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/jit.c

@@ -0,0 +1,156 @@
+/*-------------------------------------------------------------------------
+ *
+ * jit.c
+ *	  Provider independent JIT infrastructure.
+ *
+ * Code related to loading JIT providers, redirecting calls into JIT providers
+ * and error handling.  No code specific to a specific JIT implementation
+ * should end up here.
+ *
+ *
+ * Copyright (c) 2016-2018, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/backend/jit/jit.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+
+#include "fmgr.h"
+#include "jit/jit.h"
+#include "miscadmin.h"
+#include "utils/resowner_private.h"
+#include "utils/fmgrprotos.h"
+
+
+/* GUCs */
+bool		jit_enabled = true;
+char	   *jit_provider = "llvmjit";
+
+static JitProviderCallbacks provider;
+static bool provider_successfully_loaded = false;
+static bool provider_failed_loading = false;
+
+
+static bool provider_init(void);
+static bool file_exists(const char *name);
+
+
+/*
+ * SQL level function returning whether JIT is available in the current
+ * backend. Will attempt to load JIT provider if necessary.
+ */
+Datum
+pg_jit_available(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_BOOL(provider_init());
+}
+
+
+/*
+ * Return whether a JIT provider has successfully been loaded, caching the
+ * result.
+ */
+static bool
+provider_init(void)
+{
+	char		path[MAXPGPATH];
+	JitProviderInit init;
+
+	/* don't even try to load if not enabled */
+	if (!jit_enabled)
+		return false;
+
+	/*
+	 * Don't retry loading after failing - attempting to load JIT provider
+	 * isn't cheap.
+	 */
+	if (provider_failed_loading)
+		return false;
+	if (provider_successfully_loaded)
+		return true;
+
+	/*
+	 * Check whether shared library exists. We do that check before actually
+	 * attempting to load the shared library (via load_external_function()),
+	 * because that'd error out in case the shlib isn't available.
+	 */
+	snprintf(path, MAXPGPATH, "%s/%s%s", pkglib_path, jit_provider, DLSUFFIX);
+	elog(DEBUG1, "probing availability of JIT provider at %s", path);
+	if (!file_exists(path))
+	{
+		elog(DEBUG1,
+			 "provider not available, disabling JIT for current session");
+		provider_failed_loading = true;
+		return false;
+	}
+
+	/*
+	 * If loading functions fails, signal failure. We do so because
+	 * load_external_function() might error out despite the above check if
+	 * e.g. the library's dependencies aren't installed. We want to signal
+	 * ERROR in that case, so the user is notified, but we don't want to
+	 * continually retry.
+	 */
+	provider_failed_loading = true;
+
+	/* and initialize */
+	init = (JitProviderInit)
+		load_external_function(path, "_PG_jit_provider_init", true, NULL);
+	init(&provider);
+
+	provider_successfully_loaded = true;
+	provider_failed_loading = false;
+
+	elog(DEBUG1, "successfully loaded JIT provider in current session");
+
+	return true;
+}
+
+/*
+ * Reset JIT provider's error handling. This'll be called after an error has
+ * been thrown and the main-loop has re-established control.
+ */
+void
+jit_reset_after_error(void)
+{
+	if (provider_successfully_loaded)
+		provider.reset_after_error();
+}
+
+/*
+ * Release resources required by one JIT context.
+ */
+void
+jit_release_context(JitContext *context)
+{
+	if (provider_successfully_loaded)
+		provider.release_context(context);
+
+	ResourceOwnerForgetJIT(context->resowner, PointerGetDatum(context));
+	pfree(context);
+}
+
+static bool
+file_exists(const char *name)
+{
+	struct stat st;
+
+	AssertArg(name != NULL);
+
+	if (stat(name, &st) == 0)
+		return S_ISDIR(st.st_mode) ? false : true;
+	else if (!(errno == ENOENT || errno == ENOTDIR))
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not access file \"%s\": %m", name)));
+
+	return false;
+}