[Backtracing] Add support for looking up paths for auxiliary executables.

We need to be able to locate `swift-backtrace` relative to the current
location of the runtime library.

This needs to work:

* In a Swift build directory.
* On Darwin, where we're installed in /usr/lib/swift and /usr/libexec/swift.
* On Linux, where we're in /usr/lib/swift/linux and /usr/libexec/swift/linux.
* On Windows, where we may be in a flat directory layout (because of limitations
  of Windows DLL lookups).

rdar://103071801

Author: al45tair

include/swift/Runtime/EnvironmentVariables.h

@@ -24,6 +24,10 @@ void initialize(void *);
 
 extern swift::once_t initializeToken;
 
+// Define a typedef "string" in swift::runtime::environment to make string
+// environment variables work
+using string = const char *;
+
 // Declare backing variables.
 #define VARIABLE(name, type, defaultValue, help) extern type name ## _variable;
 #include "../../../stdlib/public/runtime/EnvironmentVariables.def"

include/swift/Runtime/Paths.h

@@ -0,0 +1,77 @@
+//===--- Paths.h - Swift Runtime path utility functions ---------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// Functions that obtain paths that might be useful within the runtime.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_RUNTIME_UTILS_H
+#define SWIFT_RUNTIME_UTILS_H
+
+#include "swift/Runtime/Config.h"
+
+/// Return the path of the libswiftCore library.
+///
+/// This can be used to locate files that are installed alongside the Swift
+/// runtime library.
+///
+/// \return A string containing the full path to libswiftCore.  The string is
+///         owned by the runtime and should not be freed.
+SWIFT_RUNTIME_EXPORT
+const char *
+swift_getRuntimePath();
+
+/// Return the path of the Swift root.
+///
+/// If the path to libswiftCore is `/usr/local/swift/lib/libswiftCore.dylib`,
+/// this function would return `/usr/local/swift`.
+///
+/// The path returned here can be overridden by setting the environment variable
+/// SWIFT_ROOT.
+///
+/// \return A string containing the full path to the Swift root directory, based
+///         either on the location of the Swift runtime, or on the `SWIFT_ROOT`
+///         environment variable if set.
+SWIFT_RUNTIME_EXPORT
+const char *
+swift_getRootPath();
+
+/// Return the path of the specified auxiliary executable.
+///
+/// This function will search for the auxiliary executable in the following
+/// paths:
+///
+///   <swift-root>/libexec/swift/<platform>/<name>
+///   <swift-root>/libexec/swift/<name>
+///   <swift-root>/bin/<name>
+///   <swift-root>/<name>
+///
+/// It will return the first of those that exists, but it does not test that
+/// the file is indeed executable.
+///
+/// On Windows, it will automatically add `.exe` to the name, which means you
+/// do not need to special case the name for Windows.
+///
+/// If you are using this function to locate a utility program for use by the
+/// runtime, you should provide a way to override its location using an
+/// environment variable.
+///
+/// If the executable cannot be found, it will return nullptr.
+///
+/// \param name      The name of the executable to locate.
+///
+/// \return A string containing the full path to the executable.
+SWIFT_RUNTIME_EXPORT
+const char *
+swift_getAuxiliaryExecutablePath(const char *name);
+
+#endif // SWIFT_RUNTIME_PATHS_H

stdlib/public/runtime/CMakeLists.txt

@@ -62,6 +62,7 @@ set(swift_runtime_sources
     MetadataLookup.cpp
     Numeric.cpp
     Once.cpp
+    Paths.cpp
     Portability.cpp
     ProtocolConformance.cpp
     RefCount.cpp

stdlib/public/runtime/EnvironmentVariables.cpp

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/Runtime/Debug.h"
+#include "swift/Runtime/Paths.h"
 #include "swift/Runtime/EnvironmentVariables.h"
 
 #include <string.h>
@@ -24,6 +25,12 @@ using namespace swift;
 
 namespace {
 
+// This is required to make the macro machinery work correctly; we can't
+// declare a VARIABLE(..., const char *, ...) because then the token-pasted
+// names won't work properly.  It *does* mean that if you want to use std::string
+// somewhere in this file, you'll have to fully qualify the name.
+typedef const char *string;
+
 // Require all environment variable names to start with SWIFT_
 static constexpr bool hasSwiftPrefix(const char *str) {
   const char prefix[] = "SWIFT_";
@@ -124,6 +131,14 @@ static uint32_t parse_uint32_t(const char *name,
   return n;
 }
 
+static string parse_string(const char *name,
+                           const char *value,
+                           string defaultValue) {
+  if (!value || value[0] == 0)
+    return strdup(defaultValue);
+  return strdup(value);
+}
+
 // Print a list of all the environment variables. Lazy initialization makes
 // this a bit odd, but the use of these variables in the metadata system means
 // it's almost certain to run early.

stdlib/public/runtime/EnvironmentVariables.def

@@ -81,4 +81,9 @@ VARIABLE(SWIFT_BINARY_COMPATIBILITY_VERSION, uint32_t, 0,
 VARIABLE(SWIFT_DEBUG_FAILED_TYPE_LOOKUP, bool, false,
          "Enable warnings when we fail to look up a type by name.")
 
+VARIABLE(SWIFT_ROOT, string, "",
+         "Overrides the root directory of the Swift installation. "
+         "This is used to locate auxiliary files relative to the runtime "
+         "itself.")
+
 #undef VARIABLE