[YSQL] #2509 Force network library initialization on postmaster start…

…up on macOS

Summary:
Add a DNS lookup of the local hostname to postmaster startup to force
macOS network libraries to get initialized before any fork() calls
happen.  This fixes failures of ~20 tests in macOS debug mode. Without
this, PostgreSQL backends would frequently crash with SIGSEGV and dump
cores when trying to do the same DNS lookup.

Here is a SIGSEGV stack trace that we would previously get without this
fix:

```
frame #0: 0x00007fff7bd53e34 libsystem_trace.dylib`_os_log_cmp_key + 4
frame #1: 0x00007fff7bbfcb74 libsystem_c.dylib`rb_tree_find_node + 53
frame #2: 0x00007fff7bd52021 libsystem_trace.dylib`os_log_create + 368
frame #3: 0x00007fff7bc5b127 libsystem_info.dylib`gai_log_init + 23
frame #4: 0x00007fff7bd37ce3 libsystem_pthread.dylib`__pthread_once_handler + 65
frame #5: 0x00007fff7bd2daab libsystem_platform.dylib`_os_once_callout + 18
frame #6: 0x00007fff7bd37c7f libsystem_pthread.dylib`pthread_once + 56
frame #7: 0x00007fff7bc5a4ab libsystem_info.dylib`gai_log + 27
frame #8: 0x00007fff7bc5b33f libsystem_info.dylib`_gai_load_libnetwork_once + 63
frame #9: 0x00007fff7bd37ce3 libsystem_pthread.dylib`__pthread_once_handler + 65
frame #10: 0x00007fff7bd2daab libsystem_platform.dylib`_os_once_callout + 18
frame #11: 0x00007fff7bd37c7f libsystem_pthread.dylib`pthread_once + 56
frame #12: 0x00007fff7bc5b29b libsystem_info.dylib`_gai_load_libnetwork + 27
frame #13: 0x00007fff7bc5b64f libsystem_info.dylib`_gai_nat64_v4_address_requires_synthesis + 31
frame #14: 0x00007fff7bc5aaa0 libsystem_info.dylib`_gai_nat64_second_pass + 512
frame #15: 0x00007fff7bc39847 libsystem_info.dylib`si_addrinfo + 1959
frame #16: 0x00007fff7bc38f77 libsystem_info.dylib`_getaddrinfo_internal + 231
frame #17: 0x00007fff7bc38e7d libsystem_info.dylib`getaddrinfo + 61
frame #18: 0x000000011512f8e5 libyb_util.dylib`yb::GetFQDN(hostname="...") at net_util.cc:371:20
```

Test Plan: Jenkins

Reviewers: mihnea, dmitry

Reviewed By: dmitry

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D7757

src/postgres/src/backend/postmaster/postmaster.c

@@ -944,6 +944,19 @@ PostmasterMain(int argc, char *argv[])
 	}
 
 	YBReportIfYugaByteEnabled();
+#ifdef __APPLE__
+	if (YBIsEnabledInPostgresEnvVar()) {
+		/* 
+		 * Resolve local hostname to initialize macOS network libraries. If we
+		 * don't do this, there might be a lot of segmentation faults in
+		 * PostgreSQL backend processes in tests on macOS (especially debug
+		 * mode).
+		 *
+		 * See https://github.com/yugabyte/yugabyte-db/issues/2509 for details.
+		 */
+		YBCResolveHostname();
+	}
+#endif
 
 	/*
 	 * Create lockfile for data directory.

src/yb/common/ybc_util.cc

@@ -30,6 +30,8 @@
 #include "yb/util/status.h"
 #include "yb/util/version_info.h"
 
+#include "yb/util/net/net_util.h"
+
 #include "yb/gutil/stringprintf.h"
 
 using std::string;
@@ -258,6 +260,15 @@ const char* YBCGetStackTrace() {
   return YBCPAllocStdString(yb::GetStackTrace());
 }
 
+void YBCResolveHostname() {
+  string fqdn;
+  auto status = GetFQDN(&fqdn);
+  if (!status.ok()) {
+    LOG(WARNING) << "Failed to get fully qualified domain name of the local hostname: "
+                 << status;
+  }
+}
+
 } // extern "C"
 
 } // namespace yb

src/yb/common/ybc_util.h

@@ -42,6 +42,8 @@ const char* YBCStatusCodeAsCString(YBCStatus s);
 
 bool YBCIsRestartReadError(uint16_t txn_errcode);
 
+void YBCResolveHostname();
+
 #define CHECKED_YBCSTATUS __attribute__ ((warn_unused_result)) YBCStatus
 
 typedef void* (*YBCPAllocFn)(size_t size);