Add support for setting the path to llvm-symbolizer through an environment variable

[mehdi_amini]

Summary

This allows to point to an executable that isn't named exactly
"llvm-symbolizer" and not necessarily in the current PATH.

[mehdi_amini]

Tweak variable name

[MaskRay]

llvm/lib/Support/Signals.cpp

123–126

if (const char *Path = getenv(LLVMSymbolizerPathEnv))

[MaskRay]

llvm/lib/Support/Signals.cpp

124

And perhaps you want to add a pair of braces because ...

You wrote https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements :)

// Use braces for the `if` block to keep it uniform with the else block.

[mehdi_amini]

Address comments

[MaskRay]

LGTM.

llvm/lib/Support/Signals.cpp

 // Use explicit storage to avoid accessing cl::opt in a signal handler.
 static bool DisableSymbolicationFlag = false;
 static cl::opt<bool, true>
     DisableSymbolication("disable-symbolication",
                          cl::desc("Disable symbolizing crash backtraces."),
                          cl::location(DisableSymbolicationFlag), cl::Hidden);
 
 constexpr char DisableSymbolizationEnv[] = "LLVM_DISABLE_SYMBOLIZATION";
+constexpr char LlvmSymbolizerPath[] = "LLVM_SYMBOLIZER_PATH";
 
 // Callbacks to run in signal handler must be lock-free because a signal handler
 // could be running as we add new callbacks. We don't add unbounded numbers of
 // callbacks, an array is therefore sufficient.
 struct CallbackAndCookie {
   sys::SignalHandlerCallback Callback;
   void *Cookie;
   enum class Status { Empty, Initializing, Initialized, Executing };
static bool printSymbolizedStackTrace(StringRef Argv0, void **StackTrace,
   if (Argv0.find("llvm-symbolizer") != std::string::npos)
     return false;
 
   // FIXME: Subtract necessary number from StackTrace entries to turn return addresses
   // into actual instruction addresses.
   // Use llvm-symbolizer tool to symbolize the stack traces. First look for it
   // alongside our binary, then in $PATH.
   ErrorOr<std::string> LLVMSymbolizerPathOrErr = std::error_code();
-  if (!Argv0.empty()) {
+  if (getenv(LlvmSymbolizerPath))
+    LLVMSymbolizerPathOrErr =
+        sys::findProgramByName(getenv(LlvmSymbolizerPath));
+  else if (!Argv0.empty()) {
     StringRef Parent = llvm::sys::path::parent_path(Argv0);
     if (!Parent.empty())
       LLVMSymbolizerPathOrErr = sys::findProgramByName("llvm-symbolizer", Parent);
   }
   if (!LLVMSymbolizerPathOrErr)
     LLVMSymbolizerPathOrErr = sys::findProgramByName("llvm-symbolizer");
   if (!LLVMSymbolizerPathOrErr)
     return false;