Basic JIT provider and error handling infrastructure.

This commit introduces:

1) JIT provider abstraction, which allows JIT functionality to be
   implemented in separate shared libraries. That's desirable because
   it allows to install JIT support as a separate package, and because
   it allows experimentation with different forms of JITing.
2) JITContexts which can be, using functions introduced in follow up
   commits, used to emit JITed functions, and have them be cleaned up
   on error.
3) The outline of a LLVM JIT provider, which will be fleshed out in
   subsequent commits.

Documentation for GUCs added, and for JIT in general, will be added in
later commits.

Author: Andres Freund, with architectural input from Jeff Davis
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

src/backend/jit/llvm/llvmjit_error.cpp

@@ -0,0 +1,141 @@
+/*-------------------------------------------------------------------------
+ *
+ * llvmjit_error.cpp
+ *	  LLVM error related handling that requires interfacing with C++
+ *
+ * Unfortunately neither (re)setting the C++ new handler, nor the LLVM OOM
+ * handler are exposed to C. Therefore this file wraps the necesary code.
+ *
+ * Copyright (c) 2016-2018, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/backend/jit/llvm/llvmjit_error.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+extern "C"
+{
+#include "postgres.h"
+}
+
+#include <llvm/Support/ErrorHandling.h>
+
+#include "jit/llvmjit.h"
+
+
+static int fatal_new_handler_depth = 0;
+static std::new_handler old_new_handler = NULL;
+
+static void fatal_system_new_handler(void);
+#if LLVM_VERSION_MAJOR > 4
+static void fatal_llvm_new_handler(void *user_data, const std::string& reason, bool gen_crash_diag);
+#endif
+static void fatal_llvm_error_handler(void *user_data, const std::string& reason, bool gen_crash_diag);
+
+
+/*
+ * Enter a section in which C++ and LLVM errors are treated as FATAL errors.
+ *
+ * This is necessary for LLVM as LLVM's error handling for such cases
+ * (exit()ing, throwing std::bad_alloc() if compiled with exceptions, abort())
+ * isn't compatible with postgres error handling.  Thus in section where LLVM
+ * code, not LLVM generated functions!, is executing, standard new, LLVM OOM
+ * and LLVM fatal errors (some OOM errors masquerade as those) are redirected
+ * to our own error handlers.
+ *
+ * These error handlers FATAL, because there's no reliable way from within
+ * LLVM to throw an error that's guaranteed not to corrupt LLVM's state.
+ *
+ * To avoid disturbing extensions using C++ and/or LLVM, these handlers are
+ * unset when not executing LLVM code. There is no need to call
+ * llvm_leave_fatal_on_oom() when ERRORing out, error recovery resets the
+ * handlers in that case.
+ */
+void
+llvm_enter_fatal_on_oom(void)
+{
+	if (fatal_new_handler_depth == 0)
+	{
+		old_new_handler = std::set_new_handler(fatal_system_new_handler);
+#if LLVM_VERSION_MAJOR > 4
+		llvm::install_bad_alloc_error_handler(fatal_llvm_new_handler);
+#endif
+		llvm::install_fatal_error_handler(fatal_llvm_error_handler);
+	}
+	fatal_new_handler_depth++;
+}
+
+/*
+ * Leave fatal error section started with llvm_enter_fatal_on_oom().
+ */
+void
+llvm_leave_fatal_on_oom(void)
+{
+	fatal_new_handler_depth--;
+	if (fatal_new_handler_depth == 0)
+	{
+		std::set_new_handler(old_new_handler);
+#if LLVM_VERSION_MAJOR > 4
+		llvm::remove_bad_alloc_error_handler();
+#endif
+		llvm::remove_fatal_error_handler();
+	}
+}
+
+/*
+ * Reset fatal error handling. This should only be called in error recovery
+ * loops like PostgresMain()'s.
+ */
+void
+llvm_reset_after_error(void)
+{
+	if (fatal_new_handler_depth != 0)
+	{
+		std::set_new_handler(old_new_handler);
+#if LLVM_VERSION_MAJOR > 4
+		llvm::remove_bad_alloc_error_handler();
+#endif
+		llvm::remove_fatal_error_handler();
+	}
+	fatal_new_handler_depth = 0;
+}
+
+void
+llvm_assert_in_fatal_section(void)
+{
+	Assert(fatal_new_handler_depth > 0);
+}
+
+static void
+fatal_system_new_handler(void)
+{
+	ereport(FATAL,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("out of memory"),
+			 errdetail("while in LLVM")));
+}
+
+#if LLVM_VERSION_MAJOR > 4
+static void
+fatal_llvm_new_handler(void *user_data,
+					   const std::string& reason,
+					   bool gen_crash_diag)
+{
+	ereport(FATAL,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("out of memory"),
+			 errdetail("While in LLVM: %s", reason.c_str())));
+}
+#endif
+
+static void
+fatal_llvm_error_handler(void *user_data,
+						 const std::string& reason,
+						 bool gen_crash_diag)
+{
+	ereport(FATAL,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("fatal llvm error: %s",
+					reason.c_str())));
+}