Fix multithreaded postmaster on macOS

curl_global_init() with an IPv6 enabled curl build on macOS will cause
the calling program to become multithreaded. Unfortunately for
shared_preload_libraries, that means the postmaster becomes
multithreaded, which CANNOT happen. There are checks in Postgres to make
sure that this is not the case.

pgxn/neon/control_plane_connector.c

@@ -35,16 +35,16 @@
 #include "utils/memutils.h"
 #include "utils/jsonb.h"
 
+#include "neon_utils.h"
+
 static ProcessUtility_hook_type PreviousProcessUtilityHook = NULL;
 
+static const char *jwt_token = NULL;
+
 /* GUCs */
 static char *ConsoleURL = NULL;
 static bool ForwardDDL = true;
 
-/* Curl structures for sending the HTTP requests */
-static CURL *CurlHandle;
-static struct curl_slist *ContentHeader = NULL;
-
 /*
  * CURL docs say that this buffer must exist until we call curl_easy_cleanup
  * (which we never do), so we make this a static
@@ -226,6 +226,8 @@ ErrorWriteCallback(char *ptr, size_t size, size_t nmemb, void *userdata)
 static void
 SendDeltasToControlPlane()
 {
+	static CURL		*handle = NULL;
+
 	if (!RootTable.db_table && !RootTable.role_table)
 		return;
 	if (!ConsoleURL)
@@ -236,29 +238,57 @@ SendDeltasToControlPlane()
 	if (!ForwardDDL)
 		return;
 
-	char	   *message = ConstructDeltaMessage();
-	ErrorString str = {};
+	if (handle == NULL)
+	{
+		struct curl_slist *headers = NULL;
 
-	curl_easy_setopt(CurlHandle, CURLOPT_CUSTOMREQUEST, "PATCH");
-	curl_easy_setopt(CurlHandle, CURLOPT_HTTPHEADER, ContentHeader);
-	curl_easy_setopt(CurlHandle, CURLOPT_POSTFIELDS, message);
-	curl_easy_setopt(CurlHandle, CURLOPT_URL, ConsoleURL);
-	curl_easy_setopt(CurlHandle, CURLOPT_ERRORBUFFER, CurlErrorBuf);
-	curl_easy_setopt(CurlHandle, CURLOPT_TIMEOUT, 3L /* seconds */ );
-	curl_easy_setopt(CurlHandle, CURLOPT_WRITEDATA, &str);
-	curl_easy_setopt(CurlHandle, CURLOPT_WRITEFUNCTION, ErrorWriteCallback);
+		headers = curl_slist_append(headers, "Content-Type: application/json");
+		if (headers == NULL)
+		{
+			elog(ERROR, "Failed to set Content-Type header");
+		}
+
+		if (jwt_token)
+		{
+			char		auth_header[8192];
+
+			snprintf(auth_header, sizeof(auth_header), "Authorization: Bearer %s", jwt_token);
+			headers = curl_slist_append(headers, auth_header);
+			if (headers == NULL)
+			{
+				elog(ERROR, "Failed to set Authorization header");
+			}
+		}
+
+		handle = alloc_curl_handle();
+
+		curl_easy_setopt(handle, CURLOPT_CUSTOMREQUEST, "PATCH");
+		curl_easy_setopt(handle, CURLOPT_HTTPHEADER, headers);
+		curl_easy_setopt(handle, CURLOPT_URL, ConsoleURL);
+		curl_easy_setopt(handle, CURLOPT_ERRORBUFFER, CurlErrorBuf);
+		curl_easy_setopt(handle, CURLOPT_TIMEOUT, 3L /* seconds */ );
+		curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, ErrorWriteCallback);
+	}
+
+	char	   *message = ConstructDeltaMessage();
+	ErrorString str;
+
+	str.size = 0;
+
+	curl_easy_setopt(handle, CURLOPT_POSTFIELDS, message);
+	curl_easy_setopt(handle, CURLOPT_WRITEDATA, &str);
 
 	const int	num_retries = 5;
-	int			curl_status;
+	CURLcode	curl_status;
 
