Include the process PID in assertion-failure messages.

This should help to identify what happened when studying the postmaster
log after-the-fact.

While here, clean up some old comments in the same function.

Discussion: https://postgr.es/m/1568983.1601845687@sss.pgh.pa.us

src/backend/utils/error/assert.c

 /*-------------------------------------------------------------------------
  *
  * assert.c
- *	  Assert code.
+ *	  Assert support code.
  *
  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -10,9 +10,6 @@
  * IDENTIFICATION
  *	  src/backend/utils/error/assert.c
  *
- * NOTE
- *	  This should eventually work with elog()
- *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
@@ -24,6 +21,10 @@
 
 /*
  * ExceptionalCondition - Handles the failure of an Assert()
+ *
+ * We intentionally do not go through elog() here, on the grounds of
+ * wanting to minimize the amount of infrastructure that has to be
+ * working to report an assertion failure.
  */
 void
 ExceptionalCondition(const char *conditionName,
@@ -31,20 +32,21 @@ ExceptionalCondition(const char *conditionName,
 					 const char *fileName,
 					 int lineNumber)
 {
+	/* Report the failure on stderr (or local equivalent) */
 	if (!PointerIsValid(conditionName)
 		|| !PointerIsValid(fileName)
 		|| !PointerIsValid(errorType))
-		write_stderr("TRAP: ExceptionalCondition: bad arguments\n");
+		write_stderr("TRAP: ExceptionalCondition: bad arguments in PID %d\n",
+					 (int) getpid());
 	else
-	{
-		write_stderr("TRAP: %s(\"%s\", File: \"%s\", Line: %d)\n",
+		write_stderr("TRAP: %s(\"%s\", File: \"%s\", Line: %d, PID: %d)\n",
 					 errorType, conditionName,
-					 fileName, lineNumber);
-	}
+					 fileName, lineNumber, (int) getpid());
 
 	/* Usually this shouldn't be needed, but make sure the msg went out */
 	fflush(stderr);
 
+	/* If we have support for it, dump a simple backtrace */
 #ifdef HAVE_BACKTRACE_SYMBOLS
 	{
 		void	   *buf[100];
@@ -55,12 +57,12 @@ ExceptionalCondition(const char *conditionName,
 	}
 #endif
 
-#ifdef SLEEP_ON_ASSERT
-
 	/*
-	 * It would be nice to use pg_usleep() here, but only does 2000 sec or 33
-	 * minutes, which seems too short.
+	 * If configured to do so, sleep indefinitely to allow user to attach a
+	 * debugger.  It would be nice to use pg_usleep() here, but that can sleep
+	 * at most 2G usec or ~33 minutes, which seems too short.
 	 */
+#ifdef SLEEP_ON_ASSERT
 	sleep(1000000);
 #endif