC/MCSectionCOFF.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCSectionMachO.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbolCOFF.h"
 #include "llvm/MC/MCSymbolELF.h"
 #include "llvm/MC/MCSymbolMachO.h"
 #include "llvm/Support/COFF.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ELF.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/SourceMgr.h"
 
 using namespace llvm;
 
+static cl::opt<char*>
+AsSecureLogFileName("as-secure-log-file-name",
+        cl::desc("As secure log file name (initialized from "
+                 "AS_SECURE_LOG_FILE env variable)"),
+        cl::init(getenv("AS_SECURE_LOG_FILE")), cl::Hidden);
+
+
 MCContext::MCContext(const MCAsmInfo *mai, const MCRegisterInfo *mri,
                      const MCObjectFileInfo *mofi, const SourceMgr *mgr,
                      bool DoAutoReset)
     : SrcMgr(mgr), MAI(mai), MRI(mri), MOFI(mofi), Allocator(),
       Symbols(Allocator), UsedNames(Allocator),
       CurrentDwarfLoc(0, 0, 0, DWARF2_FLAG_IS_STMT, 0, 0), DwarfLocSeen(false),
       GenDwarfForAssembly(false), GenDwarfFileNumber(0), DwarfVersion(4),
       AllowTemporaryLabels(true), DwarfCompileUnitID(0),
       AutoReset(DoAutoReset), HadError(false) {
-  SecureLogFile = getenv("AS_SECURE_LOG_FILE");
+  SecureLogFile = AsSecureLogFileName;
   SecureLog = nullptr;
   SecureLogUsed = false;
 
   if (SrcMgr && SrcMgr->getNumBuffers())
     MainFileName =
         SrcMgr->getMemoryBuffer(SrcMgr->getMainFileID())->getBufferIdentifier();
 }