 	for (int i = 0; i < num_retries; i++)
 	{
-		if ((curl_status = curl_easy_perform(CurlHandle)) == 0)
+		if ((curl_status = curl_easy_perform(handle)) == 0)
 			break;
 		elog(LOG, "Curl request failed on attempt %d: %s", i, CurlErrorBuf);
 		pg_usleep(1000 * 1000);
 	}
-	if (curl_status != 0)
+	if (curl_status != CURLE_OK)
 	{
 		elog(ERROR, "Failed to perform curl request: %s", CurlErrorBuf);
 	}
@@ -266,13 +296,11 @@ SendDeltasToControlPlane()
 	{
 		long		response_code;
 
-		if (curl_easy_getinfo(CurlHandle, CURLINFO_RESPONSE_CODE, &response_code) != CURLE_UNKNOWN_OPTION)
+		if (curl_easy_getinfo(handle, CURLINFO_RESPONSE_CODE, &response_code) != CURLE_UNKNOWN_OPTION)
 		{
-			bool		error_exists = str.size != 0;
-
 			if (response_code != 200)
 			{
-				if (error_exists)
+				if (str.size != 0)
 				{
 					elog(ERROR,
 						 "Received HTTP code %ld from control plane: %s",
@@ -835,34 +863,10 @@ InitControlPlaneConnector()
 							 NULL,
 							 NULL);
 
-	const char *jwt_token = getenv("NEON_CONTROL_PLANE_TOKEN");
-
+	jwt_token = getenv("NEON_CONTROL_PLANE_TOKEN");
 	if (!jwt_token)
 	{
 		elog(LOG, "Missing NEON_CONTROL_PLANE_TOKEN environment variable, forwarding will not be authenticated");
 	}
 
-	if (curl_global_init(CURL_GLOBAL_DEFAULT))
-	{
-		elog(ERROR, "Failed to initialize curl");
-	}
-	if ((CurlHandle = curl_easy_init()) == NULL)
-	{
-		elog(ERROR, "Failed to initialize curl handle");
-	}
-	if ((ContentHeader = curl_slist_append(ContentHeader, "Content-Type: application/json")) == NULL)
-	{
-		elog(ERROR, "Failed to initialize content header");
-	}
-
-	if (jwt_token)
-	{
-		char		auth_header[8192];
-
-		snprintf(auth_header, sizeof(auth_header), "Authorization: Bearer %s", jwt_token);
-		if ((ContentHeader = curl_slist_append(ContentHeader, auth_header)) == NULL)
-		{
-			elog(ERROR, "Failed to initialize authorization header");
-		}
-	}
 }

pgxn/neon/extension_server.c

@@ -14,6 +14,8 @@
 
 #include "utils/guc.h"
 
+#include "neon_utils.h"
+
 static int	extension_server_port = 0;
 
 static download_extension_file_hook_type prev_download_extension_file_hook = NULL;
@@ -31,15 +33,19 @@ static download_extension_file_hook_type prev_download_extension_file_hook = NUL
 static bool
 neon_download_extension_file_http(const char *filename, bool is_library)
 {
-	CURL	   *curl;
+	static CURL	   *handle = NULL;
+
 	CURLcode	res;
 	char	   *compute_ctl_url;
 	char	   *postdata;
 	bool		ret = false;
 
-	if ((curl = curl_easy_init()) == NULL)
+	if (handle == NULL)
 	{
-		elog(ERROR, "Failed to initialize curl handle");
+		handle = alloc_curl_handle();
+
+		curl_easy_setopt(handle, CURLOPT_CUSTOMREQUEST, "POST");
+		curl_easy_setopt(handle, CURLOPT_TIMEOUT, 3L /* seconds */ );
 	}
 
 	compute_ctl_url = psprintf("http://localhost:%d/extension_server/%s%s",
@@ -47,28 +53,22 @@ neon_download_extension_file_http(const char *filename, bool is_library)
 
 	elog(LOG, "Sending request to compute_ctl: %s", compute_ctl_url);
 
-	curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
-	curl_easy_setopt(curl, CURLOPT_URL, compute_ctl_url);
-	curl_easy_setopt(curl, CURLOPT_TIMEOUT, 3L /* seconds */ );
+	curl_easy_setopt(handle, CURLOPT_URL, compute_ctl_url);
 
-	if (curl)
+	/* Perform the request, res will get the return code */
+	res = curl_easy_perform(handle);
+	/* Check for errors */
+	if (res == CURLE_OK)
 	{
-		/* Perform the request, res will get the return code */
-		res = curl_easy_perform(curl);
-		/* Check for errors */
-		if (res == CURLE_OK)
-		{
-			ret = true;
-		}
-		else
-		{
-			/* Don't error here because postgres will try to find the file */
-			/* and will fail with some proper error message if it's not found. */
-			elog(WARNING, "neon_download_extension_file_http failed: %s\n", curl_easy_strerror(res));
-		}
-
-		/* always cleanup */
-		curl_easy_cleanup(curl);
+		ret = true;
+	}
+	else
+	{
+		/*
+		 * Don't error here because postgres will try to find the file and will
+		 * fail with some proper error message if it's not found.
+		 */
+		elog(WARNING, "neon_download_extension_file_http failed: %s\n", curl_easy_strerror(res));
 	}
 
 	return ret;

pgxn/neon/neon_utils.c

@@ -1,6 +1,9 @@
-
 #include <sys/resource.h>
 
+#ifndef WALPROPOSER_LIB
+#include <curl/curl.h>
+#endif
+
 #include "postgres.h"
 
 #include "lib/stringinfo.h"
@@ -114,3 +117,48 @@ disable_core_dump()
 		fprintf(stderr, "WARNING: disable cores setrlimit failed: %s", strerror(save_errno));
 	}
 }
+
+#ifndef WALPROPOSER_LIB
+
+/*
+ * On macOS with a libcurl that has IPv6 support, curl_global_init() calls
+ * SCDynamicStoreCopyProxies(), which makes the program multithreaded. An ideal
+ * place to call curl_global_init() would be _PG_init(), but Neon has to be
+ * added to shared_preload_libraries, which are loaded in the Postmaster
+ * process. The Postmaster is not supposed to become multithreaded at any point
+ * in its lifecycle. Postgres doesn't have any good hook that I know of to
+ * initialize per-backend structures, so we have to check this on any
+ * allocation of a CURL handle.
+ *
+ * Free the allocated CURL handle with curl_easy_cleanup(3).
+ *
+ * https://developer.apple.com/documentation/systemconfiguration/1517088-scdynamicstorecopyproxies
+ */
+CURL *
+alloc_curl_handle(void)
+{
+	static bool curl_initialized = false;
+
+	CURL *handle;
+
+	if (unlikely(!curl_initialized))
+	{
+		/* Protected by mutex internally */
+		if (curl_global_init(CURL_GLOBAL_DEFAULT))
+		{
+			elog(ERROR, "Failed to initialize curl");
+		}
+
+		curl_initialized = true;
+	}
+
+	handle = curl_easy_init();
+	if (handle == NULL)
+	{
+		elog(ERROR, "Failed to initialize curl handle");
+	}
+
+	return handle;
+}
+
+#endif

pgxn/neon/neon_utils.h

@@ -1,6 +1,12 @@
 #ifndef __NEON_UTILS_H__
 #define __NEON_UTILS_H__
 
+#include "lib/stringinfo.h"
+
+#ifndef WALPROPOSER_LIB
+#include <curl/curl.h>
+#endif
+
 bool		HexDecodeString(uint8 *result, char *input, int nbytes);
 uint32		pq_getmsgint32_le(StringInfo msg);
 uint64		pq_getmsgint64_le(StringInfo msg);
@@ -8,4 +14,10 @@ void		pq_sendint32_le(StringInfo buf, uint32 i);
 void		pq_sendint64_le(StringInfo buf, uint64 i);
 extern void disable_core_dump();
 
+#ifndef WALPROPOSER_LIB
+
+CURL *		alloc_curl_handle(void);
+
+#endif
+
 #endif							/* __NEON_UTILS_H__ */